How the guttering calculator works
Each gutter run is split into standard 4m lengths. The calculator then adds the fittings each run needs: brackets at no more than about 900mm apart, a union wherever two lengths meet, a stop end at each open end, a running outlet for each downpipe, and any corners. Downpipes are made up from 2.5m lengths with a clip roughly every 1.8m, a pair of offset bends to clear the eaves overhang, and a shoe at the bottom.
Gutter capacity check
If you enter the roof plan area, the calculator compares the rain falling on the roof with a typical outlet capacity for your gutter profile, using a design rainfall of 75mm per hour (about 0.021 litres per second per m²). Capacities shown are typical figures for an end outlet on a level gutter. Central outlets and a slight fall increase capacity. Always check the manufacturer's drainage tables.
| Profile | Approx. flow (end outlet) | Roof plan area per outlet |
|---|---|---|
| Half round 112mm | ~0.8 l/s | ~38 m² |
| Ogee 122mm | ~0.9 l/s | ~43 m² |
| Square 114mm | ~1.0 l/s | ~48 m² |
| Deep flow 115mm | ~1.3 l/s | ~62 m² |
Fitting tips
- Set a string line with a slight fall towards each outlet and fix the end brackets first.
- Leave the expansion gap marked inside unions and fittings. PVC expands a lot in summer sun.
- Keep gutters as close to the roof edge as possible so water runs into the centre of the gutter.
- Take downpipes into a gully, water butt or soakaway, never just onto the ground next to the wall.
Worked example: front and back of a semi
Two 8m runs of 112mm half-round gutter (16m in total), each with one downpipe 5m high:
- Gutter: each 8m run is two 4m lengths, so 4 lengths and 2 unions.
- Brackets: 8m ÷ 0.9m = 9, plus one = 10 per run, so 20 brackets.
- Ends and outlets: 4 stop ends and 2 running outlets.
- Downpipes: each 5m drop is two 2.5m lengths, so 4 lengths, 2 sockets, 8 clips, 4 offset bends and 2 shoes.
Enter the roof plan area to check capacity. If each slope covers about 30m², 30 × 0.021 = 0.62 litres per second per outlet, which is within the roughly 0.8 l/s a half-round end outlet handles. Drain both slopes (60m²) through one downpipe, and it would overflow in a heavy storm.
Repair, replace or upgrade?
Leaking joints are often just failed seals in the unions. A new union or a gutter sealant can fix them cheaply. Sagging gutters usually mean missing or broken brackets. If the PVC is brittle, faded and leaking in many places, or the gutter overflows in normal rain, replacing the whole run, perhaps with a deep-flow profile, is worth it. Matching the profile of next door's gutter on a semi or terrace keeps the joint between houses neat and watertight.
Working safely at height
Most gutter work is done from a ladder, but only for short tasks where you can keep three points of contact and the ladder is footed or tied. For a whole house, a scaffold tower with outriggers is far safer and quicker. If the gutter is above a conservatory, flat roof or busy path, or you're not comfortable working at height, get a professional.
Common mistakes
- Too much or too little fall. About 2mm per metre towards the outlet is plenty.
- Brackets too far apart. Gutters sag and joints leak, especially under snow. Keep within 1m.
- Not cleaning them. Leaves and moss block outlets, and overflowing gutters soak walls and cause damp. Clear them each autumn, and fit leaf guards near trees.
Replacing the roof too? The roof tile calculator works out tiles, battens and membrane. See all roofing calculators.