roof drainage


Roofs exist to protect buildings and products from external elements. From school buildings to garage units to residential homes, the primary function of a roof is to keep the weather out.

Not only is water damaging to materialistic interiors but it can also lead to structural damage and reduced building integrity. From mould growth to rising damp through to structures falling apart, wet interiors of buildings can have expensive, long consequences ranging from health issues to failing business proactivity.

Sometimes, even with roofing, damage can occur to premises when water is retained and wears away the roof layer. This mainly occurs on flat roofing as water can’t flow away. In this guide, we’ll cover how you can ensure your flat roof has appropriate drainage and the roofing standards you need to be aware of.


What is roof drainage? 

As part of Building Regulation Part H, it is a requirement that adequate provision is made for rainwater to be carried away from the roof of the building. It is also specified that this should occur as quickly as possible to prevent damage. Roof drainage should be designed and installed with the following British Standards in mind.

BS 6229:2018 provides an overall recommendation for the design, specification and briefly the installation and later maintenance of a flat roof. The BS 6229 standards outline that all flat roof surfaces should be designed with a fall of 1:40 to ensure finished drainage falls of 1:80 are achieved. BS8217:2005 complies closely with BS 6229 and gives recommendations for the selection of reinforced bitumen membranes.

Both standards indicated that the design should allow for all factors that could reduce or hinder the drainage, eliminating the risk of ponding on roofs. For upstand heights, this standard recommends a clear upstand of at least 150mm above the finished roof surface, including any paving, inverted insulation or green roofing.

These standards are applied in the industry to manufacture and design roofs that allow the water to flow away from the surface and into suitable run-off so that water does not collect or puddle.


roof water


How do flat roofs drain water? 

What is done to prevent puddling you may be wondering. And how can a flat roof have a fall? Surely that would mean…they’re not flat. Got it in one. The cat is out of the bag.

Flat roofs aren’t flat but they are designed with such a small fall that they have the appearance that they are. Then aren’t they pitched roofs?  Not at all. Even if they are designed with a slight pitch, flat roofing is more economical and efficient and when first installed it saves the owner money and energy with the right roofing tools.

An efficient low-slope roof drainage system works entirely by gravity because water always seeks the lowest level. By making sure your drainage is at the lowest points on your flat roof, you can avoid water infiltration and subsequent flat roof repair.


How to slope a flat roof for drainage? 

To determine the slope required, the roof span must be measured to make sure a place for the new peak of the roof can be appointed. This will be the highest point of your flat roof and from there the level should fall away toward the drainage.

Whether you intend to use scuppers, inner drains or guttering as your water drainage solution, you need to make sure that the incline of your roof makes sure water reaches them.

As the minimum fall for a roof run is 1:40, a precise process will need to be carried out and marked to ensure the drops occur in the correct places and levels should be checked throughout the installation process rather than after. Dependent on how much rain an area received, an increased fall is recommended.

To design the fall, fitting strips or tapered insulation is the most appropriate choice of material to create the slope. When laying the roofing covering, make sure to lay it completely flat against the material so that no divets or bumps inhibit the fall.

Smooth off any leadwork by making it completely flush with a bending stick. Create clean surfaces and seamless edges with brushes and rollers. Make sure no space is left uncovered with sealants and tapes for those final functional touches.

Ensure better drainage with vents and outlets that are designed to lay flush against EPDM, felt or liquid surfaces to maintain the fall.


roofing drainage


Why roof drainage is important

Roof drainage is important because it reduces the amount of damage from collections of water that can cause structural damage and wear. Standing water increases the load on the roof structure which can cause the deck to deflect. These deflections make drainage even harder.

It can also lead to a build-up of debris on roofs which in turn can lead to blockages of drainage outlets, resulting in unsightly roofs and increased risk of flooding.

If a roof is allowed to become damaged by any of the reasons outlined above then the interior could suffer from greater water ingress.

If your roof does not drain appropriately then you are at risk of causing structural damage to your building which puts your property and residents/staff at risk. Replacing roofing or guttering is also expensive and be a huge drain on your resources. If only draining had been done properly in the first place….


Tools for roof drainage

Waterproof liquid roofing systems are highly advised for helping to keep any style of roofing waterproof. Even with the slope, water can cause damage and wear. The Lancastria waterproof system THX, is incredibly easy to apply and can have a seamless finish to keep a flat roof smooth for great drainage.

Hot melt systems are extremely popular for this area of work. They give builders the diversity for different styles of roofs and with tactile tools that make sure no layer goes unsealed, roofs can be waterproofed and covered to the highest standards. From bitumen coatings to liquid roofing, Lancastria can provide all the tools you need to get the job done.

At Lancastria, we never stop trying to achieve a solution. We’ve got the tools, resources and knowledge to make sure your project goes by smoothly. We’re experts in waterproofing and tools and if you need some help getting to you and your materials to your roofing projects safely then we even offer a great selection of hoists for sale or hire.

Need a solution for your flat roof project? Contact Lancastria today on 0161 777 9009 or email [email protected]

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