Green roofs have a growing medium, typically grass or succulents. The growing medium can bind water and thereby reduce the amount of rainwater that needs to be dealt with on your property or that needs to be diverted to the sewer system. However, the water can lead to damage from damp.
The ability of the growing medium to bind and retain water is vital for how much water needs to be led away and the types of plants that can thrive on the roof.
The weight of a green roof depends on with the amount of water which the layer can retain. You should be aware of whether the roof structure can carry the load of the green roof.
Damage may be caused by damp if the roofing membrane beneath the humid growing medium is penetrated so that water reaches the roof construction.

If you want to establish a green roof, you will have to make sure that the roof structure has been dimensioned to carry the load of the entire roof, including a growing medium filled with water and the plants themselves. If the roof can absorb and retain a lot of water, for example in a thick topsoil layer, the roof will become very heavy.
Typically, a roofing membrane is used to separate the humid growing medium from the roof structure itself. It is important that the humid environment of the growing medium is kept separate from other parts of the building structure. You must therefore prevent roots etc. from penetrating the roofing membrane. Removing weeds and saplings may be required.
The growing medium can be kept in place by a grid to avoid ultraviolet degradation of the roofing membrane.

Strong winds or storms may cause gable ends and detached walls to collapse or roof tiles to break loose. The wind can also lift roof tiles or entire roof surfaces off the roof. Strong winds or storms may also tear off and displace solar shading, awnings, or temporary constructions such as scaffolding. Furthermore, the wind may get a hold of garden trampolines, garden furniture or lightweight buildings such as playhouses, rabbit hutches or hen houses. Detached objects may pose a danger to people and may cause damage to buildings. Damaged buildings, as well as objects and material that have become wedged, pose a safety risk to people in the area.

Usually, because they are insecure, structures can collapse, roof tiles and entire roofs can become detached, and objects can be swept away by the wind. So far, it has been sufficient to secure building components in accordance with the current building regulation requirements. A building expert can examine your house and assess whether everything has been properly secured and anchored. Over time, joints and anchoring may become weak. Detached objects such as garden trampolines and garden furniture can be packed away before a storm. Take shelter while the storm takes its course.

Trees are vulnerable during strong winds and storms. Especially during summer and early spring, when the trees have leaves and the wind can get a good grip on the tree top. Strong winds or storms are often followed by rain. If the ground near a tree becomes damp the risk of windfall increases. You should be aware that older trees may be weakened. Trees or branches on trees can be weakened from previous storms, fungus, disease or collision by other falling trees.

Trees need to be taken care of. Sick, dead and possible broken branches must be removed. Most trees have to be pruned at some point during their life time in order to encourage healthy and harmonious growth. The pruning techniques differ from species to species. You should consult a specialist before you start pruning.
It is important that trees develop a healthy and strong root system matching their size. If you have to perform excavation work near a tree when establishing a path or similar, it is important that you avoid damaging the roots of the tree. Following construction work or heavy-duty traffic near the tree, you should ensure that the tree does not suffer permanent damage from water deficiency.
Trees should be cut down before they pose a safety risk.

Powerful wind and tidal water can cause sea water levels to rise and result in a storm surge. You may experience water level increases in coastal areas, areas near fjords, rivers and watercourses connected to the sea. If the water level increases at sea, it may prevent the discharge of water from fjords, rivers and watercourses, and this, in turn, may lead water pooling up further inland. Large quantities of water may spill over into the terrain, including around houses. If your house has not been sufficiently secured, the water may intrude into your home.

During storm surges some areas will be under water and, usually, it will not be possible to divert the water away. You can prevent the backflow of sewage water with a backflow blocker.
Pipe penetrations and other penetrations in the buildings at ground level must be tightly sealed. You should ensure that basement windows and doors, as well as groundfloor doorways are waterproofed. Sand bags or watertight barriers that can be closed or established at short notice may be used to ensure that skylights, basement gutters and groundlevel access are all waterproof.
Property of value, such as family photos and documents, should be stored in watertight boxes, in particular, if you store then in the basement or other low-lying rooms. Another, less safe solution, is to store your effects in storage racks at an appropriate distance from the floor.

