WHAT TYPES OF WATERPROOFING CAN BE USED ON INVERTED FLAT ROOFS?
Inverted flat roofs can be waterproofed with bituminous membranes, single ply membranes, or liquid-applied membranes
By far the most common solution, for reasons that we’ll discuss in this post, is the type of liquid-applied waterproofing known as hot melt. However, the choice of waterproofing depends on what material the structural deck is constructed from.
During construction of an inverted roof, the structural deck is waterproofed prior to installation of the thermal insulation and roof finish (which also acts as ballast). This protects the roof structure and building from rain sooner, but also subjects the waterproofing layer to more foot traffic. The type of waterproofing selected should therefore also reflect the conditions during both installation and use.
What is the most common structural deck for inverted roof waterproofing?
The overwhelming majority of inverted roofs are concrete decks (or metal liners with a concrete topping). There are a variety of reasons for this being the case.
- The roof structure must be capable of bearing the weight of the ballast.
- Many inverted roofs are designed to bear the weight of foot traffic, mechanical plant, or green and blue roof systems. The roof structure must also be capable of bearing these loads.
- As a non-combustible material, there is less risk associated with concrete and a hot-applied waterproofing material like hot melt.
Timber decks provide a flat substrate for the waterproofing, but are much less commonly specified than concrete. Metal decks, meanwhile, cannot be used by themselves in an inverted roof because of their profiled shape hindering the waterproofing installation.
Properties of different inverted roof waterproofing options
With the structural deck chosen and installed, a variety of factors contribute to the choice of system to waterproof it. Method of attachment to the deck, speed of installation, and overall system performance must all be taken into account.
If the waterproofing product requires mechanical fixings then a concrete deck has to be pre-drilled, which adds time and cost to the operation. An adhered membrane obviously avoids the needs for mechanical fixings but, depending on how adhesion is achieved, does not necessarily achieve a bond over the whole roof area.
A complete bond between waterproofing and deck is a desirable characteristic in an inverted roof build-up. Should the waterproofing become damaged, especially by follow-on trades and subsequent work on the roof, any leak is confined to the isolated area. Water cannot track through the system and enter the building elsewhere, avoiding difficult and costly leak investigations.
This is a significant reason why liquid applied solutions, and hot melt in particular, are usually preferred.
Selecting inverted roof insulation
Nevertheless, it remains the case that multiple waterproofing options are available for inverted roofs. Extruded polystyrene (XPS) insulation can be used with all of them. No fixing of the insulation is required thanks to the ballast installed on top of the insulation.
Polyfoam XPS’s Roofboard can be used in conjunction with our Upstand Board to create a complete inverted roof system. Visit our technical support page to download a copy of our inverted roof BBA certificate, or contact us to discuss your specific project requirements or request an inverted roof U-value calculation.