Roof construction and drainage
Roof construction and drainage
A good green roof starts with the basics: a strong roof structure and a reliable drainage system. Whether you’re having a new green roof installed or maintaining an existing green roof, it’s crucial that your roof is strong enough and allows rainwater to drain properly. This is because a green roof can become quite heavy when saturated with water, and without proper drainage, that water will stay on your roof like a swimming pool. In this section you will learn what to look out for in the roof construction and drainage of your green roof, so that your roofing and home remain safe and durable.
Considerations for roof construction and drainage
Bearing capacity of the roof
Check whether the structure of your roof can support the weight of a soaked green roof. An extensive sedum roof easily weighs 50-80 kg per m² when fully saturated with water. Does your roof have a wooden structure or are you in doubt? Then call in a construction specialist to calculate the load-bearing capacity. Better to be sure in advance, than discover afterwards that your roof is sagging.
Waterproof and root resistant layer
Proper waterproofing is essential under your green roof. Make sure there is a solid roof covering (for example, EPDM or bitumen) that is also root-resistant or a root canvas on top. This layer protects the structure from moisture as well as roots that want to grow through it. Leaks can cause enormous damage, so during maintenance it is important to always check to see if the roofing is still intact, especially around edges and penetrations.
Drainage and drainage
An effective drainage system drains excess rainwater. Under the substrate of your green roof should be a drainage layer, such as drainage mats or slabs with grooves, or a layer of hydro pellets. Also check the drainage pipe(s): are the drain openings free of leaves and debris? Clogged drainage can lead to flooding and extra weight on your roof. Therefore, keep the rainwater drains clean at least twice a year.
Inclination angle of the roof
Flat roofs (up to about 5º slope) benefit from extra attention to drainage because water stays there longer. With a nearly flat roof, an emergency overflow or additional drainage layer is not a luxury. With pitched roofs, you must ensure that the green roof cannot slide down. Anti-sliding measures such as grids or plant mats with anchoring are used for this purpose. With a sloping roof, you must also ensure that the substrate is not washed away during heavy rain – a combination of drainage mats and substrate mats helps here.
Maintenance accessibility
Make sure you (or a professional) can get onto the roof safely for inspection and maintenance. For example, through a roof hatch, roof hooks for a harness or a safe ladder arrangement. Access may seem like an afterthought, but safe maintenance is only possible if you can access the roof properly. A green roof that no one can ever access can eventually cause problems if something does go wrong (blockage, leak). So think about this during construction and, if necessary, schedule regular maintenance intervals with a specialist.
Is your roof structure strong enough? Do you want to be 100% sure, or do you notice signs of problems (for example, sagging or long standing water)? Then schedule an inspection with our specialists. We will be happy to visit you to check your green roof and roof construction. That way you can avoid surprises and have peace of mind about the future with your green roof.
Benefits of professional inspection and maintenance
Maintaining the roof structure and drainage of a green roof is not a typical do-it-yourself job. It involves construction knowledge and often requires working at heights. The benefits of outsourcing this to a professional:
A professional knows exactly where the weak points may be. He or she thoroughly checks for cracks in the roofing, subsidence in the structure or blockages in the drain that you easily overlook yourself.
Timely inspection will help you discover small problems before they become major leaks. For example, a professional can immediately see if the sealant seams around a drain are starting to dry or crack – something that can lead to a leak later. Preventive maintenance thus saves you expensive repairs.
Working on a roof can be dangerous. A specialist has the right equipment and experience to work safely at heights, even in awkward places. So you won’t risk a fall or damage to your roof by accidentally stepping on the wrong spot.
After a professional inspection, you get clear advice. It may turn out that your roof structure could use some extra reinforcement, or that an extra drainage point would come in handy. An expert can make specific recommendations to make your green roof even better. That kind of advice is worth its weight in gold for the life of your roof.
With a maintenance contract, you don’t have to think about anything yourself. The green roof experts come by at agreed times, check everything and perform maintenance immediately. This way you can be sure that both the construction and the drainage remain in good order, year after year, without you having to think about it.
“We came to a customer for routine maintenance and discovered that the downspout was almost completely clogged. Fortunately, we were able to clean it immediately – thus preventing water damage to the roof!” ~ Roderick
Is my roof suitable for a green roof?
One of the most frequently asked questions about green roofs is: will my roof hold the weight? When constructing a green roof, this is the very first check. A structural engineer or green roof specialist will calculate how many kilos per square meter your roof can handle. As an indication: a simple sedum roof weighs about 40-60 kg/m² dry, and can go up to 80-100 kg/m² after a lot of rain. Most modern houses can carry this without any problems, but with old houses, sheds or extensions it is wise to check this.