A milder climate can exacerbate the development of rot and wood beetles. Stronger sunshine and more hours of sunshine result in increased ultraviolet radiation. Increased ultraviolet radiation will require a greater extent of maintenance of outside surfaces, because organic building materials, including wood, plastic and paint, are degraded faster. You can expect reduced durability and life span of exterior materials and surfaces. Awnings perish, plastic goods turn brittle and painted surfaces peel.

Greater exposure to ultraviolet radiation and increased risk of rot and insects such as wood beetles place greater demands on building materials. Organic material, in particular, such as wood, some types of plastic, paper, cardboard and paints are vulnerable. It may be necessary to choose other materials for structures and surface treatment. Furthermore, more frequent maintenance may be required. In special circumstances, it may be relevant to modify the house to provide climate protection, or to establish mechanical ventilation, dehumidification and water diversion.

New pests and diseases are the result of increased globalisation and climate change. Globalisation creates spread, and climate change changes the local conditions for animals and plants. The balance of species, animal as well as plant, is constantly changing, and new challenges may weaken a local species before a new balance is created. Known species may come under threat or may disappear locally.
History is full of examples of the negative consequences for biodiversity. The introduction of rats and cats to Australia and the Iberian slug to Denmark are good examples of this.

All animals that are a nuisance to humans are pests. Apart from parasites, no animal is harmful by design. Most pests are merely plant eaters and their niche in the great scheme of things is to eat our plants. Invasive species often develop into pests because the biological balance shifts. As a garden owner you should avoid using pesticides and herbicides. Consult with local government pest management service for guidance and advice.
Your local government will have a greater knowledge about organisms used in forestry, nurseries, parks, landscapes and urban environments. You will be able to get information from your local government about known pests that have changed behaviour due to changed climatic conditions.

Water can collect around your house. If the water cannot drain away, it will be able to make outer walls and foundations damp and run through cracks and unsealed joints. Water can reduce the life of painted surfaces. At the same time, the risk of decay of materials increases. If water enters, there will be a risk of mould growing, of discoloration and peeling.

Ensure that water can drain away from the house. Terrain can be landscaped so that it falls away from the house. If necessary, you can dig ditches to help direct water away. A bank or stone wall around the house will prevent water from running in.

During wet periods, it can be difficult for soil to absorb more water. If rain falls on saturated soil, water will begin to collect and run on the surface of the soil. Large volumes can run to low-lying areas. The extent of the problem depends on the slope of the land and the size of the area.

Water must be channelled away in a controlled manner. It is possible to collect water by altering the slope of land and using ditches, grates or pipes. Water can be directed towards flower beds, depressions, ponds and fascine drains on the site. If water is channelled towards flower beds, these can be linked to a depressed area in a lawn which in turn can be linked to a fascine drain.

Drainage systems collect and channel water, typically to an underground catch basin. It is important that the entire system is designed to carry the volumes of water that may arise during heavy rainfall. This also applies to the parts of the system hidden below ground. If a catch basin has collapsed or is not large enough, water will collect, overflow and find other routes.

It is important to maintain the entire drainage system to avoid the accumulation of water. This means you must ensure underground sections are working correctly. It is important that drains and catch basins have the correct dimensions to collect and direct water away. Collapsed wells must be replaced. Wells that are too small should be replaced with larger ones.

Drain sizes should be designed to cope with the volumes of water that may arise during heavy rainfall. If drains, ditches or grates become blocked, water will accumulate, overflow and find other routes. The same will happen if bottlenecks occur in the drainage system due to a collapsed drain or one that is too narrow.

It is important to maintain the drains and keep them free of leaves and dirt. It is also important that roof gutters, grates and downpipes have the correct dimensions to collect and direct water away. Collapsed drains must be replaced. Drains that are too small should be replaced with larger drains.

Paving, asphalt and other surface coverings laid directly on the soil inhibit the infiltration of surface water. The same effect may occur where the ground consists of hard or saturated clay soil. Water in these areas will col-lect and run on the surface of the soil. Large volumes of water can – depending on the slope of land and size of area – collect in low-lying areas and constitute a considerable nuisance.

