Free UK construction calculators Metric and imperial 2026 UK prices
Work out what size gutter your roof needs and exactly what to buy. Enter the length of guttering, the number of runs, corners and downpipes, and the roof size and pitch, then press Calculate for the gutter size to Approved Document H and a full parts list with prices.
| Part | Qty | Each | Cost |
|---|---|---|---|
| Total | £0.00 |
FloPlast 112 mm half-round and 68 mm round downpipe at typical 2026 UK prices including VAT. Screws, hoppers and drain connectors are extra.
The result shows the effective roof area and rainfall, the gutter and outlet size Approved Document H asks for, whether a standard 112 mm gutter is big enough, and a full parts list with quantities and prices. The drawing shows one run with every bracket, joint and downpipe.
In England and Wales, Approved Document H (Drainage and waste disposal), Section H3, sets out how big gutters should be for a design rainfall of 75 mm an hour. Table 2 gives the largest effective roof area that each size of level half-round gutter can drain through an outlet at one end:
| Max effective roof area | Gutter size | Outlet size | Flow |
|---|---|---|---|
| 6 m² | No gutter needed | – | – |
| 18 m² | 75 mm | 50 mm | 0.38 l/s |
| 37 m² | 100 mm | 63 mm | 0.78 l/s |
| 53 m² | 115 mm | 63 mm | 1.11 l/s |
| 65 m² | 125 mm | 75 mm | 1.37 l/s |
| 103 m² | 150 mm | 89 mm | 2.16 l/s |
Most UK homes use a 112 mm half-round plastic gutter with a 68 mm round downpipe, which is bigger than the 100 mm row. Makers such as FloPlast publish their own tested figures: their 112 mm half-round with an end outlet carries about 0.82 l/s laid level and 1.05 l/s with a fall, rising to 1.62 and 2.34 l/s with a running outlet in the middle of the run. Deep-flow and high-capacity profiles of 115 to 125 mm carry more, and 150 mm gutters are used on large and commercial roofs.
Rain falls at a slant in wind, so a steep roof catches more than its plan area suggests. Approved Document H allows for this with a pitch factor (Table 1), which is the same as the formula used by gutter makers:
Effective area = (W + H ÷ 2) × L
where W is the flat depth from gutter to ridge, H is the height from gutter to ridge and L is the length of the gutter. For a 35° roof 4 m deep, H is 2.8 m, so each metre of gutter drains (4 + 1.4) = 5.4 m² of effective roof.
| Roof | Effective area |
|---|---|
| Flat | Plan area × 1.0 |
| 30° pitch | Plan area × 1.29 |
| 45° pitch | Plan area × 1.50 |
| 60° pitch | Plan area × 1.87 |
| Over 70° or a wall | Elevation area × 0.5 |
The flow to carry is then the effective area × 0.0208 litres a second (75 mm an hour). A 43 m² roof slope gives about 0.9 l/s.
Each downpipe can only take the water a gutter can bring to it, so a long or steep roof needs more than one. Putting the outlet in the middle of a run instead of at the end roughly doubles the roof area one downpipe can serve, because water flows to it from both directions and each half of the gutter carries only half the water.
| Gutter run | Effective area | Downpipes needed |
|---|---|---|
| 6 m | 32 m² | 1 at the end |
| 8 m | 43 m² | 2 at the ends, or 1 in the middle |
| 12 m | 65 m² | 2 at the ends, or 1 in the middle |
| 16 m | 86 m² | 3, or 2 in the middle |
The figures follow the Approved Document H table, which is cautious for modern plastic gutters. Where the calculator says a 112 mm gutter needs more downpipes, you can either add them or switch to a deep-flow gutter of the size shown.
Plastic guttering is cheap: the calculator uses typical 2026 UK prices for FloPlast 112 mm half-round, including VAT.
| Part | Price |
|---|---|
| Gutter, 4 m | £9.85 |
| Fascia bracket | £0.80 to £1.00 |
| Union bracket | £2.90 |
| Stop end / stop end outlet | £2.70 / £4.90 |
| Running outlet | £4.00 |
| 90° angle | £4.40 |
| Downpipe, 2.5 m | £8.20 |
| Offset bend / clip / shoe | £3.80 / £1.00 / £4.00 |
The parts for a typical semi-detached house, front and back, come to around £150 to £200. Black, anthracite and cast-iron effect colours cost about the same; deep-flow systems cost roughly a third more, and aluminium or cast iron several times more. Fitting by a roofer, with scaffold, usually costs far more than the materials.
If the water goes to a soakaway, size it with our soakaway calculator.
Most houses use 112 mm half-round plastic guttering with 68 mm downpipes. A large or steep roof may need a deep-flow 115 to 125 mm gutter, or more downpipes. The calculator checks this against Approved Document H.
No more than 1 m apart for plastic half-round gutters, 800 mm in areas with heavy snow, and within 150 mm of every joint, angle and outlet.
Enough that the roof area per downpipe stays within the gutter's capacity. For a standard 112 mm gutter that is roughly 40 m² of effective roof per end outlet, or about double with a running outlet in the middle.
About 1 in 350 to 1 in 600 towards the outlet: 3 to 5 mm for every 2 m. A gutter can be fitted level, but it then carries less water.
Use (W + H ÷ 2) × L, where W is the flat depth from gutter to ridge, H the height from gutter to ridge and L the gutter length.
About £9.85 for a 4 m length of 112 mm half-round plastic gutter in 2026, including VAT.
Like-for-like replacement of guttering does not normally need approval. New roofs and extensions must have gutters and downpipes that meet Approved Document H.
Rainwater should go to a surface water drain, a soakaway or a water butt. Do not connect it to a foul drain unless it is a combined system and you have permission.
Gutter sizes and pitch factors from Approved Document H, Section H3, Tables 1 and 2 (75 mm/h design rainfall). FloPlast flow capacities and bracket spacing from FloPlast product data. Prices from UK merchant listings for FloPlast 112 mm half-round, 2026, including VAT. Reviewed November 2026.