Do you already have a green roof in place and have doubts about the construction afterwards? Then it is smart to still seek advice. Sometimes it is possible to apply a lighter green roof system (for example, an extra thin substrate layer or lightweight sedum mats) if your construction is just strong enough. A structural engineer can also see if reinforcing beams or installing additional support is possible. Safety comes first: you want to enjoy your green roof, not worry about it coming through your ceiling.
The importance of good drainage
Green roofs hold water – that’s one of their advantages, as it relieves sewers and provides water buffering. But retaining too much is not good. Drainage provides the balance: after a rainstorm, some water remains in the substrate and plants as nutrients; the rest should be able to flow away. If drainage doesn’t work properly, you get puddles on the roof. A brief puddle of water is no big deal, but if water still remains after a day, you have a problem.
A clog is often the culprit. Leaves, gravel or dirt can block the drain pipe. That’s why it’s so important to check and clean the drain at every maintenance service. A simple action that potentially prevents damages of thousands of dollars. At worst, standing water can corrode the roofing and roots will rot in that standing water, which won’t do your sedum any good either.
When the green roof is installed, a drainage layer is usually installed. This consists of plastic panels or mats with channels, or a layer of coarse material that stores and drains water. Make sure this layer does not silt up. Preferably, also use a filter cloth (separation membrane) between the substrate and the drainage, so that soil particles do not sink into the drainage. During maintenance, we also often check that this filter fleece is still in place and intact.
Get your roof inspected early to avoid costly repairs.
Frequently asked questions about roof construction and drainage
An extensive green roof with sedum typically weighs around 50-80 kilograms per square meter when soaked from rain. In dry conditions it is lighter, often 20-30 kg/m². More intensive green roofs with thicker substrate layers or grass and shrubs can be even heavier – sometimes as much as 100-150 kg/m² or more. The weight also depends on the structure and water buffer. That’s why it’s so important to know what your roof structure can handle. Fortunately, most residential roof structures can support a sedum roof just fine, but if in doubt, have it checked to be sure.
The best way to do this is to consult a structural engineer. They can calculate the load-bearing capacity based on your current roof structure and materials. Do you have the construction drawings of your house? If so, they often indicate the maximum load per square meter. For a shed or extension you can sometimes tell how solid it is from the beams, but appearances can be deceptive. Green roof specialists can put you in touch with a structural engineer or make their own assessment during an inspection. The bottom line: when in doubt, always have it checked – you’ll sleep a lot more peacefully that way.
Don’t panic, there are options. First, you can opt for a lighter green roof system. Sedum roofs come in different thicknesses; for example, there are packages with extra-light substrate or even systems with foam mats that hold water instead of heavy soil. These can reduce the overall weight considerably. Second, you can consider having the roof structure reinforced. Consider installing additional beams or struts in the roof. This is an investment, of course, but it can open the way to having a green roof. Get expert advice on this. Sometimes it is also an option to make part of the roof green (for example, only the edges) to reduce weight.
Drain maintenance is basically simple: keep it clean. Make sure no piles of leaves, twigs or debris remain on the drainpipe grate. During your regular maintenance (spring/autumn) you can use a hand shovel or just by hand to remove that debris. If necessary, flush some water down the drain to check for proper flow. If you have a gravel rim around your drain, swirl that around as well to loosen any debris caught in it. If you find that the water still drains poorly, the blockage may be deeper – in which case it’s better to call in a professional or get to work with an unclogging spring if you’re handy. In our maintenance services, of course, we include this as standard.
A small puddle on your roof immediately after a rain is not uncommon, especially on flat roofs. But in principle, the water should have sunk into the green roof or drained through the downspout within a few hours to a day at most. If a layer of water remains for a long time (days), it is a sign that the drainage is not optimal. This can be due to a clogged drain, insufficient drainage or because the roof does not have enough slope. Prolonged standing water is not good: It can lead to rot in the sedum and extra stress on your roof structure. If you see puddling every time, take action: clean the drainage and see if it gets better. If not, have an expert assess the situation. If necessary, an additional drain or drainage layer may be the solution.
Do you have other questions about maintaining your sedum or green roof? Then feel free to contact us. We will be happy to help you – whether it’s advice on how to get started, or to take care of all the maintenance for you. That way you can keep your roof beautifully green and healthy year after year.