Water must be channelled away in a controlled manner. It is possible to collect water by changing the slope of the land and using ditches and grates. Water can be directed towards flower beds, ponds or fascine drains which are typically subterranean chambers. Check that it is possible to collect and channel the volumes of water that may arise during heavy rainfall.

Sudden thawing can convert large amounts of snow and ice into melt water very quickly. If melt water collects against the outside of your house, there is a risk your foundation and basement walls will absorb water and become damp, or that water will intrude via the garage, basement windows and doors, or ground-level and balcony doorways. Water can cause damage to your house in the form of discolouration and peeling wall coverings. Damp material exposed to heavy frost risks frost damage. Snow can gather in basement entrances, skylights, or against wall fronts. Snowdrift can enter attics or accumulate on the sheltered side of rooftops where it will eventually melt.

Make sure that water can run away from your house, possibly by regrading the ground so that it slopes away from the house. If this is not enough, you can dig gullies to divert the water away. Remove snow piling up near or against the house. Remove leaves, dirt and ice from drains and grates. Check that roof gutters, downpipes and ground level grates are working properly and remove snowdrift from the attic. A perimeter drain may be a good solution for outer foundation walls if there is a need to reduce the risk of water entering the basement through the walls.

Icicles often form when freezing sets in again after a period of thawing. The sun can cause the ice and snow to melt even during frosty weather. Furthermore, the heat loss from poorly insulated roofs can cause the snow on rooftops to melt and form icicles along the edge of the roof, because the surrounding temperature is below freezing. A continued feed of water will cause the icicles to grow over time. Icicles can eventually become very large, pointed and heavy. If hanging from rooftops, they may drop and cause damage and injury to people or objects. Homeowners have a duty to remove any icicles or to cordon off the area below them, so that no one can get hurt.

Gutters, downpipes and grates must be kept clean, so that melt water can be diverted away and does not freeze to ice in the gutters etc. You can limit the risk of icicles from rooftops by reducing the loss of heat through your roof. Icicles can be prevented by installing electric heating cables inside gutters and downpipes. Some types of heating cables will only turn on if the temperature drops towards freezing, in order to save electricity. You can remove icicles using a garden broom or rake to knock them down. You should be aware that icicles can be dangerous, also during removal. It is therefore important to cordon off the area below the icicles before you remove them. As a homeowner, you are liable if someone is injured because of icicles that fall from your roof.

In particular, low pitched roofs on broad buildings can be exposed to greater snow loads than they have been dimensioned for, so there could be a risk of structural collapse. A risk of collapse can occur, for example, after a longer period of wind and snowdrift from the same direction, and when the snow lies on the roof for long periods and turns hard and compact. Compacted snow on low-pitched roofs may prevent the water from draining off. If the water accumulates, it can flow back and into the loft and cause damage. Damage can also occur after sudden thawing if large quantities of water are released at once.

You should be aware of when snowdrifts form on your roof and make sure to remove the snow before the load becomes greater than the roof can cope with. When you remove snow, you must dispose of it so that it does not obstruct melt water from the roof from running away from the house, or obstruct pedestrians, bicycles and cars from passing on pavements and roads. You should also remember that moving about on the roof is dangerous.

When it freezes after thaw, black ice may form on surfaces such as pavements, driveways, garden paths and galleries. Typically, this happens when ice or snow melts during the day in sunny weather and then, subsequently, is exposed to temperatures below freezing. A continued feed of water will cause the black ice to grow thicker and more extensive over time. Black ice on pedestrian path surfaces is a potential safety risk, as people may slip and fall.
Black ice may pose a special risk and nuisance to people impaired through age or disability, as well as service employees bringing mail and newspaper carriers and collecting refuse. Homeowners are therefore responsible for removing snow and ice from their own walkways and on pavements in front of their house so that no one is injured.
Black ice can also cause damage to paved areas, and damp material may crack if exposed to severe frost.

