Gutter profiles, manufacturer-specific falls and support centres, with effective roof area and safe discharge checks.
Rainwater drainage systems capture run-off water from roofs and discharge it safely into surface water drains or soakaways, protecting the building's fabric. Use Approved Document H, Part H3 for the drainage guidance in England, together with the selected manufacturer’s instructions:
Gutter Profiles
- Half-Round: A common domestic profile; check its rated capacity for the catchment and outlet layout.
- Square-Line: An alternative profile; compare the actual capacity rather than inferring it from the name.
- Deep-Flow: A deeper section can offer more capacity for a large effective roof area. Compare the selected profile’s capacity and outlet layout.
- Parapet or Box Gutters: Built into the roof structure rather than hung at the open eaves, so if their outlets cannot cope or become blocked the water backs up and can enter the building; they need adequately sized outlets and often an overflow to guard against this.
Setting Falls (Gradients)
Use the selected system’s installation and capacity instructions. A level gutter can have a valid design capacity; where a fall is specified, set it towards the outlet and maintain the roof-edge catching position. A local sag is different from an intentionally level run.
- Worked gradient: 1 in 600 means 1 mm drop in 600 mm of horizontal run, or about 1.7 mm per metre. Over 6 metres the drop is 6000 ÷ 600 = 10 mm. This is a conversion example, not a universal minimum.
- Manufacturer examples: FloPlast’s cited PVCu instructions recommend 1 in 350. Alumasc’s Apex cast-iron guide specifies a fall from 1 in 600 to 1 in 350, or level where that is not possible. Use the applicable profile’s instructions and rated layout.
Gutter Bracket Support Spacing
Support the fully loaded gutter and maintain its designed line. The profile, fixings, fascia, water/snow loading and manufacturer’s instructions determine the centres; heavier material does not automatically mean closer spacing than every plastic system.
For the cited FloPlast PVCu systems:
- Straight runs have fascia brackets at no more than 1.0 metre, reduced to 800 mm for Niagara and 600 mm for Xtraflo.
- With an 80 mm round downpipe on Hi-Cap or Niagara, the guide specifies 800 mm maximum fascia-bracket spacing.
- In areas with possible high snowfall, the maximum is 400 mm.
- Provide a fascia bracket within 150 mm of angles and stop ends. Fix unions, outlets and other fittings as their instructions require; the 150 mm rule is not a universal rule for every fitting.
- Support downpipes below an offset and at no more than 1.8 metres in this system; support shoes, branches and bends where needed.
Alumasc’s cited Apex cast-iron gutter guide generally uses 915 mm centres, with additional support at the gutter joints. These are system examples, not interchangeable specifications.
Roof Area and Flow Capacity
The size of the gutter and downpipe required is determined by the effective roof area, which allows for the roof's pitch. Approved Document H multiplies each pitched slope's plan area by a set factor: 1.29 at 30 degrees, 1.50 at 45 degrees and 1.87 at 60 degrees. A flat roof keeps its plan area, and a wall, or a roof pitched over 70 degrees, counts as half its elevational area.
Positioning, discharge and soakaways
Set the gutter so it sits under the edge of the roof covering, catching the water as it comes off the tiles; a gutter set too low or too far back lets heavy rain leap straight over it.
Route discharge away from the building rather than letting it fall at the wall or across a path: against the foundations it saturates the ground beneath them and can cause damp or erosion, and across a path it can freeze into a slip hazard. Take it to a proper gully, drain or soakaway instead.
Where the destination is a soakaway, the ground must be permeable enough to accept the water, with the water table below the base of the device, and it is kept at least 5 metres from a building or road. For a small soakaway serving 25 square metres or less, Approved Document H permits an assumed design storm of 10 mm of rain in 5 minutes for its small-soakaway method. Check ground suitability and the applicable sizing method; it is not a guarantee against every possible storm.