As one of the top construction companies in Kenya, INTEGRUM has worked with a wide range of waterproofing methods, solutions, and products.
Drawing from this experience, our construction project managers and building contractors have compiled the following guide to help you choose the best flat roof waterproofing methods for your construction project.
Follow along as we explain the different methods of waterproofing flat roofs, their pros and cons, the appropriate waterproofing products and solutions to use in each case, plus the estimated costs of waterproofing flat roofs.
Issues with Flat Roof Designs in Kenya
While they offer a sleek, modern look, flat roofs are prone to water leaks – which can lead to significant damage and costly repairs if not properly managed.
You see, flat roof house designs don’t offer much of a slope. This means that instead of rainwater quickly running off, it tends to pool on the surface. Over time, that standing water may break down the roofing materials to make way for seepage fissures.
Kenya’s intense equatorial sun also adds to the problem of flat roof leaks. The UV radiation here is known to degrade roofing membranes, ultimately causing cracks and blisters that compromise their waterproofing capabilities.
Another reason for frequent leaks in flat roofs is shoddy installation. Many roofing contractors in Kenya just don’t have the right training to properly put in a flat roof system. They mess up the seams, skimp on the flashing, and overlook the drainage levels. All those mistakes leave weak points in the roof’s waterproofing barrier, many of which may not be apparent at the time of handing over the project.
Other times, it’s the building owners themselves who invite trouble by dropping the ball on basic roof maintenance. They let debris pile up, forget to clear out clogged drains, and ignore small issues until they snowball into big leaks. Without routine inspections and proper maintenance, even minor problems can spiral out of control.
The consequences of flat roof leaks can be severe, ranging from unsightly water stains and dangerous mould growth to major structural damages and even roof collapse in extreme cases.
5 Best Methods of Waterproofing Flat Roofs in Kenya
#1. Reinforced Bitumen Membranes (RBM) for Flat Roofs
Reinforced Bitumen Membranes (RBM) consist of two to three layers of bitumen that are bonded together and laid across the entire surface of the flat roof. The layers are then reinforced with a polyester carrier to create a strong, flexible waterproof membrane.
Reinforced Bitumen Membranes. Courtesy: Sika Waterproofing
As a waterproofing method, RBM is ideal for small to mid-sized flat roofs, commonly found in residential properties. It works particularly well in warm climates like Kenya, as the layers are tough enough to resist damage from UV rays.
Before rolling out the RBM sheets, the roof surface needs to be prepped. This means cleaning it well, filling any cracks, and levelling things out. Primers like Sika Primer BF 4 or Kenbro Bituseal T5 are then painted onto the roof to help the RBM adhere tightly.
With the roof primed, the RBM sheets are placed on the flat roof with overlaps of at least 10 cm on the sides and 15 cm at the ends. These sheets can be bonded to the roof by either melting the bitumen with a blowtorch or using a cold adhesive.
For a watertight seal, the mineral surface of the top sheet is melted away at the ends with an intense flame. Colour-matched granules are then sprinkled on the seam for a clean, professional look.
When installed properly, this method of waterproofing flat roofs can last 20-30 years with minimal maintenance. You just need to check it for damage once a year and maybe sweep off any leaves or debris.
Popular brands for RBM waterproofing products in Kenya include Sika, Kenbro, Bitumat, Index, Soprema, and Polyglass. Whichever you choose for your flat roof waterproofing, you can expect to spend around 1,000 to 1,500 shillings per square metre.
#2. APP Bitumen Membranes for Flat Roofs
Unlike Reinforced Bitumen Membranes (RBM) which use a polyester carrier or fibreglass, APP contains a plastic polymer called atactic polypropylene. This special ingredient gives APP membranes better resistance to heat, UV rays, and cold temperatures.
APP is perfect for flat roofs that get lots of direct sunlight, as the added polymer prevents the bitumen from melting at high temperatures. They work best on regular, smooth roof surfaces with few details of protrusions.
To install APP, waterproofing contractors start by cleaning the flat roof surface and applying a bituminous primer. Next, they unroll the APP membrane and use a propane torch to heat the bottom until it gets shiny.
The heated membrane is then firmly pressed onto the primed roof, with each new roll overlapping the previous one by 10cm on the sides and 15cm on the ends.
In Kenya, popular APP membrane solutions for waterproofing flat roofs include SikaBit PRO P-40-0 MG and Kenflex APP Modified Membrane. They come in thicknesses of 3mm to 5mm and rolls of 1m by 10m.
For a good flat roof waterproofing job, you’re probably looking at paying between KES 1,200 and 2,000 per square metre. The resultant APP will last you 15 to 20 years with minimal upkeep.
#3. Liquid Waterproofing in Kenya
Liquid waterproofing is favoured for flat roofs in Kenya due to its seamless application and flexibility. While bitumen membranes come in rolls, this method of waterproofing flat roofs is applied directly as a fluid coating. The liquid then cures to form a durable, elastomeric membrane that is fully bonded to the substrate.
You get a continuous barrier over your entire flat roof, eliminating any joints or seams that could be potential weak points for water ingress. The liquid membrane can even bridge small cracks, as well as withstand structural movements and temperature changes without losing its waterproof qualities.
Liquid coatings work great on roofs with lots of penetrations, upstands, and intricate details – since the liquid can easily flow around them. They’re also good for fixing up old roofs, as the bond is still strong over pre-existing roofing materials like tar, concrete, or metal.
During application, your roofing contractor will first clean the surface. They may then apply a primer if needed, before pouring the liquid waterproofing membrane in multiple coats using a roller, brush, or spray equipment. Each coat should be allowed to cure before applying the next, with your final flat roof waterproofing adding up to a total thickness of around 1-2mm.
