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NFRC Technical Talk SUMMARY OF CHANGES TO BS 6229 PART 2: LOADING & DRAINAGE Gary Walpole, NFRC Technical Officer, continues his review of the changes to BS 6229, the Code of Practice for flat roofs. BS 6229 (Flat roofs with continuously supported flexible waterproof coverings. Code of Practice) was last updated in 2003 and much has changed within the industry in the last 15 years. The revised BS 6629:2018 describes best current practice in the design, construction, care and maintenance of roofs with a flat or curved surface, at a pitch not greater than 10 degrees to the horizontal, with a continuously supported flexible waterproof covering. Last month I explained what the main changes were in relation to the different flat roof buildups; this month I look at loading and drainage: Loading The roof structure should be designed for the strength of the specified waterproofing and take into account any added surfacing including green roofs, paving slabs, gravel, blue roofs for rainwater retention, or any mechanical or electrical equipment which may be situated on the finished roof. Achieving drainage through roof fall Ponding water adds a dead load to the roof structure and in exposed warm roofs increased stresses in the waterproofing layer, therefore, a minimum 1:80 finished fall is recommended to both the general area of the roof and to any formed internal gutters. BS 6229 recommends that all flat roofs should be designed with a fall of 1:40 to ensure a 1:80 fall BS 6229 has been revised and there are a number of changes that roofers need to be aware of. is achieved once the roof is completed. This design criteria includes internal gutters and should take account of construction tolerances, settlement and for deflection under load, both during and post construction. Zero falls Certain third-party certified waterproofing and insulating systems are approved for use with zero falls, but back falls are not acceptable. To ensure a finished surface with a zero fall, a design fall of 1:80 should be used and a detailed structural analysis should account for construction tolerances, settlement and for deflection under load. Where areas are found by a site level survey to have negative falls, ponding water will occur. This will need to be corrected through the introduction of an additional outlet or localised screed to falls. To prevent ponding caused by waterproofing system lap, build ups around rainwater outlets, rainwater outlets should be recessed into the slab / deck or fitted in sumps when it is practicable to do so. Rainwater disposal The design of most flat roofs should ensure rainwater is drained from the roof as quickly as possible. However, some roof finishes are designed to control the disposal of rainwater from the roof. For example, green roofs are designed to support planting, and blue roofs are designed to attenuate the drainage of rainwater. These roofs Left: Gary Walpole, NFRC. may require additional rainwater design considerations. All blue roofs and roofs that drain into a single internal rainwater outlet or combined outlets connected to a single downpipe, should also be fitted with a visible overflow to drain and alert the building user of any blockages to the rainwater outlet. Door threshold and upstand details The minimum height of all upstands and abutments from the finished waterproofing system remains at 150mm. However, if level access is required from within the building, as in the case of a door opening onto a balcony or terrace: • The height of the clear upturn under the door threshold may be reduced to not less than 75mm. • The waterproofing should be fitted before fixing the door threshold. It is important that the roof is designed with the rainwater falling away from the door, and in cases where there is an internal outlet or through-wall outlet: • An overflow must be fitted a minimum 25mm below the height of the door threshold to prevent water ingress into the dwelling in situations of blockages to the outlet. • The height of the upstand must return to 150mm above the finished roof level beyond the door reveals. Contact the NFRC 020 7638 7663 www.nfrc.co.uk @TheNFRC 18 TC MAY <strong>2019</strong>
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May 2019

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