The short answer
Roof tile gauge is the distance from one batten to the next, measured up the slope. It decides how much of each tile is left on show and how far each tile overlaps the ones below, which is the headlap. The right figure comes from the tile maker's data for your tile and your pitch. Set it wrongly and rain can be driven under the overlap.
Gauge is a small measurement with a big effect. It is fixed when the battens go on, long before the first tile is laid.
Gauge, battens and what shows on the slope
Tiles hang from battens, narrow timber laths fixed across the rafters, with a membrane called the underlay laid beneath them. The gauge is the spacing between those laths, taken from the top edge of one lath to the top edge of the next. Every course of tiles hangs from a batten, so the gauge fixes how much of a tile is left exposed to the weather.
What does the gauge actually control?
Two things at once. One is the visible face of each tile, which gives the roof its pattern and rhythm. The other is the overlap each tile has over the course below, which you cannot see. A wider gauge shows more tile and hides less, so the overlap shrinks. A tighter gauge does the opposite. The roofer works inside a window, and the tile maker publishes it.
Gauge is marked out working down from the highest course, so the rows finish evenly at the eaves and meet the ridge cleanly. The ridge is the crest where the two slopes join. Getting this right on the day saves a fudged course at the end, and it keeps the lines of the tiles straight when seen from the road.
Headlap and why it keeps water out
Headlap is the amount a tile overlaps the tile two courses beneath it. It is the lap that stops rain, pushed up the slope by wind, from reaching the batten and the underlay.
How does pitch change the headlap needed?
The shallower the slope, the more overlap a tile needs, because water runs off more slowly and wind finds an easier path under the edge. A steep roof sheds fast. That is why the maker's data ties each gauge range to a pitch. Marley's published minimums show how much tile type matters: its Double Roman is 22.5 degrees (smooth), its plain concrete tile is 35 degrees, and its Mendip is 15 degrees (smooth). For natural slate, Cupa quotes BS 5534 as indicating a minimum roof pitch of 20 degrees.
Gauge and headlap are two ends of one measurement. The maker's own table is the source to trust, because the sum depends on how the particular tile is designed to lap. Once you know the length of the tile and the lap it needs, the batten spacing follows from the maker's rules. If the maker's data and the pitch disagree with what is on the roof, the figures win, not the habit of the person laying it.
Where the right figure comes from
No single gauge fits every roof. Each tile has its own length, pitch range and lap, and the manufacturer sets them out on a data sheet. The code of practice for laying tiles, BS 5534, sets the wider method, though Marley points out that installing to BS 5534 is not required by law.
Where can I find the figure for my own tiles?
On the product data sheet, or by measuring an existing roof that has stayed dry for years. A roofer measures the tile length, checks the pitch with a level, then reads the spacing from the maker's table. If the tile is old, discontinued or hand-made, the spacing already on the roof is worth recording before anything is stripped.
Our answer on matching old roof tiles covers choosing a similar profile. On a re-roof the battens are renewed and set to the gauge for the covering that goes back on, which can differ from the old one if the tile has changed. Measuring first avoids a roof that looks right from the road and leaks in the first gale.
Why the wrong gauge leaks
A gauge that is too wide leaves too little headlap. In ordinary rain nothing shows. In a storm with driving rain, water gets past the shortened overlap, wets the underlay and finds a nail hole or a join. The ceiling stains weeks later, far from the true cause.
What happens when the batten spacing is off?
If the battens are too close, the courses stack up and tiles may not sit flat, and the roof can end short of the ridge, forcing a fudge at the top. If they are uneven, tiles rock and crack. In both cases a patch is a poor remedy, because the fault runs across the whole slope.
Our guide on whether roof battens need replacing shows the signs, and why roof tiles slip covers a related cause. The pitched roofing page sets out the rest. We check the gauge at the survey, and the written total follows what we find. If the spacing already on your roof is wrong for the tile, you see that in the pictures before anything is agreed, along with what it would take to put it right.
More on gauge and headlap
Can one gauge be used for any tile?
No. Each tile has its own length and pitch range, so its maker publishes its own batten spacing. Borrowing the spacing from a different tile can leave the headlap short. A roofer reads the data sheet for the tile being laid and measures the pitch before marking out the battens.
Is gauge the same as the visible length of a tile?
They are closely related. Gauge is the spacing between battens, and on a laid roof that is also the exposed length of each tile. The difference between that and the full length of the tile is the lap. If you can read the tile length, you can work out the lap from the gauge.
The full picture: Pitched roofing in Taunton ยท More short answers