A new OSB flat roof needs a waterproofing system that can protect the deck properly from the start. In the video above, we show how a failed roof is stripped back, rotten timber is replaced with fresh OSB, and the new deck is waterproofed using the Owl LAVA 20 system. LAVA 20 is a liquid-applied polyurethane waterproofing membrane that forms a continuous, seamless layer over the prepared surface. It can be used on OSB and plywood as well as a range of other roofing substrates. Our Owl LAVA 20 Waterproofing Repair Kit gives you the materials needed for this type of waterproofing project. Here's how the process works. Prepare the OSB flat roof Before you start with the LAVA 20, have a good look at the OSB first. Check the boards over properly before you start. They should be firm and well fixed, with no damp or rotten sections. If some of the OSB has seen better days, replace it first. You don't want to waterproof over damaged boards and then have the same problem coming back. This is particularly important when refurbishing an older flat roof. In the video, the damaged timber is removed before the new OSB deck is installed. There's no point putting the new waterproofing over boards that are already on the way out. Get the base right first. Also, make sure the roof has enough fall for the water to run off into the outlets instead of sitting there. Check the joints, edges and penetrations The main roof area is only part of the job. Joints, corners, roof edges, drains and anything passing through the membrane all need proper attention. Owl provides dedicated technical details for areas including roof edges, drains and pipe penetrations, so the waterproofing system can be detailed correctly rather than relying on a generic patch or sealant. How to waterproof OSB with LAVA 20 Once the deck is prepared, the LAVA 20 system can be applied. 1. Fill holes and defects Any holes, cracks or gaps should be dealt with before the main membrane is applied. Our LAVA 20 system includes PU Mastic for this type of preparation. Getting these areas sorted first gives the membrane a much better surface to work with. 2. Prime the surface The prepared OSB should then be primed with the appropriate LAVA 20 primer. Priming is an important part of the system because it helps provide the right surface for the membrane to bond to. Always use the primer specified for your substrate and follow the current application instructions supplied with the system. 3. Reinforce vulnerable areas Joints and other detail areas should be reinforced before waterproofing the main roof. LAVA 20 can be used with reinforcement tape in a wet-on-wet application, helping strengthen areas where movement or stress is more likely. Pay particular attention around corners, edges, penetrations and other changes in the roof detail. 4. Mix and apply LAVA 20 Once the preparation is complete, mix the LAVA 20 according to the manufacturer's instructions. Apex's current product guidance specifies mixing for around four minutes before application. Apply the membrane evenly across the prepared roof, making sure there is sufficient material to achieve the recommended coverage. For wood substrates such as OSB and plywood, we recommend planning for around 2.5 kg/m². Coverage requirements vary between substrates, so don't assume the same quantity will be suitable for every roof. The aim is to create a continuous waterproof membrane across the entire roof, including the areas already reinforced. How quickly does LAVA 20 become rain resistant? LAVA 20 is designed for relatively fast application and curing compared with many traditional roofing systems. When correctly applied at around 20°C, Apex states that it can become rain resistant in approximately 30 minutes. Actual curing times will vary with temperature, humidity and conditions on site. That doesn't mean you should start a roof when rain is already on the way. A suitable weather window and proper preparation are still essential. Do you need a topcoat? A topcoat can be used where a particular colour or finish is required. In the video, a dark grey LAVA 20 topcoat is applied to the finished roof. This is optional and is separate from the main waterproofing membrane. Owl provides dedicated LAVA 20 topcoat products in its technical range, while our GRP Complete Topcoat Refurb Kits are intended for refurbishing existing GRP roofs rather than being a substitute for the LAVA 20 topcoat system. LAVA 20 or a traditional GRP roof? You’ll often see LAVA 20 mentioned alongside GRP roofing, but they’re actually two different systems. GRP uses resin and fibreglass mat, whereas LAVA 20 is a liquid polyurethane membrane that goes on as a coating and cures to form the waterproof layer. We offer both options through our GRP roofing kits, so the best choice depends on the roof construction, substrate and type of project. For a LAVA 20 installation, the key advantage is the liquid membrane, which can follow awkward details and create a continuous waterproof layer when correctly applied. Don't overlook the roof edges Waterproofing the main roof surface is only part of getting a flat roof right. Edges, drainage points, corners and abutments need to be detailed as carefully as the rest of the membrane. For projects using GRP systems, our range of fibreglass roofing trims and corners provides options for finishing and directing water away from the roof structure. For LAVA 20 projects, follow the relevant Owl detail for the specific edge or penetration rather than treating every roof in exactly the same way. Common mistakes when waterproofing an OSB roof A few avoidable mistakes can cause problems later: Covering damaged OSB: Replace rotten or unsound decking first. Applying to a dirty or damp surface: The substrate needs to be properly prepared. Skipping the primer: Use the correct primer for the substrate and system. Ignoring joints and edges: These areas need proper reinforcement and detailing. Using too little material: Keep to the recommended coverage for OSB. Working in unsuitable conditions: Always check the weather and current product instructions before starting. One other thing to check before starting is whether the job needs Building Regulations approval. It can depend on what you're changing, so have a look at the current GOV.UK guidance before you begin. Check the current building regulations guidance. A properly prepared deck makes all the difference It's not just a case of getting the coating on and calling the roof done. The deck needs to be sound, dry and properly prepared. Once that is in place, the LAVA 20 system provides a continuous liquid-applied membrane, with reinforcement and detailing used where the roof needs extra protection. Watch the video at the top of the page to see the complete process in practice, then take a look at our Owl LAVA 20 Waterproofing Repair Kit for the materials required for your project. If you're unsure about the coverage, materials or which system is right for your roof, give us a call on 0800 612 7903 or get in touch with the Apex team. We're happy to help.
