Advanced options
Standard 112mm half-round PVC gutter handles roughly 50 m2 of roof per downpipe with the outlet at the end (more with a central outlet). Fall about 1:350 (roughly 3mm per metre) toward the outlet, brackets at 800mm centres (closer in snow areas), and stop ends, corners and a running outlet per downpipe complete the parts list.
What The Guttering Calculator Works Out
- Gutter lengths: the run divided by the 4 m stock length, rounded up, with one union bracket between each pair of lengths. A 12 m run is three lengths and two unions, plus stop ends and whatever corners the roofline demands.
- Brackets: one every 800mm along the run plus one to finish. Set the first bracket by string line with the fall built in, because flat gutters pond and grow gardens while overdone falls look drunk from the pavement.
- Downpipes: standard 112mm half-round PVC with an end outlet handles roughly 50 m2 of roof plan area per downpipe, so a 50 m2 roof wants one. Each needs a running outlet, two pipe brackets per storey and a shoe or drain connection; discharge into a drain or soakaway, never against the wall base.
- Gutter profile (advanced): deeper profiles carry more water: 114mm square line roughly 65 m2 per downpipe and 170mm deep flow about 105 m2, which buys headroom under big or steep roofs (pitched roofs shed more water per plan area than the flat figure suggests).
- Outlet position (advanced): an outlet mid-run lets water travel both ways and roughly doubles the capacity of each downpipe, often saving a pipe on a long elevation.
- Bracket spacing and corners (advanced): tighten brackets to 600mm in snow-prone areas or on long unsupported fascias, and count the internal and external corners so they appear in the parts list.
- Storeys and downpipe lengths (advanced): each downpipe runs about 2.7 m per storey, bought in 2.5 m lengths and rounded up, with offset bends to clear the eaves on top.
- Fall to set (advanced): the total drop from the high end to the outlet at 1:350, about 3mm per metre; 36mm over a 12 m run, split either side of a mid-run outlet.
- Capacity check (advanced): how much of the installed downpipe capacity the roof uses. Above about 85% there is little margin for cloudbursts; add a pipe or step up a profile.
- Estimated materials cost (advanced): gutter and downpipe lengths at your prices plus typical PVC fitting prices (GBP 1.50 brackets, GBP 4 unions, GBP 6 outlets and so on), itemised in the parts table. PVC is cheap, adequate and brittle after decades of UV; aluminium and steel cost more and outlast several PVC generations.