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Basics
Roof area and gutters: measuring and sizing
You can work out the area of a roof and the gutters it needs without climbing onto it. This guide explains roof pitch, slope area, how many tiles or shingles to order and how to plan gutters and downspouts.
1. Measure the footprint
Measure the length and width of the building from the ground and add the eave overhang on each side. For a simple gable roof, this footprint is the horizontal area that the roof covers.
2. Roof pitch and slope area
Pitch is given as an angle in degrees, as a percentage (rise per 100 horizontal) or, in North America, as rise per 12 (for example 6/12 ≈ 26.6°). The sloped area is larger than the footprint:
Roof area = footprint ÷ cos(pitch)
For 30°, divide by 0.866, so a 100 m² footprint becomes about 115 m² of roof. The Roofing calculator does this and adds waste.
3. How much roofing to order
Clay or concrete tiles are counted per square meter, typically about 10–12 per m² for interlocking tiles; check the manufacturer’s figure. Asphalt shingles are sold by the square (100 ft²), usually three bundles per square. Add 10–15% for cutting at hips, valleys and edges, plus ridge pieces and underlayment.
4. Gutter size and fall
Gutter size depends on the roof area draining into it and local rainfall intensity. Typical house gutters are 100–125 mm (5–6 in) wide. Give gutters a slight fall towards the outlets, often around 2–5 mm per meter, and support them with brackets about every 60 cm (24 in), closer where snow loads are heavy.
5. Downspouts
As a rule of thumb, plan one downspout for about every 10 m (35–40 ft) of gutter, and at least one per separate run. Large or steep roofs and heavy-rain areas need more or larger downspouts. Lead the water at least 1–2 m away from the foundations, or into a drain or rain barrel. The Gutter calculator counts gutter length, downspouts and brackets.
6. Working at height
Most measurements can be taken from the ground or plans. Work on a roof needs proper fall protection; for steep or high roofs, hire a professional roofer.
Common mistakes
- Using the footprint instead of the sloped area.
- Mixing up pitch in degrees, percent and rise per 12.
- Too few downspouts for a long gutter run.
- Gutters installed level, so water stands in them.
- Discharging downspouts right next to the foundation.
Frequently asked questions
How do I calculate roof area from the ground?
Measure the footprint including overhangs and divide by the cosine of the pitch angle.
How many roof tiles per square meter?
Usually about 10–12 for interlocking tiles; small plain tiles need many more. Check the manufacturer’s data.
What does 6/12 pitch mean?
The roof rises 6 units for every 12 horizontal units, about 26.6°.
How many downspouts do I need?
Roughly one per 10 m (35–40 ft) of gutter, more for large roofs or heavy rain.
How much slope should a gutter have?
A small fall towards the outlet, typically a few millimeters per meter.