Zero Pitched Falls
Hot Melt Monolithic Membranes may be laid to zero pitched falls in accordance with certification from the British Board of Agrément (BBA). In such applications thermal calculations should be carried out in accordance with ETAG 031 (Guideline for European Technical Approval of Inverted Roof Insulation Kits). Further guidance can be found in the BBA Information Bulletin No. 4 ‘Inverted Roofs – Drainage and U-Value Calculations’.
Zero pitched roofs are defined as flat roofs with a slope between 0 – 0.7°. It is essential to identify suitable drainage points to facilitate the effective removal of rainwater. Existing structures may require a detailed analysis to determine overall and local deflection as well as the direction of falls. It is considered good practice to design new roofs with a 1:40 slope to compensate for future settlement or deflection.
Flat roofs are usually constructed with slight falls, typically 1:80, which allow water to drain away. Minor constructional variations in the plane of the surface will allow the collection of water which has no way of draining. It is therefore advisable to design falls greater than the required finished falls to allow for construction tolerances and dead and imposed loads.
However, Hot Melt Monolithic Waterproofing systems are not constrained by the same falls requirements as warm flat roofs, where the waterproofing is the upper visible layer. Hot Melt Monolithic Waterproofing systems are commonly constructed as inverted roofs, the waterproofing is beneath the insulation and any applied finish. A loose laid water control layer is typically installed above the insulation and beneath the ballast/paving/green roof layer, this controls the travel of water within the roof build-up enabling drainage at insulation level and deck level.
NOTE: for the water control layer to operate effectively, the inverted roof should be designed with a positive fall and in accordance with BBA Information Bulletin No. 4 and the manufacturer’s instructions (Hot Melt Code of Practice January 2014 - LRWA).