Remove snow and ice and dirt from gullies and grates, so that melt water can be diverted away and does not freeze to ice. Use a shovel, spade or an old-fashioned iron snow scraper. It is a good idea to apply salt or gravel on walkways. The salt will cause the ice to melt and only small amounts are needed for an effect. Apply the salt by hand or a salt-shovel. However, remember that the salt is not good for your plants, bushes and trees.
If people are injured on the pavement outside your house, even though you have followed the above advice, you must notify the injury to your insurance company. Your insurance company is responsible for assessing whether the rules on slippery roads etc. have been breached and whether the injured person is eligible for indemnification. You should be aware of whether your insurance has homeowner liability coverage.

Concealed pipes in roofs, walls and flooring are vulnerable. Water pipes in the building envelope or in unheated rooms may burst during hard frost, leading to severe water damage when thaw sets in. If you change your heating system or how you heat your home, there is a risk of frost damage to concealed pipes. If you have an outside water tap, the tap and the connection to the water installations in the house may be at risk. Water can cause damage to furniture and result in discolouration and peeling. Furthermore, damp may entail a risk of mould growth.

Up to 1972, concealed pipe connections in water installations were allowed. The concealed pipes may run through roof slopes or inside walls and in attics. You should be wary of pipes in unheated rooms and pipes close to poorly insulated outer walls and attics. Some of these pipes may need better insulation. You can install a water alarm that will alert you or close down the water system if it detects water consumption beyond the usual.

During periods with heavy rain, the infiltration of water through the soil layers may be so extensive that the layers become saturated with water. This causes a build-up of pressure. If your house has a basement, water may intrude through cracks and fissures, for example around grates, pipes, cables etc. below ground. In a worst-case scenario, the pressure that builds up may push the floor up and the basement walls in, if they have not been dimensioned to resist the water pressure.

The pressure under floors and against walls can be reduced. In most cases, a well functioning perimeter drainage system will be enough. However, if the floor area is very large, or if the water cannot be diverted away from a perimeter drainage system, a groundwater pump may be the solution. A groundwater pump is suitable for draining clean water.
The pump must be placed in a pit, for example in the basement. The sides of the pit must allow the water from the soil to run into the pit. This will lower the water pressure during periods with water-saturated soil.

Note that due to soil conditions, the water which seeps into the pit may come from your neighbour's property.

A groundwater pump can also drain sewage water with more fluid particles, and some groundwater pumps can handle slightly aggressive liquids.
A groundwater pump requires regular service inspections.

Water can gather and exert pressure on cellar walls and floors. It may come from the surrounding soil or accumulate because it is not being drained away from the house. If there are floor drains and supply pipes under the cellar for example, unsealed wall and floor joints will allow further penetration. Water can cause damp-related problems such as mould, discoloration and peeling.

Installations may include water supply pipes for the house or floor drains linked to the sewerage network. If you have any installations routed through cellar floors or outer walls below ground level, you should ensure that all joints are sealed.

If you are unfortunate enough to have water enter a cellar, you should minimise the risk of subsequent damage. Water should reach as few items as possible. Electrical equipment and installations are particularly sensitive to water, which can enter routings, short connections and destroy electronics and contacts.

It is a good idea to install cabling along a cellar ceiling. Electrical outputs and switches can be placed at an appropriate height. Electrical equipment, such as washing machines and tumble-dryers, can also be installed on plinths to avoid damage. Electrical tools can be placed on shelves or racks at a suitable distance from the floor.

If your house has no perimeter drain and there has been a lot of rain over a period, the earth around the house can become saturated. When the earth can no longer absorb water, it will press on cellar walls and push through cracks. Water can cause damp, with the resultant risk of discoloration, mould and peeling on the inside of walls.

Water should be directed away from the outer cellar wall. The best way of doing this is to construct a perimeter drain around the house. The drain should be able to collect and channel water away. Ensure that the drain is working properly and that the water-tightness of cellar walls is intact.

Problem: Sewage water can flow back through the sewerage system when this becomes overloaded. Used water including domestic, bath and toilet water and to some extent rainwater is sent for treatment via the sewerage network. If sewerage pipes fill up during heavy rainfall, water can flow back through the network. A typical point of entry is a floor drain or toilet that is in a low location.

