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It is increasingly common to see reports like this making headlines in MSM.
Whether you are a climate change believer or a not, what cannot be denied is we do seem to be experiencing more frequent periods of concentrated heavier than normal rainfall.
Many of the UK’s roofs are not coping with this increased drainage load, in particular flat roofs and/or pitched roofs with discreet valleys.
What is exacerbating the situation is the now common practice, with the majority of membrane roof system suppliers, single-ply, bitumen sheet and/or cold applied liquids, to use refurbishment rainwater outlet sleeves. Leaving existing rainwater outlets in place and simply using a so called ‘refurb outlet’ that sits inside the original outlet. This can reduce drainage capacity of a roof or discreet gutter by >20%, significantly increasing the risk of flooding.
Add to this the oh-so popular trend of putting tapered (cut to falls) insulation onto existing roofs or within existing gutters. With the popular but not necessarily accurate belief that this will make your roof drainage better. Unless you undertake a professional drainage flow calculation, you cannot begin to understand the adverse effect this can have on existing drainage capacities.
It will be rare to find a system supplier representative aware of these issues and even rarer to find one who knows how to do this calculation.
EVERY specification should be accompanied with an evidenced drainage calculation or you risk roof failure of a different and possibly more catastrophic type that you might be aware of.
Be aware that the roofing sales rep, wants to sell you the most expensive products in their range, this will include accessories such as refurbishment rainwater outlets and tapered insulation. If they are advising this and have not undertaken a drainage calculation to BS 12506-3:2000, then they have not exercised due professional diligence.
At RAM Building Consultancy, roof drainage calculations to BS 12506-3:2000 are standard within all our roofing specifications. Professional and independent specifications do not cost you money, they invariably save you money.
#footnote: when carrying out the drainage calculation, avoid the other common mistake of not taking account other roofs that may drain onto the roof you are surveying.
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