What Wind Speed Damages a Roof?

Why there is no magic number

Wind does not push a roof evenly. It creates pressure on the side facing the wind and suction on the leeward side, the roof edges and the ridge. The suction lifts tiles and can strip a covering from the top down. How well a roof copes depends on the fixings, the condition of the timber, the pitch, the shape of the building and its exposure. A tight modern roof in a sheltered street may be untroubled by a gale. An old roof on an exposed hill can lose tiles in lower winds. For the same reason, one house in a street may have damage while its neighbours are fine.

Mean speed and gusts

Weather forecasts often give a mean wind speed and a gust speed. The gust is the figure that does the damage, because roofs are loaded by sudden peaks rather than the average. A forecast of mean winds in the 30s with gusts well above that is a warning. The Met Office issues yellow, amber and red warnings for wind, and an amber or red warning is a signal to check loose items and expect damage. The Beaufort scale describes gale force as 39 to 46 miles per hour, severe gale as 47 to 54, and storm as 55 to 63. Damage to buildings tends to appear in these bands, particularly to weaker roofs.

Where damage starts

Wind pressure is highest at the edges of a roof, so these parts are fixed more heavily in good practice and fail first when they are not.

• The ridge. Ridge tiles bedded in mortar crack as the roof flexes and lift in suction. Current standards call for mechanical fixing rather than relying on mortar alone.
• The verge. The edge of a gable roof gets strong uplift. Mortar verges can crumble and let the edge tiles go.
• The eaves. The bottom course of tiles, and the gutter, face strong wind and are easily lifted.
• Corners and hips. Wind spills round corners and creates local suction.
• Chimneys, dormers and flat roofs. Weak flashings lift, and loose membrane edges flap and tear.

Things that make a roof weaker

Several faults make wind damage more likely. Rusted nails and missing tile clips let tiles lift. Perished battens do not hold fixings. Old mortar at the ridge and verge dries out and cracks. Thin or missing felt allows wind-driven rain in once a tile moves. Moss and age make tiles brittle. And earlier patch repairs that used sealant or mortar in place of nails often fail first. A roofer can spot most of these from a close inspection. Our guide to what to do if tiles blow off in a storm covers the immediate response.

Exposure and the shape of the building

Where a house stands changes its wind load. Open ground, a hilltop, a gap between buildings and the end of a terrace see higher local speeds than a sheltered garden. Tall buildings create turbulence around them. Large overhangs at the eaves and open porches catch the wind from underneath. A hipped roof, which slopes on all four sides, tends to cope better than a gable with exposed verges. Many parts of north Hampshire and Berkshire are open farmland, so the houses on the edge of town often feel the full force of a south-westerly.

Reducing the risk

A check of the roof before winter pays for itself. Have loose, cracked or slipped tiles replaced, ridge and verge fixings inspected, and gutters cleared. A roof with sound fixings, clips on the exposed edges and a mechanical ridge system copes better. Trim overhanging branches so they cannot strike the roof. Secure garden furniture, trampolines and sheds, which can become missiles. Our page on storm damage to roofs lists what to do first when something goes wrong.

Leaks that appear in wind

Strong wind can also drive rain into roofs that are fine in calm weather. If you find a leak that only comes on in a gale, our guide to why a roof leaks only in heavy wind covers the likely causes.

What we do

We inspect the roof at height, photograph the fixings, ridge, verges and tiles, and show you what we find. If a few repairs will make the roof sound, we say so. If the roof has reached the end of its useful life and the next gale will take more, we explain that honestly and set out the options with a price in writing. The repairs we carry out are listed under emergency and general roof repairs.