Sealing flat roofs

One of the most common ways of waterproofing a flat roof is by using bitumen and welding. This involves heating the underside of the bitumen sheet using a gas burner or a heat gun. Bitumen membranes are favoured for their high resistance to aggressive substances and longevity, making it a highly effective material for renovating or repairing a flat roof.

One key advantage of the welding process is that the heat generated can simultaneously dry a slightly damp roof, making it ready to be worked on. If you would rather work without a heat gun or gas burner, you can use cold liquid bitumen as an alternative.

To renovate a flat roof properly, patience and an eye-for-detail are essential. This is the only way to avoid damage and problems further down the line. Health and safety precautions should also be taken seriously. As you are not working at ground level, but on a roof, take all occupational safety steps necessary to protect your health and safety. Please refer to our overview of safety precautions for DIY for further information.

Are you familiar with our 18-volt system? It includes countless compatible power tools for a wide variety of applications. Multiple tools can be used effortlessly by switching out a single battery. This is particularly handy for DIY projects such as flat roof sealing, where you often need multiple tools to complete the job.

If you are applying the bitumen sheets in two layers, ensure the first layer has completely cooled before applying the second layer. Always place the second layer offset so that there is never a seam on top of a seam. Important: The roof must not get wet between the application of layers.

If the bitumen membrane is porous, coat it with liquid bitumen beforehand. Before you attach the new bitumen sheet, let it dry a bit. For flat roof repairs, you can also seal the edges of the new bitumen membrane with thick liquid bitumen. This will provide an extra-secure flat roof sealing.

A flat roof should be renovated as rarely as possible. So, when constructing a new building, decide from the beginning which material is going to be the best for the long term. Take into account what stress the roof will be under, the typical weather conditions, as well as what chemicals or other substances the roof will come into regular contact with.

If you are renovating a flat roof, first determine which material was used previously. Plastic roofing is particularly susceptible to heat damage. Heating bitumen sheets with a gas burner during your work could damage the existing roofing, especially if it's made of plastic.

Once you have determined the initial material used, you can now consider the advantages and disadvantages of all the possible materials that can be used for flat roof sealing. These include liquid sealants (hot and cold), foils, bitumen membranes (hot and cold), roofing felt, self-adhesive membranes and sheets.

Possible materials for waterproofing a flat roof:

Now, let’s look closer at some of the advantages and disadvantages of the most popular materials, based on different requirements.

If long-term durability and little need for renovation are important to you for your flat roof, sheet metal might just be the material of choice for you. Alternatively, EPDM films can also last up to 50 years and effectively seal your flat roof. Bitumen membranes laid in several layers are also suitable. In addition to the material, durability depends on the workmanship, regular maintenance and external influences such as exposure to corrosive elements.

The difficulty and costs associated with repairing a flat roof leak depend largely on the material used. Bitumen membranes, liquid bitumen and liquid plastic can be easily repaired. It may be sufficient to simply lay another bitumen membrane across the existing one or to apply another layer of liquid waterproofing. For pond liners or EPDM foils, you will need a special glue for repair.

Whether sealing a roof for the first time or performing a repair, it is important to pay attention to the operating temperature of your material. Some recommended material temperatures include:

• Bitumen membrane: min. 5 °C, max. 150 °C (after which bitumen liquefies)
• Liquid plastic and liquid bitumen: min. 5 °C
• EPDM and roofing felt: min. 10 °C
• Pond liner: min. 15 °C

 Sheets and foils are, of course, the least complicated to dry. Liquid plastic is typically completely dry after 24 hours.

In general, the drying time for flat roof waterproofing depends on the outside temperature and humidity. Bitumen membranes that have been glued together with a gas burner need three to seven days to dry through, depending on the weather. The same applies to liquid bitumen.

If you work in several layers, make sure that your flat roof does not get wet between the individual work steps.

Pay attention to how environmentally friendly the individual materials are, as well as any additional health and safety measures you may have to take.

When it comes to flat roof sealing, the drier the weather, the better. But what if your flat roof leaks in winter?

Unfortunately, most leaks are going to occur during the cold and wet seasons, after a period of heavy rainfall or even snow. To avoid more damage, you could apply a temporary cover of pond liners or EDPD and sheet metal without any problems, even in the rain. However, keep in mind that the layers underneath will dry less quickly. Depending on the ventilation, there may be a risk of mould once the weather gets warmer.

Hot-glued bitumen membranes can also be applied to slightly damp ground as the gas burner dries the surface during the process.

A flat roof often isn't as flat as it might appear at first glance. It likely has connections, edges, chimneys, rain pipes and other components – all of which are potential trouble areas when it comes to flat roof waterproofing.

In general, you can either you use special moulded parts, another sealing membrane, or you can reseal with a liquid sealing material. In many hardware shops, you can buy various moulded parts for roof sealing or you can make your own with the aid of templates.

For example, if you have a wall connection, an angled sealing plate can be placed underneath and coated with liquid bitumen. For a ventilation pipe, liquid sealant can work well. Simply apply several layers around the pipe. Don't forget the transitions!

After securing all connections and edges against moisture and completing the flat roof sealing, regularly monitor your new roof. Undetected leaks can cause serious damage, including water or moisture penetrating the substructure. This can lead to mould infestation, loss of insulation, and structural issues.

The longer the damage persists, the higher the repair costs become. For more on general maintenance, see our guide on looking after your home and car.

The cost of flat roof waterproofing depends on the size of the area to be sealed. Here are average prices of the materials mentioned in this guide:

• Bitumen: £8–£12/m2
• Liquid bitumen: £15/m2
• Liquid plastic: £25–£40/m2
• Plastic sheets: £6–£15/m2
• EPDM: £10–£30/m2
• Connections: £10–£20/m
• Corrugated iron £25/m2
• Copper sheet: £100/m2
• Roofing felt: £1–£3/m2

If you noticed too late that your flat roof is leaking, you may also have to repair the subsequent damage. You can expect to pay approx. £120–£240/m2 for full renovation. A new flat roof building, on the other hand, costs around £75/m2. But don't worry, if you seal your flat roof using our tips and maintain it regularly, you will save yourself a lot of money in the long run.

Before beginning any construction work, you must consult the best practices and standards for flat roof waterproofing as set out by the British Standards Institution and the National House-Building Council (NHBC). In general, the BS 6229:2018 describes best practices for designing, constructing and maintaining roofs with a flat or curved surface and replaces the 2003 version. In particular, the BS 85250:2011 and A1:2016 code of practices applies to the control of condensation in buildings. Depending on whether you choose bitumen shingles with mineral and/or synthetic reinforcements, the European Standard, BS EN 544:2011 may also apply.

We have many more guides and tutorials ready and waiting to make your DIY projects easier. Whether you need to remove carpet glue, sand a wall or repair a roller shutter, we have what you need.

Are you more of a creative type? Then you might be interested in our "Build your own furniture" DIY section.