Home Contractor's CornerAn Inspector Calls Don’t be the fall guy when it comes to flat roofing

Don’t be the fall guy when it comes to flat roofing

by Matt Downs

This month, the Inspector looks at how updates in BS 6229:2018 can stop you being the fall guy when it comes to flat roofing.

One of the most eye-catching amendments in the revision of BS 6229:2018 was the reference to falls. This is now much clearer, stating that “all flat roof surfaces (including gutter beds) should be designed with a fall of 1:40 to ensure finished drainage falls of 1:80 are achieved. This should take account of construction tolerances, permitted deviations and deflection under load, and account for deflections/settlement.”

In other words, any design should allow for all factors that could reduce or hinder the drainage, eliminating the risk of ponding on roofs. Without these considerations, it is highly likely standing water will occur. Although improvements in membrane technology and performance have increased significantly, standing water may still result in additional and unnecessary stresses in the membrane, particularly in the winter when that standing water freezes. Furthermore, in the event of a defect being present in the area of the ponding, greater water ingress will occur when compared to a well-drained roof.

Standing water due to lack of fall

Accelerate the ageing process

Standing water is typically defined as water that remains on the roof for longer than 48 hours, and whilst this is not an immediate threat, it can accelerate the natural ageing process and have a detrimental effect on the membrane’s lifespan.

It’s possible that standing water can still occur even when the structural deck has a fall within it, as compressed insulation, blocked or faulty drainage and damaged membranes can all contribute towards this issue too. Regular maintenance can therefore reduce this risk, alongside reducing the number and weight of items permanently stored on the roof. Another contributing factor could be the sequencing of membrane laps and edge details sitting proud of the finished floor level, consequently reducing the water’s ability to navigate past the resulting water-check.

The reference to falls is all the more pertinent given that, these days, there are certain third-party certified waterproofing and insulating systems that have gained approval for use with zero falls. Hot melt systems in particular are popular for this area of work. For these systems, zero falls are acceptable, but negative falls are not, so should be corrected. It is not really acceptable in this day and age for any contractor to install roof decks with large depressions, back falls and non-draining areas.

To ensure a zero fall finished surface i.e. one that is totally flat, a design fall of 1:80 should be used, along with a detailed structural analysis to account for construction tolerances, settlement and deflection under load.

Ponding can shorten the life of a roof

Remedial action

If sites have negative falls, thereby increasing the likelihood of ponding, then remedial action has to be taken before the roof system is applied. This could be by laying a localized screed to falls and firings, fitting tapered insulation or fitting additional rainwater outlets at the lowest points.

As a result, the roofing contractor should expect a flat, properly drained surface on which to lay the specified system and the finished roof should not suffer from ponding or inadequate drainage.


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