Setting Out Roof Battens: Gauge and Headlap

The first line is set by the gutter, not by the roof

The eaves batten is the datum for every mark above it, and where it goes is decided by where the tail of the first course has to finish. That tail has to discharge into the gutter: far enough over the front bead that a gust cannot carry the run-off past it, not so far across that the water lands on the far side of the gutter or the tail sits down inside it collecting leaves. There is no published dimension for that relationship — BS 5534 requires the covering to discharge into the gutter and leaves the setting to the eaves detail in the tile maker's fixing instructions, which is where you should take it from when the maker draws one. The common workshop setting is the tail landing roughly a third of the way across the gutter, and that is a rule of thumb rather than a specification.

Work the projection on the flat before you work anything on the slope. How far the covering stands past the face of the fascia is a plan dimension, because everything the projection is for lives on the flat: where the gutter hangs, how much of the wall head the eaves shelters, how close the roof comes to a boundary. Only once that number is agreed does it become a cutting length, and the cutting length is bigger, because a rafter tail runs down the slope rather than horizontally. Three hundred millimetres of projection past the wall face is more than three hundred millimetres of timber, and the steeper the roof the wider the gap between the two figures. Cutting tails to the plan dimension is a mistake that shows up as an eaves that finishes short of the gutter with the fascia already fixed.

Then the tilt. The tail of the first course has to be lifted or it stands proud on its own nose and the eaves course rocks instead of bedding — a tilting fillet on the fascia does it, or a thicker or doubled eaves batten. That lift is a real thickness and it moves the batten's effective line, so it belongs in the set-out rather than in the fixing. Slate and plain tile put a second constraint on the same batten: the under-eaves course is a short piece of gauge-plus-lap length, so the eaves batten has to land where a short piece of an obtainable length can reach it. That piece is either a product you order or full ones cut down at full price, and deciding which is a Tuesday decision rather than a Friday one.

If the gutter is staying, measure what it gives you rather than what it ought to give you: fascias bow, gutters get re-hung at a fall somebody guessed at, and a bead sitting fifteen millimetres high moves the whole acceptable band for the tail. If it is coming off, then the eaves set-out and the gutter set-out are one job — capacity and layout under BS EN 12056-3, position off the eaves detail, both settled before the first batten rather than after.

1. Agree the projection past the fascia face as a plan dimension, with the gutter position and the boundary in front of you.
2. Convert that plan figure into an along-slope length before any tail is cut or lengthened.
3. Set the tilting fillet or the thickened eaves batten, and treat its lift as part of the datum rather than as a fixing detail.
4. Position the eaves batten so an under-eaves piece of gauge-plus-lap length can reach it without being invented.
5. Lay a straightedge from the batten line down the tail and check where the water actually leaves, against the gutter as it is hung today.
6. Repeat the whole check at the far gable, because one bowed fascia will give you two different answers.

Dividing what is left into equal steps

With the eaves batten fixed and the top batten line known, the distance to divide is finally a real measured number. It is not the rafter length and it is not eave to ridge on the drawing. It is eaves batten to top batten, taken along the slope: call it the field run, because it is the part of the roof the field courses have to cover. Divide it by the maximum gauge the lap requirement left you and you will get something like 17.6 courses. Round up to 18. Divide the field run by 18 and that is the gauge you will nail. It is smaller than the maximum, which means the lap is larger than the minimum, and that is the only direction it is ever safe to be wrong in.

Rounding the other way is how roofs get quietly lost. Seventeen courses instead of eighteen saves a row of battens, a course of slate and half an hour, and opens the gauge past the maximum by a few millimetres. Nobody can see it, nothing leaks for a decade, and then a night of driven rain finds every head joint at once — and the callback is a strip, because the fault is not in one place. The temptation is worst on a long slope, where one course in twenty looks like rounding rather than a decision.

The remainder has three honest homes and one dishonest one. Spread it equally across every course by tightening the gauge fractionally — the default, and the one that reads best from the ground, because nothing about the roof looks different. Take it at the eaves, by moving the tail projection a few millimetres inside the band the gutter allows. Or take it at the top, letting the ridge covering sit over a slightly deeper lap on the last course. What you may not do is absorb it in one course somewhere in the middle of the slope. A single course laid at a different margin is a line straight across the building that low winter sun picks out from the pavement for the rest of the covering's life, and it is the fault every roofer can name on somebody else's job.

Do all of this per plane. Two sides of a hip rarely measure the same. A dormer cheek is its own slope with its own short run, its own rounding and often its own answer. A rear addition re-plated at one end will hand you two different eave-to-ridge lengths across what looked like one rectangle from the scaffold. Each plane gets its own division and its own gauge, and where two planes meet at a hip you then decide on purpose which one's course lines carry across the intersection — because the eye follows those lines over the hip, and a mismatch there is the first thing anyone notices about a roof they were not looking at.

