August 2018

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Flat Roof Falls FALLS IN CONCRETE FLAT ROOFS – TAPERED INSULATION OR SCREED? By Daniel Bosworth, Design Manager at SIG Design & Technology. Cut to Falls is often the most expensive part of the roof system and we’re often asked how to achieve affordability. It might be possible to reduce costs by engaging in consultation and / or discussion with specialists at an early design stage to investigate alternatives and their benefits and impacts on methodology, buildability, sequencing and interfaces. As roofing designers, we enjoy the challenge of making roofs work harder and be more cost efficient. There is a balance to consider between the product cost per square or cubic metre, the labour input and the time or sequencing benefits associated with each of the options. “As roofing designers, we enjoy the challenge of making roofs work harder and be more cost efficient” Falls are introduced into a concrete flat roof mainly by: • Putting the fall in the structure. • Putting the fall in a screed. • Putting the fall in the insulation. While each roof is different, if you can introduce falls in the structure, this can be the simplest, most cost-effective option. 1. Introduce falls in the roof structure if possible If you can put falls into the flat roof structure rather than rely on screed or tapered insulation, do so. Falls in structure are usually very easy with precast concrete roofs, with steel frame and metal decking, and with many timber structures. So why don’t all roofing projects put the falls in the structure? It may be for ease of initial design, but this can lead to issues during installation. Height clashes with door thresholds or reduced / non-compliant upstand heights at parapets which can cause delays, cost time and money, and achieve a sub-standard result. It might be a misunderstanding of the term “flat”, but we sometimes need to explain to customers that there are minimum falls required by BS 6229:2003, even if manufacturers are happy to “accept” zero falls. Or it may just be an oversight. However, if the holistic approach of detailing all interfaces based on 100% compliance is undertaken early enough, then costs, time and mistakes can all be minimised. 2. Screed is heavy Laying a screed over a flat roof structure to introduce falls is sometimes a simple practical solution. However, screed is a heavy, dense material, and therefore requires consideration in the structural calculations. It may impact on overall design. Installed by wet trades, standard screeds can have drying times in excess of 100 days. Even quick drying screeds require a minimum of approximately four hours and often substantially longer. Consider the pros and cons of using screeds within your project. Daniel Bosworth is Design Manager at SIG Design & Technology. 3. But tapered insulation can also be expensive Manufacturers of tapered insulation argue that it costs less per m² than screed. Insulation is much lighter than screed and therefore may save on structural costs. It is also quicker to install and allows for the creation of falls and satisfying the thermal performance of the roof. However, tapered insulation isn’t always going to be the cheapest option. In a large roof with very long runs, or in a complicated roof, tapered insulation can be less attractive and is not always the best use of space either. “Screed doesn’t only serve the purpose of introducing falls, it also acts as a smoothing surface, evening out the bumps in concrete beam and plank roof constructions” Consider the bigger picture By omitting the screed layer at a school in Twickenham, SIG Design & Technology was able to save our client thousands of pounds in costs caused by the additional weight and drying time, and time for the wet trades to install it. We were only able to do this by being asked to undertake “value engineering” at an early stage by the design team and were happy to do so to ensure 52 TC AUGUST <strong>2018</strong>
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