Some of the leading liquid waterproofing products available in Kenya are:
- Sika Sikalastic Liquid Applied Membranes
- BASFMasterseal Roof Waterproofing Systems
- Mapei Aquaflex Liquid Membranes
- Fosroc Polyurea APE
- Bostik Watertite Liquid Applied Waterproofing
- Kerakoll Bioscud Liquid Membranes
- Cico Tapecrete P151 Acrylic Waterproofing
- Dr. Fixit Pidiproof
The building costs are usually around KES 1,500-2,500 per square meter for the materials and labour. While this may be slightly higher than some bitumen-based methods of waterproofing flat roofs, the long-term performance and ease of application often justify the investment.
A liquid waterproofing system may proceed to serve you for 25 to 30 years or more.
#4. Single-Ply Membranes for Flat Roofs
Single-ply membranes are a top waterproofing solution for projects that require lightweight and quick construction. Unlike bitumen membranes made from modified asphalt, single-ply membranes are manufactured from synthetic rubber or thermoplastic materials like PVC, TPO, or EPDM.
They come in large, pre-fabricated sheets that are rolled out and joined using heat welding or adhesives.
This method of waterproofing flat roofs creates a continuous, impermeable barrier over the entire surface. These membranes are pretty flexible, with the capacity to handle building movement and heat expansion better than stiff bitumen sheets. They can also stand up to UV rays, fire, and extreme temperatures.
Roofing contractors in Kenya use them on roofs with complex shapes, numerous penetrations, or limited load-bearing capacity. You’ll appreciate that they are lightweight, thus exerting less structural load than multi-layer bitumen systems. T
he membranes can be loose-laid with ballast, mechanically fastened, or fully stuck to the substrate.
Some of the single-ply waterproofing products that you may find in Kenya include:
- Sika Sarnafil PVC membranes
- IKO Armourplan PVC
- Protan SE polyester-reinforced PVC
- Renolit Alkorplan PVC
- Fatra Fatrafol PVC
They are typically engineered to protect flat roofs for about 25 to 30 years. The waterproofing cost itself varies based on your membrane type, roof size and complexity, and the installation method. On average, though, Kenyans are spending between KES 1,400 and 2,300 on each square metre of the single-ply waterproofing membrane.
#5. Waterproofing Sealants in Kenya
Sealants are a quick and easy solution for waterproofing flat roofs, especially for small areas or repairs. Unlike liquid waterproofing that covers the entire roof, sealants are applied as a paste to fill cracks, joints, and penetrations. They block water by sealing gaps and sticking to the surfaces around them.
You can use sealants for localized repairs on existing roofs, especially around vents or small leaks. They also work well in spots prone to cracking, like corners or joints.
To apply this waterproofing method, clean the selected roof area first, add primer if needed, and then use a caulking gun or spatula to spread the sealant. You should smooth it out to make sure the paste sticks well, with a thickness of about 2-3mm. And when you’re done, let it sit for 24-48 hours to harden completely.
You may consider solutions like:
- Sika Sikaflex polyurethane sealant
- Bostik Seal N Flex polyurethane sealant
- Alchimica Hyperseal polyurethane sealant
- Mapei Mapeflex PU40 polyurethane sealant
- BASF MasterSeal NP1 polyurethane sealant
- TKK Tekaflex polyurethane sealant
They are mostly available in 300ml cartridges, priced at KES 500-1,000 for a coverage of 5-10 linear meters.
It’s worth noting that while sealants are less expensive than completely revamping your roof’s waterproofing system, they may not have quite as long of a lifespan as some other waterproofing methods for flat roofs. Polyurethanes and silicone-based options can stretch to 10-20 years, whereas the more common bituminous sealants go for 5-10 years.
Tips for Choosing the Best Flat Roof Waterproofing Method
- Check the roof’s slope and drainage: Even though they’re called “flat”, most flat roofs actually have a slight slope of at least 1:80 to facilitate water runoff. Make sure the waterproofing method you choose works well with this subtle slope to prevent water pooling.
- Consider your local climate: Select waterproofing materials that perform well in your climate. Reflective coatings are ideal for sunny areas, while flexible materials suit colder regions with frequent temperature fluctuations.
- Examine the existing roof’s condition: If you’re waterproofing an older roof, thoroughly inspect the surface for any cracks, blisters, or signs of moisture damage. Some roof waterproofing materials – like liquid-applied coatings – can be applied directly over the existing roof, while others may require removing the old layer. Fix any issues before applying the new waterproofing over your flat roof.
- Plan for maintenance: Understand the maintenance needs of your waterproofing system. Liquid-applied coatings may need reapplication every 10-15 years, while high-quality single-ply membranes can last over 20 years.
- Account for rooftop usage and traffic: If your flat roof doubles as a deck, garden, or houses heavy equipment, choose a waterproofing solution that can withstand foot traffic and the weight of any installations. Good options include reinforced bitumen and single-ply membranes.
- Ensure proper ventilation: Choose a flat roof waterproofing method that allows for good air circulation without blocking vents. This will help to regulate temperature and moisture levels, thus extending the life of your waterproofing system.
- Pay attention to the details: Edges, corners, and penetrations are vulnerable spots. Pick a waterproofing solution that includes flashing and detailing accessories to seal these areas well.
And lastly, remember not to make any assumptions. Flat roof waterproofing is highly intricate, and a single miscalculation can potentially jeopardise the entire project.
Send us your flat roof specifications today, and our team of building contractors will provide free quotes for the best waterproofing methods and products.