If your house has a backflow blocker on the sewerage drain, you should check that this is legal, correctly installed and working properly. It should be inspected and cleaned at least once a year. Items should be kept in watertight boxes at a reasonable distance from the ground and on racks or shelves that are fixed to the wall. Carpets, furniture and other items should be kept away from the reach of water.

Heavy rainfall can result in water entering light wells and cellar stairs. If the water cannot drain away, it will collect around foundation walls, windows or cellar doors. Water can infiltrate joints, fillings, mouldings and crevices and can find a way inside, even when windows and doors and closed. Wooden frames and plinths may become damp and their life thereby shortened.

Keep light wells and stairways clean. Clean drains and keep them free of leaves and dirt. You should ensure that drains can carry the volumes of water that may arise during heavy rainfall. Wooden frames should be healthy and free from rot. Joints and weather strips must close and be water-tight, including during heavy rain.

Long heatwaves, in particular, can give rise to problems for people as well as animals. The strong sun and heat can give physical discomfort to some people during heatwaves. The heat may be life-threatening for the weak or elderly and for the sick. You should be aware that also pets can suffer heatstroke. Furthermore, some food, electrical appliances, building materials, or equipment do not tolerate extreme heat.

Usually, indoor temperatures can be lowered using solar shading and natural ventilation. If this is not enough, you can use mechanical ventilation. You should consider using photovoltaic solar modules to run such mechanical ventilation and cooling. One advantage of this is the close match between the demand for energy and the supply of energy. Solar shading must be placed so that it shades your house. For example, you can apply exterior Venetian blinds or shutters. You should be aware that cooling in the summer, also through natural ventilation, may result in a relatively high indoor humidity and thus a risk of mould, which can cause allergies.

Long periods with warm and humid weather can lead to relatively high indoor humidity. In particular rooms that you keep at a lower temperature than other rooms to save energy, have a higher risk of mould due to the high humidity. Mould may, ultimately, cause allergies. You should also note that cooling your house during summer, even using only natural ventilation, could increase the risk of mould.

Typically, mould will occur in rooms with low temperatures and high humidity levels, such as toilets, baths and basement rooms. At the back or behind cupboards are also areas susceptible to mould growth, if the cupboards are placed against outer walls or in cold rooms, for example stairways. You should be especially aware of grey spots from mould growth in corners that meet outer walls. Keep an eye on the humidity level and reduce the humidity if required by increasing ventilation, heating or dehumidification. It may be necessary to combine several solutions, as summer cooling - also when using natural ventilation - can result in high humidity levels.

A long-lasting heatwave will increase the temperature in your home. The addition of heat from solar radiation through windows may increase this heating effect further. Temperatures may reach a level that interferes with your sleeping and which may even cause electrical appliances, such as freezers and refrigerators, to break down. Strong sunshine and ultraviolet radiation may also cause building materials and furniture etc. to discolour, degrade or perish.

Strong solar radiation through windows can be reduced effectively with solar shading. The solar shading solutions must be installed so that they provides shading for the window. For example, you can apply exterior Venetian blinds or shutters. Solar shading systems can be controlled mechanically. You may have to reduce the indoor temperature through natural or mechanical ventilation as well.
You could consider using photovoltaic solar modules to provide energy to run your cooling and mechanical ventilation system. One advantage of this is the close match between the demand for energy and the supply of energy.
You should be aware that cooling in the summer, also through natural ventilation, may result in a high indoor humidity and thus a risk of mould, which can cause allergies.

Air tightness and the possibility to regulate the ventilation of your home influence energy consumption and indoor climate as well as the effect of humidity on the building envelope. The level of air tightness of the building envelope is vital for the amount of air coming in and out of your house, in particular when it is windy. If your house has not been constructed so that it is air tight, air leakages can account for 20-30% of your energy consumption.
Leakages may also reduce comfort due to nuisance from draught, a cold floor and cold interior surfaces. If there is leakages in the building envelope hot moist air may cool off on its way through the construction causing humidity to increase. Humidity can lead to condensation and, ultimately, to rot and mould.

