Application of a Liquid Waterproofing System to Existing Concrete Flat Roof

Liquid Waterproofing onto Concrete Flat Roof | Roofing BlogLast month, we discussed how the latest liquid waterproofing system can be applied to an existing felt roof – catch up if you missed it here.

Next, in part two of our series, we are going to take a look at how liquid waterproofing works over an existing concrete surface. To view and download this useful PDF document from a UK-based liquid waterproofing system provider, click here.

Important first steps and substrate preparation for liquid waterproofing application to concrete

Commercial and domestic surfaces and flat roofing can benefit from the application of liquid waterproofing systems, which also act as non-slip surfaces for additional safety. As with most installations, preparation of the flat roof is the most critical step, and can make application easier, faster and long-lasting. Failure to properly prepare the concrete can result in failure of the system.

Inspection of the existing concrete flat roof or surface should be minute and thorough. First brush it clear of debris and loose chippings, making sure it is clear. At this point, decide whether you need to jet wash or scrub the concrete surface to remove any instances of grease or oil patches, moss and even mildew or dust. Let the area dry, then remove any outlets or drain covers and clean them too.

Applying a primer coat

During application of a primer onto the existing concrete flat roof or surface, any small cracks or uneven points should be smoothed. This can be done by following the instructions in the PDF, or according to the supplier’s instructions. Often, primer is mixed with a PU additive and a mineral agent, such as silica sand to create a suitable filler.

For larger areas of damage, (or for cracks, joins, holes etc) our favourite supplier recommends use of specialised matting and accelerated liquid membrane. Significant cracks should also be treated with a polyurethane-based mastic (never silicone-based).

Accelerating and applying the liquid membrane

This part of the process is pretty time-critical, as once the accelerator is added, the mixture will begin to ‘go off’. This can occur as quickly as 20-30 minutes after mixing, so it is important to be properly prepared so that you can begin application as soon as possible. Only mix the quantity you will need for each area. Once applied, the liquid waterproofing membrane should be touch-dry in 30-40 minutes.

Application of the membrane once you get going can be fast using a solvent-resistant roller. It is a self-levelling compound, so will not require overworking. You can apply 2 or 3 layers, depending on the size and use of your flat roof or surface area.

Our supplier recommends a top coat to increase UV stability. A top coat is HIGHLY recommended if the area you are working on will get high volume foot traffic. It is best if the top coat is applied no later than 48 hours after the liquid waterproofing membrane, making sure that the surface is dry and clean. The top coat also requires mixing in two parts, but has a longer life before it goes off, of about two hours. Two top coats are recommended if you are applying the liquid membrane to a car park or other public, busy area.

Other uses for liquid waterproofing over concrete

Liquid waterproofing has been proven to be ideal for balcony areas and delivers a great, non-slip finish when anti-slip beads, silica sand or other granules are added.

It is also suitable for vertical surfaces, as other specialist types of liquid waterproofing membrane are available. It is a good idea to read through the PDF, or contact our suppliers to find out more.

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