Headlines about extreme weather and widespread flooding in the UK are becoming increasingly common:
- UK to be hit by more heavy rain as Met Office issues new weather warning
- Severe flood warning issued as the UK faces more weather disruptions
- Major incidents declared in Midlands as flooding hits UK
Regardless of one’s stance on climate change, the reality is that many regions are seeing more concentrated and intense rainfall. This shift in weather patterns is putting immense pressure on buildings, especially their roofs.
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.
The Critical Role of Drainage Flow Calculations
Far too often, these systems are specified without any supporting data. In most cases, no one has actually calculated whether the roof can handle the volume of water it’s likely to see.
Why You Need BS 12506-3:2000-Compliant Calculations
This British Standard sets out how to properly calculate drainage flow on roofs. It’s a rigorous approach that ensures water is moved off the roof as fast as it lands. If your specification doesn’t include a calculation like this, it’s an omission that could cost you thousands in future repair and disruption.
Unfortunately, it’s rare to find a manufacturer’s rep who’s even aware of this standard - let alone one who applies it.
Avoiding the Sales-First Mentality
EVERY specification should be accompanied with an evidenced drainage calculation or you risk roof failure of a different and possibly more catastrophic type than 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.
How RAM Building Consultancy Can Help
At RAM Building Consultancy, we include drainage calculations to BS 12506-3:2000 as standard in all of our roof specifications. This ensures that:
- Every outlet and fall is designed for real-world rainfall,
- The risk of water pooling, leaks, or catastrophic roof failure is minimized,
- You’re not left with expensive “fixes” that cause more problems than they solve.
Our approach is professional, independent, and rooted in engineering, not sales. Proper specifications don’t cost money - they save it.
Final Thought: Don’t Forget the Bigger Picture
One last point: When calculating roof drainage, always account for other roofs that drain into the one you’re assessing. It’s a common mistake that can drastically skew your figures and lead to unexpected overflows.