It is important that your house is air tight according to the requirements of the building regulations. If you follow the requirements, you will be sure that your house is being ventilated in an economical and energy-friendly fashion that also meets your demand for good indoor air quality. In order to achieve a good indoor climate, your should set your ventilation on the basis of the level of humidity and pollution in your indoor climate.
Indoor air pollution can originate from activities in the home and from the release of gases and particles from building material and furniture etc. You can regulate ventilation of your home mechanically, either through mechanical exhaustion or by establishing a balanced ventilation system. You can also regulate ventilation manually or through air bleeder valves in the exterior walls. You should take care that the pressure inside the house does not become considerably lower than the pressure outside the house. You should also take care that overpressure does not build up inside the house, as warm, humid indoor air will then be forced out through the building envelope.

A quick and easy way to relieve the pressure on the sewerage system and to benefit from the rainwater is to place a rainwater barrel close to your house. When it is empty, it takes the first water from a downpipe and if there is more, this will run out to the sewer. Most barrels can contain 200-300 litres of rainwater, but they are available with a capacity of up to 1000 litres.

Even though, on the face of it, rainwater may seem clean, in fact it contains a lot of different substances. These collect in the atmosphere and on roofs and car parks, and they include eroded particles from cars and particles from other surfaces. The rainwater can be used to water the garden or wash your car, saving the groundwater and money. Water from a rainwater barrel can be polluted by dirt, and if it is also exposed to light algae can grow. This may lead to bad odours from the barrel. If the water in the barrel stands still, the dirt and dead algae will fall to the bottom, where they will settle and rot so that the water smells of rotten vegetables. Therefore you should periodically empty and clean your rain barrel. It is also a good idea to put a lid on the barrel. This will help discourage algae and you will prevent mosquitoes from laying eggs in the water, with subsequent mosquito problems when they hatch.

A pump well can collect sewage water during heavy rainfall to prevent flooding in your cellar. It is the most reliable solution, but also the most expensive. A pump well can always help get rid of wastewater, even when the sewer is full.

The well is placed so that the wastewater from the cellar to be pumped out runs towards the well. There is a pump in the well which pumps the water out to the sewer. Even if the sewer system is overloaded, the water can be pumped onwards and you can still get rid of the water. Installation must be by an authorised sewer contractor, and a well pump costs around DKK 60,000.

If you want to reuse rainwater to a greater extent, you could invest in a rainwater installation with a large tank buried underground. This will provide enough rainwater for toilets and washing clothes. The installations usually sold for this purpose have a capacity of either two or four cubic metres of water.

Manufacturers recommend that you choose a size which corresponds to three-weeks' water consumption by your family on washing clothes and flushing the toilet. This will mean that you will rarely run out of water. A tank holding two or four cubic metres will suit most families. If the rainwater is collected in an underground tank, you will need a filter to clean the water before it ends in the tank. The filter will need cleaning two-four-times a year. The tank will need inspecting once a year. The tank is emptied and the inside is cleaned, possibly with a high-pressure sprayer. There is no light in the tank and the temperature is maintained at less than 18 degrees so the water stays clean without further treatment. About three-five times a year there will be more rainfall than a two or four cubic meter tank can contain. Therefore the tank must have an overflow pipe to make sure that excess rainwater is led out to the sewer or a dry well, where the water can be collected temporarily and then drain down into the soil. If you lead the water to the sewer, it is very important that you fit a return valve to ensure that sewage water does not flow back into your tank. You will have to contact your municipality before starting installation of a rainwater tank. There may be special requirements and regulations for subsidies from your municipality. Source: Bolius

A backflow blocker protects your house against water from the sewer during a cloudburst. In daily use, wastewater passes through the backflow blocker. During a cloudburst (flooded or blocked sewer) the backflow blocker will close automatically so that water cannot flow into your house.

