Polyurethane is a simpler process to apply but can be a little more technical in regards to preparation, priming and weather conditions.
The main factor that can influence the quality of a polyurethane waterproofing membrane on a roof are;
- High Wind
- High temperatures
- Forecast rain
We tend to only apply polyurethane membranes on a roof if the conditions are perfect to do so or if the area has a lot of machinery or sensitive materials that do not allow for torch-on or PVC Sheet membrane.
The usual warranty period for a UV stable polyurethane waterproofing membrane is 10 years although when applied with a fleece layer impregnated within the polyurethane the warranty can be extended to 15 years.
Some polyurethane products state that they are self priming and a primer is not needed on the substrate before application. While this can work we always use a 2 part EPOXY primer to seal the substrate before applying the membrane.
The reason for this is there are unknown factors such as residual moisture that can be trapped within the substrate, chemicals and oils that could also be present in the substrate. If the polyurethane is applied to a substrate and there are contaminants then the membrane can fail prematurely (i.e instead of 10 years we have seen roofs fail after just 3-4 years).
Once a membrane begins to fail by cracking or peeling then water is able to enter the substrate and soak the substrate up to 30cm from the entry point. Once the water has seeped under the membrane due to capillary action it causes a situation where heat from the sun can cause the moisture to evaporate and detach the membrane outwards from the original failure point causing the full failure of the waterproofing membrane.
We do apply polyurethane membranes when necessary but we prefer to use factory manufactured sheet membranes such as PVC heat welded membranes which can carry a product warranty up to 20 years or the bitumen torch-on membranes which also carry a product warranty up to 20 years depending on the system chosen.