Interactive lesson

Roofs, step by step

Interactive lesson: shape and pitch to suit the covering, snow and wind, rafters and holding-down, battens, metal sheet and tiles, gutters and safe work on the roof.

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The whole lesson as text

1. Shape and pitch

You don't choose the roof pitch by eye but by the covering that goes on it.

Shape to suit the building — A mono-pitch roof is the simplest, for a lean-to or woodshed against a wall. A gable roof has a ridge and a loft, for an animal house, store or hen house.
Metal sheet takes a low pitch — Profiled sheet goes from 5–10°, in one length from ridge to eaves. Corrugated sheet needs 10–15°.
Tiles and shingles steeper — Clay interlocking tiles roughly from 22–30°, timber shingles from 30–35°. The steeper the shingles, the longer they last.
A steep roof in snow — Where a lot of snow falls, go steeper than the minimum. Snow slides off a steep roof by itself and doesn't sit on the frame.

Why: Below the minimum pitch, wind drives water back under the laps and snow lies longer. In snowy regions go steeper, because snow slides off a steep roof by itself.

Always check the minimum pitch in the manufacturer's instructions for that sheet or tile.
Quick calculation: height difference = roof width × 0.18 for 10°, × 0.36 for 20°, × 0.58 for 30°.

2. Snow, wind and weight

A roof carries its own weight, snow and wind.

Wet snow is heavy — 30 cm of wet snow weighs 100–120 kg per m² (20–25 psf). On a 20 m² roof that is 2–2.4 t.
Snow where you live — Build the roof for the snow where you live, not for an average winter. Ask an engineer, building control or the building department for the value for your site and altitude.
Metal sheet or tiles — Sheet with battens weighs 5–10 kg per m², tiles 45–60 kg. An old frame built for sheet isn't for tiles without checking.
Wind lifts the roof — Wind acts hardest at corners, edges and overhangs. On exposed sites (coasts, hills, open plains, hurricane regions) fix the covering closer and hold the roof down harder.

Why: Snow is the most common reason homestead roofs fail, especially wet snow after rain or snow drifted by wind onto one side. Wind lifts a light metal sheet roof like an aircraft wing.

A span over 4–5 m, tiles or stone on a new frame, heavy snow areas and exposed windy sites need calculations from a structural engineer.
The same goes for any building where people or large livestock stay.