There are several types of backflow blocker, which can be placed either inside or outside your house. It is recommended you seek the advice of a sewer contractor for the type that best suits your own house. Installation must be done by an authorised sewer contractor and a backflow blocker usually costs between DKK 8,000 and DKK 30,000 to install. The cheapest types usually fit to the parts of the sewer system which are not connected to a toilet. In addition to this is excavation work, and in many cases subsequent repair of the cellar floor as well as possible redirection of the pipe system so that rainwater is removed behind the backflow blocker. A backflow blocker requires regular maintenance because there must be no dirt or grime in the valve.

If you have had flooding in your cellar from clean water (not sewage water), or if you can see or feel that the foundations or footings are damp because of excessive amounts of water against the house, a perimeter drainage system may be a good idea to prevent damp in your cellar or dry rot etc. However, perimeter drainage is a lot of work and very costly, so it is important to have a preliminary investigation to determine whether the problem will be solved with a perimeter drainage system. Perimeter drainage is like a buried gutter along the foundations and outer wall of a house and it leads excess water away from the ground to a rainwater well, then on to the sewer and drainage system. If the walls of your cellar are damp, this may lead to damage and attack from dry rot or fungus. If your house is to be renovated, a perimeter drainage system will improve the indoor climate in the cellar.

Perimeter drainage must be installed by an authorised sewer contractor but you can often do much of the work yourself and seal the cellar from the outside. It is important that you remove the earth a little at a time. If you dig away from too much of the wall at once, the wall may collapse because there is no longer enough support from the earth. According to Bolius, a perimeter drainage system costs DKK 5-10,000 per metre, depending on the soil, depth of foundations, and how much work you do yourself.

A dry well is an easy and relatively cheap way of leading rainwater away from your house. A dry well is a hole in the ground filled with gravel or stones into which the rainwater is led and where it collects before leaching into the soil.

There are many ways of installing a dry well. You can buy a complete dry well composed of a large plastic box, perforated with holes and buried in the ground. Or you can make your own dry well by digging a trench. The trench is filled with pebbles or small stones and covered with a geotextile. In addition to the dry well itself, you will also have to install the drains to lead the water away from your roof and to the dry well, as well as a sand trap to prevent leaves, dirt and soil from blocking the dry well. Certain precautions should be taken when digging a dry well. It is important to dimension the well according to the amount of water it is to receive. It is also important to take account of the ability of the soil to lead the water away. Dry wells can be filled with shingle, pellets or similar. An overflow can be established from the dry well to the sewer or other LAR installations. A permit from the municipality is required before a dry well can be installed. Note that there are distance requirements for dry wells. They must be located at least two metres from buildings and there must be at least 25 metres to drinking water wells and lakes and streams.

During heavy rainfall large volumes of water can fall on the roof, roof windows and balconies. Where strong wind accompanies rain, water can be thrown against facades and windows. Water can penetrate cracks in the facade and unsealed joints around windows and doors. Gutters and downpipes may overflow onto facades.

Water must be channelled away from the roof in a controlled manner. The roof must be watertight. Water can be directed towards grates, gutters and downpipes with the capacity to carry the water produced by heavy rainfall. Windows and flashings must be watertight. Drains and downpipes must be clean and free of leaves. Aprotective raised area can deflect water from light wells and stairways.

In periods of heavy rain, there is a risk of saturation of soil strata and a rising water table. During heavy rainfall, water pressure on cellar walls and floors increases. Water will make structures damp and force its way through cracks. Soil moisture can infiltrate and cause discoloration, peeling and mould on internal surfaces.

It is possible to remove water from external subsoil walls by constructing a perimeter drain. The perimeter drain is built on the external side of the outer wall foundation. A gully is laid along the outer wall to collect water and direct it to the perimeter drain. At the same time, heat and damp insulation should also be installed on the outer subsoil walls.

During heavy rainfall, the sewerage system can become overloaded. If this happens, there is a risk that water can flow back through pipes and into your house. Water from sewers may enter through sewerage connections, e.g., floor drains and toilets. Sewerage water can also enter a perimeter drain that is linked to the sewerage system.

During heavy rainfall, backflow valves can prevent sewage running back and into your house. A high-water valve is a piece of pipe with two flaps. The flaps work like a sluice that opens when waste water runs out of the building but close if sewage flows back towards the building from outside.