1. Measure the field run — eaves batten to top batten line, along the slope — at both gables and once in the middle.
2. Take the maximum gauge from the lap requirement for this pitch and this exposure, using the covering's measured length rather than its catalogue name.
3. Divide the run by that maximum, and round the course count up. Never down.
4. Divide the field run by the rounded count to get the gauge you will nail, then check backwards that the lap it produces is at or above the minimum.
5. Decide where the remainder goes before you mark anything: spread across all courses, or taken at one end. Not in the middle.
6. Mark both gables from the eaves datum with a tape, then chalk between the marks — never step one batten position off the last one fixed.

The battens are the permanent record of a decision made with a tape

Row count comes out one higher than the number of gauge spacings, because a row is needed at both ends of the run. That much is arithmetic and it is what the calculator below does. What the arithmetic does not know is that neither end row is an ordinary batten. The eaves row is thicker or doubled to provide the tilt, and the top row sits wherever the ridge covering needs it, which is a dimension taken off the ridge system rather than off any multiple of the gauge. Both are positioned by hand rather than by the gauge, which means the count is right about how many rows and wrong about what two of them are: the eaves row is a second batten's worth of timber or a deeper section, and the gap between the top row and the last field batten is a leftover rather than a gauge. The order has to be priced that way.

Battens are a graded structural product and not an offcut. Section against rafter spacing, strength grade, moisture content, and a marking on the timber that proves all three — BS 5534 carries the requirement in UK practice, and the fixing instructions read with the adopted code do the equivalent elsewhere. A batten too shallow for the span between rafters deflects under a roofer's weight long before it meets a storm, and the gauge sags with it permanently. Joints matter as much as section: a joint lands over a rafter, and the run is cut so joints do not stack up through consecutive courses, because a column of joints on one rafter is a hinge running straight up the slope.

Counter-battens change two numbers rather than one. Everyone remembers the first — they hold the covering clear of the underlay so a drape can drain and a horizontal batten never dams water on the membrane. Fewer remember the second, which is that they add their own depth to every fastener on the roof. A slate nail has to pass slate, batten and counter-batten and still take a proper bite of the rafter beyond, so a nail length inherited from a job with no counter-battens is short here — and short nails at the eaves are the first thing a gale finds.

Fix from the marks, not off the last batten fixed. Stepping each row off its neighbour carries that neighbour's error forward and adds a fresh one, and a consistent millimetre over eighteen courses is a course line sitting eighteen millimetres off at the ridge — enough to leave a sliver of tile under the ridge covering that nothing in the range will hide. Marks transferred from a tape run off the eaves datum keep every error independent, small, and correctable while the chalk is still wet.

Single-lap coverings halve nothing, and the settled gauge changes the order

Everything above assumes the covering doubles. Interlocking profiles and barrel tile do not. They lap the course below once and lock at the side instead of stacking a third thickness across the head, so the gauge is simply the tile's length minus the head lap the maker publishes for that profile at that pitch. No division by two. The two arithmetics use the same three words and produce answers a factor of two apart, which is why they should never be worked on the same sheet of paper on a site running both — a crew that carries the double-lap sum onto a single-lap roof lays it at roughly twice the intended exposure, and it looks generous on the first course.

The order is where the difference bites. A double-lap covering is counted per square metre out of its own geometry: one divided by the cover width times the gauge. A 265 mm plain tile at a 65 mm lap gives a 100 mm gauge, and at a 165 mm cover width that lands near sixty tiles to the square metre — the number every merchant will quote you, familiar only because those three inputs are the common case and not because it is a constant. A single-lap tile, by contrast, is usually bought against area at a published coverage, and that published coverage is quoted at the manufacturer's own gauge. Close your gauge tighter than that to land whole courses at the ridge, which is exactly what the division tells you to do, and every tile now covers slightly less roof than the brochure assumed. The count goes up. Recompute it from the gauge you settled on, using that gauge times the tile's cover width as the coverage figure, rather than from the catalogue number.

Barrel systems add their own wrinkles to the same principle. Some barrel fixing specifications set the battens out to a floating gauge with the tile hung on a nib or wired back rather than nailed through, which changes what a batten position even means and has to be read off the specification rather than assumed from a plain tile habit. The breakage allowance sits on top of everything and is not a constant either: it follows how cut-up the roof is, because clay will not cut on a curve and a hip takes a diagonal through a barrel that usually yields one usable piece.

Whatever family the covering belongs to, verify the size on the pallet standing in front of you rather than the one in your memory. Clay tile dimensions and their permitted tolerances are covered by BS EN 1304 and concrete tile by BS EN 490, and the tolerances those standards allow are real millimetres on a real tile. Pull ten off the stack, measure the hanging length on each, and set the roof out on the smallest you find — a gauge built on the average of the delivery is wrong on half of it, and it is wrong in the direction that loses lap.