Liquid Waterproofing System Application to Existing Felt Flat Roof

Liquid Waterproofing onto Felt Flat Roof | Roofing BlogThe introduction of liquid waterproofing for flat roof projects is exploding in the UK at the moment, as commercial and domestic installations can be updated and waterproofed quickly and effectively for a long-lasting finish.

Over the next few weeks, we are going to be looking at the application of liquid waterproofing over a range of existing surfaces, and will discuss felt, asphalt, concrete and more as we move through the series.

There is a handy PDF guide available for download about the entire process of preparing a felt roof, application of the liquid waterproofing compounds, and it includes some useful tips that you can use along the way.

So, let’s have a closer look…

What you need to know first about waterproofing over an existing felt flat roof

The secret of successful application of liquid waterproofing lies in the preparation. This stage of the works will undoubtedly be the most time-consuming and difficult of them all, so remember that once you have prepared your flat roof, the worst should be over.

You will need to first inspect the existing felt flat roof, brush it thoroughly to remove and loose debris from the entire surface area. Remove drain covers and thoroughly clean the outlets. You may need, additionally, to clean the roof deck to remove patches of grease, oil or moss – power washing is a good method, but you must bear in mind that if there are any current leaks, the interior of the building could be damaged.

You should treat moss patches to ensure it does not return once the liquid waterproofing has been applied.

Tears, blisters and cracks in the existing felt flat roof covering must be flattened and repaired using a polyurethane-based mastic (NOT a silicone-based product) and matting (with accelerated liquid membrane) to ensure an even, undamaged surface for application of the liquid membrane.

Outlets, both external penetrations and internal outlets, can be treated with matting and accelerator to create a perfect seal that will protect against water ingress.

Application of the liquid waterproofing membrane

The products used for the application of the liquid waterproofing membrane must be carefully mixed according to the manufacturer’s instructions. It must be noted here, that once mixed, the solution begins to ‘go off’, so should not be left to hang around for more than a few minutes before use. After 20 minutes or so, the solution should be discarded, as it may have some retraction, which can lead to cracks appearing during the rest of the curing process.

Try to maintain an even coating as you apply the solution with a roller. Topcoats help to give UV stability and can make the surface more durable, which is perfect if it will receive foot traffic on a regular basis. Top coats should be applied within 48 hours following the membrane application, and the surface should be checked for debris, which should be swept clear.

Liquid waterproofing can be applied successfully to an existing felt flat roof using the guidelines here and in the PDF guide document. It can also be applied to balconies, anti-slip surfaces and walkways. There are additional surface finish options for such applications, including the use of silica sand, anti-slip beads and more. Please refer to the PDF for additional information, tips and full application instructions.

The next article in our series will look at the application of liquid waterproofing membrane to concrete flat roofing.

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