Guttering and Rainwater Drainage for London Home Extensions

The gutter must be large enough to carry the peak storm flow from the roof area it drains without overflowing. The standard design approach for domestic guttering in the UK is set out in BS EN 12056-3 (Gravity Drainage Systems Inside Buildings — Roof Drainage, Layout and Calculation). The basic principle: the larger the roof area drained by a single gutter run and outlet, and the steeper the effective pitch of the roof, the higher the peak flow rate and the larger the gutter cross-section required. For a typical single-storey rear extension in London with a flat or shallow-pitched roof: a standard 112mm half-round gutter or 115mm ogee gutter is sufficient for a gutter run of up to approximately 6-8m serving a roof area of up to approximately 25-30m². For a wider extension or a longer gutter run, a larger gutter section (125mm half-round or equivalent rectangular profile) or additional outlets may be required. The outlet position within the gutter run affects capacity — an outlet at one end of the run has half the capacity of an outlet in the middle, because the full length of gutter must carry the entire peak flow to the end outlet. For a long gutter run serving a large roof area, two outlets — one at each end, or one centrally — are more efficient. Gutter falls: Gutters should be installed with a fall of at least 1:600 (approximately 17mm per metre run) towards the outlet, to avoid ponding of water in the gutter that accelerates debris accumulation and corrosion. Flat-bottom rectangular gutters with very small falls can accumulate debris more readily than round-bottom profiles.

Where does the rainwater go? This is a regulatory and practical question for every new extension. The options are: Connection to the surface water sewer: Many London properties have separate surface water sewers that drain to watercourses via the surface water drainage system (distinct from the foul sewer). Where a surface water sewer exists and is accessible (within reasonable cost of connection), connecting the extension downpipe to it is the straightforward approach. Thames Water consent is required for any new connection to a public sewer. Connection to the foul sewer (combined drainage): Many older London streets have combined drainage where foul and surface water are in the same sewer. Building Regulations now generally require that surface water from new extensions is not connected to the foul sewer where an alternative exists. However, in areas where only a combined sewer is available and a soakaway is not viable, connecting the surface water to the combined sewer (with Thames Water consent) is sometimes the only option. Soakaway: A drainage feature within the garden that allows surface water to percolate into the ground. A soakaway is the preferred destination for surface water where ground conditions permit (i.e., where the ground is sufficiently permeable). In London Clay conditions — which characterise most of inner and north London — the permeability of the ground is typically too low for a traditional soakaway to function effectively. A percolation test (as described in BRE Digest 365) should be carried out before designing a soakaway. Rainwater harvesting: Surface water from the extension roof can be collected in a rainwater harvesting tank for use in toilet flushing or garden irrigation — reducing both the surface water drainage load and the mains water consumption of the house.

Victorian and Edwardian London houses were built with cast iron gutters and downpipes. Cast iron, when properly maintained, lasts indefinitely — but cast iron gutters that have been neglected for decades (a common condition in London Victorian terraces) will have joints that have blown open, sections that have cracked through frost or impact, or downpipes that have rusted through internally. Failing cast iron gutters are one of the most common causes of penetrating damp in older London properties — water overflowing from a blocked or failed gutter runs down the wall behind, saturating the wall and appearing as damp patches on the interior walls at the corners or beneath window heads. Before starting a loft conversion, roof repair, or rear extension project on a Victorian terrace, the front and rear gutters should be inspected from scaffolding (including lifting joints to check the seating). Any failed sections should be replaced. Cast iron guttering is available from specialist suppliers in the traditional ogee, half-round, and moulded profiles, allowing like-for-like replacement on a period property. For rear elevations (not visible from public views), a change to aluminium or uPVC gutter is common — but for front elevations of properties in conservation areas, cast iron or cast iron-effect aluminium is typically required or strongly encouraged by the planning authority.