Building Regulations Part H covers drainage and waste disposal on new build housing, including the requirements for roof drainage. The regulations specify how roof catchment areas should be calculated, how outlets should be sized against expected rainfall intensities, and how the drainage system should discharge safely without causing flooding or nuisance.
Where specification variance appears on housing schemes
For QSs pricing housing schemes, the roof drainage sizing is one of the areas where specification variance produces very different outcomes. A contractor who sizes to the minimum acceptable standard produces a system that meets Part H at handover but has limited margin against exceptional rainfall. A contractor who sizes with proper margin produces a system that performs reliably across the range of weather events the building will experience.
The calculation itself is straightforward. Roof plan area contributing to each outlet. Rainfall intensity for the site location, typically taken as the 100-year event. Outlet flow capacity from manufacturer's data. Gutter capacity from BS EN 12056. Where the outlet is undersized against the catchment, water backs up in the gutter and eventually overtops. Where the gutter capacity is exceeded, the same happens further along the run.
Standard installation practice on UK housing tends to work for typical exposure conditions and typical rainfall. Where the location is more exposed or where rainfall intensity is higher than the average, the standard specification may be marginal. This is particularly relevant for schemes in western and coastal areas where rainfall intensities are meaningfully higher than the UK average.
Site-specific design review vs generic calculations
Globe Roofing's design review on housing schemes includes roof drainage sizing against the site's specific rainfall intensity rather than against a generic UK average. Where the design specification is marginal against the actual conditions, the discrepancy is raised with the design team. Where the specification is adequate, the install proceeds against it.
There is a specific consideration around dormer roofs and complex geometries that is worth flagging. Non-standard roof shapes concentrate runoff at particular points, and the drainage design has to account for this rather than treating the roof as a simple rectangle. Where the design does not do this properly, the concentrated runoff overwhelms the drainage at that point during heavy rain.
Downstream discharge is the other Part H consideration. Roof drainage that discharges into a combined sewer, a surface water sewer, a soakaway, or a SuDS attenuation feature has different design implications. Where the discharge type is not confirmed at design stage, the installation can be technically correct at handover but not properly connected to the intended discharge.
The commercial risk of undersized roof drainage
For developers, the specific commercial risk of undersized roof drainage is that overflow during heavy rain can cause damage to the building envelope, to internal finishes, and to neighbouring properties. Each of these produces claim liability. The cost of adequately sized drainage at first installation is significantly less than the cost of remedying flooding damage later.
On schemes where the drainage will be adopted by the water authority under Section 104, the roof drainage sizing has to meet the authority's specific technical requirements as well as Part H. These vary between authorities and should be confirmed at design stage rather than at inspection stage.
For QSs reviewing roofing tenders, the practical question is whether the bidder has verified the drainage sizing against the specific site conditions and the specific discharge arrangement. A generic answer indicates generic design attention. A specific answer indicates that the sizing has actually been calculated properly for the scheme in front of them.
Talk to Globe Roofing
To discuss roof drainage sizing to Building Regulations Part H on your scheme, contact Globe Roofing on 01223 890727 or email enquiries@theglobegroup.co.uk.
