The 20 degree figure and its conditions
Cupa, a slate maker, says in its guidance on headlap and fixing that BS 5534 points to 20 degrees as the floor for natural slate. The figure comes with stated conditions: moderate exposure, a 500 mm slate and a 115 mm lap. Change any of those and the answer is no longer assured.
A slope sheds water because the slates overlap and gravity does the rest. The shallower the pitch, the more slowly water runs off, and the easier it is for wind-driven rain to be pushed up under the lap. That is why a minimum exists.
Pitch means the angle between the slope and the horizontal, measured along the rafter line by a roofer and not estimated by eye. The standard behind the figure is BS 5534:2014+A2:2018, which covers slating and tiling. It is guidance for good work, not a statute, but roofers and manufacturers use it as the reference, and it covers re-roofing and repairs as well as new roofs.
Treat 20 degrees as the start of a calculation, not the end of one.
How exposure and lap move the figure
Exposure is how hard the weather hits a roof. Cupa grades buildings on slopes or hills, in coastal areas, and tall buildings as more exposed. A house on open ground near the sea faces more driven rain than one in a sheltered street, so the minimum pitch for the same slate can be higher.
Lap is the overlap between courses. A greater headlap gives more cover and lets a slate work at a lower slope, while a shorter lap needs more pitch. Slate length matters too, because a longer slate can be laid with more lap for the same exposed length.
| Factor | Effect |
|---|---|
| Exposure | Higher exposure raises the figure |
| Headlap | More lap allows a lower pitch |
| Slate length | A longer slate gives more cover |
These are directions of travel, not numbers. The calculation for a real roof uses the supplier's data and the site's exposure, and fixings and laps are worked out for that exposure under BS 5534. The page on slate headlap goes into the lap itself.
Hook fixings and pitches below 25 degrees
Cupa also limits fixing: it does not want hook fixings used on any pitch below 25 degrees. A hook grips the tail of the slate instead of relying on nails alone, and it is a well-known repair for slates whose nails have failed.
That has a practical effect on a roof between 20 and 25 degrees. Slates there can be laid correctly with nails, but a hook repair is not the right method, and a roofer should say so. Using hooks on a slope that shallow puts a fixing where the maker says not to.
It also bears on the choice between tingles and hooks when a slate slips on a low slope. The two are set side by side in the tingles vs hooks guide.
Below 20 degrees, natural slate is not the answer on the figure Cupa quotes, unless the maker's data sheet allows it for that slate, lap and exposure. Parts of a house often sit lower than the main slope, such as a dormer cheek, a porch or a bay roof, and the question is worth asking for each one. The point is not to rule things out but to make sure the right method is used.
Checking your own roof
You do not need to guess your pitch. A roofer measures it, and you can get a rough idea yourself. These steps help:
- Look at the roof from the end of the house. Is it steep, like a Victorian gable, or shallow?
- Check any records, such as drawings or a survey, which often give the pitch in degrees.
- Ask your roofer to measure each slope separately, because the bay, the porch or the extension may differ from the main roof.
- Ask which slate and which lap are proposed, and have them written down.
If the pitch is below the minimum for slate, there are routes. A different covering with a lower minimum may suit, and the sister page on minimum pitch for roof tiles gives published figures for several tiles. A flat-roof membrane or lead may suit a very low slope.
Do not rely on a rule of thumb from a neighbour's roof, and read the data sheet of the exact slate proposed. The slate roofing page covers the work, and a survey confirms the pitch.
