Andy Fell of Hambleside Danelaw summarises the pros and cons of different flat and low-pitched roofing membranes.

Housing maintenance is a demanding task, with cost, performance, lifespan, ease of installation and maintenance being the main governing factors. ‘Traditional’ roofing systems are still considered to be valid for certain applications, but there are alternative products that should always be appraised for their merits.


A good, robust product compatible with many other systems, with extremely good flexibility and resistance to rupture. Naked flame use however makes this a risky choice on combustible substrates, and the heavy environmental cost of sourcing and manufacture weigh against it.


Aesthetically pleasing, speedy to apply, easy to make a roof look decorative using standing seams applied to the surface. But, as well as being difficult to repair after long-term weathering, the potential for damage by following trades makes the specification of PVC waterproofing one to give serious consideration to if a roof is going to be trafficked.


Fairly robust, aesthetically pleasing, speedy to apply, and easy to make a roof look decorative using standing seams applied to the surface. TPO however is incompatible with many other membranes, especially bitumen, and can be difficult to repair after weathering. Like PVC, the potential for damage by following trades makes the specification of TPO waterproofing one which requires careful consideration.


Polyurethane and PMMA (Poly Methyl Methacrylate) liquid systems can be ideal for awkward detailing with multiple roof penetrations. Quick curing acrylics such as PMMA are ideal for trafficked surfaces, but for refurbishment, these systems rely more heavily on the quality of preparation and condition of the underlying layers and substrate to be successful, so the relatively high cost of the product, added to extensive preparation can lead to an expensive system; the cost of which exceeds many other solutions.


This can be a very hard wearing and robust waterproofing material which is usually adhered to the roof or laid loose and ballasted. However, failed EPDM roofs generally happen because detailing is more difficult to achieve, and the adhesive bonding upstand membrane can disengage from the substrate, causing flashings to fall away. Additional fixing at edge detailing is generally required, which usually means fixing through the completed membrane.


Developed from boat-building, correctly applied, wet-lay GRP systems can work very well, providing the correct weight and continuity of fibreglass reinforcement is used, and the substrate is stabilised to minimise localised movement before laying. Wet-lay GRP can be extremely robust and hard wearing, and can be installed without the need for naked flame use, but wet applied systems are prone to being affected by rain and temperature changes during the installation and curing period. This can severely restrict the amount of time available for waterproofing. As the resin used in these types of system is partially absorbed into the roof substrate, this can restrict the ability of the finished GRP membrane to expand and contract, potentially leading to high stress points at board joints and abutments.


The difference between this type of system and wet-lay GRP is that with a pre- cured membrane based product, both the membrane and the detailing trims are factory manufactured under ISO 9001 conditions and supplied to site ready for installation, all by mechanically fixing. This can be completed over an existing roof finish, providing that the substrate is in good condition. Additionally, there is no need for a naked flame to be employed during installation. The mechanical fixing can be carried in all weather conditions, however like all liquid applied and wet-lay GRP systems, the air temperature should be consistently above 5°C and the roof surface free of moisture for a successful installation to be completed.

In summary, the old adage of “horses for courses” still holds true in many circumstances, but the suitability of alternative waterproofing systems for refurbishment work does offer a wide choice.

Long-term manufacturer guarantees of water tightness covering the first 20 years of an installed roof covering, along with an insured warranty, ease of maintenance and repair contribute to a strong case for liquid and GRP waterproofing systems for housing maintenance.

Andy Fell is the dryseal manager for Hambleside Danelaw Ltd