Setting Out a Slate or Cedar Roof

The lap table is indexed to pitch, so pitch comes first

Every headlap requirement you will meet is tabulated against slope, and it moves in the same direction in all of them: the shallower the roof, the more lap it demands. Water crosses a head joint slowly on a low pitch and wind pushes it further back up the slate, so the overlap has to grow to stay ahead of it. The slate provisions in Chapter 9 of the International Residential Code set a minimum headlap against roof slope, and BS 5534 arrives at the same place through pitch, slate size and site exposure. Neither of them lets you carry a lap across from the last job unless the last job happened to share the pitch.

So the pitch has to be a measured number rather than an impression, and a stripped roof is the easiest place there is to measure it. A long level and a tape across the rafter backs, or a digital pitch gauge laid on the timber, taken at three points along the run — old buildings sag in the middle and spread at the plates, and a roof can genuinely read two degrees different at each gable. Whatever you find, convert it once. The drawing will give you degrees, the lap table wants rise in twelve, the underlay literature wants a ratio, and every conversion you do later in the day is another chance to drop a digit onto a roof you cannot easily take apart again.

Closing the slope into whole courses

The slope length will not divide evenly by the exposure. It never does, and the fix is always in the same direction: divide, round the course count up, then recalculate the gauge from that whole number and let it come out slightly tighter than the maximum. Tightening the gauge increases the headlap, which is the safe way to be wrong. Opening it out to avoid an extra course reduces the lap below what the pitch was assessed for, and that is a decision no one will find until the ceiling stains.

Do it per plane, not per building. Two sides of a hip rarely measure the same, a dormer cheek is its own slope entirely, and a roof that has been re-plated at one end will give you two different eave-to-ridge lengths on what looks like one rectangle. Each plane gets its own division and its own gauge, and where two planes meet at a hip you decide deliberately which one's course lines you are matching, because the lines will run across the hip and the eye follows them from the pavement.

The two end courses are not field courses and should be taken out of the sum before it starts. At the eave a double course is laid, the under-eave slate cut so its length equals the gauge plus the lap, which puts its head under the same line as the first full course and keeps the doubling at the most exposed edge of the roof. At the ridge the top course is cut short so its head lands where the ridge covering will sit over it. Neither of those is a full slate, and a course count that assumes they are will leave you a stunted margin under the ridge — 30 mm of slate showing, in a straight line, across the whole building.

1. Measure eave batten line to ridge along the slope on each plane separately, and record the planes that disagree.
2. Take out the eave allowance and the top course, so the number you divide is the run the field courses actually have to cover.
3. Divide by the maximum exposure the lap table permits at this pitch, then round the course count up to a whole number.
4. Recalculate the gauge from the rounded count and check the resulting headlap is at or above the minimum, never below it.
5. Mark the closed gauge on a rod, not on a scrap of paper — the rod is what goes on the roof and the paper is what blows off it.
6. Dry-lay the top three courses against the ridge before any batten above mid-slope is fixed, and confirm the last margin is one you would sign off from the ground.

Cedar divides by three, and the bundle is rated at someone else's exposure

A cedar shingle roof does the same job with a different rule of thumb, and the number that goes on the order is a bundle count rather than a piece count. Bundles are banded to a set course width, so a bundle covers a fixed length of course multiplied by whatever exposure you happen to lay it at. Lay tighter than the rating and each bundle covers less roof; lay looser and it covers more. That is a straight linear relationship and it is the whole reason a cedar order cannot be placed until the exposure is settled.

The rating printed on the band is the part people skip. Coverage is quoted at a reference exposure — commonly 5 in (127 mm) for 16 in (406 mm) shingles, which is where the familiar four-bundles-to-the-square figure comes from — and it changes with shingle length and with grade. Read the band on the product you are actually buying rather than the one you bought last year, because that reference figure is the denominator in every bundle sum you are about to do. Grade matters alongside it: the Cedar Shake & Shingle Bureau's No. 1 Blue Label, No. 2 Red Label and No. 3 Black Label are different products with different permitted exposures and different intended uses, and a roof specified in one and delivered in another is not the roof that was priced.

Shingles and shakes are not interchangeable in this arithmetic and should not share a takeoff. A shingle is sawn on both faces, thin, and laid at three-ply coverage. A shake is split, thicker, laid at a bigger exposure with only two thicknesses over a given point, and it relies on an interlayment felt woven between the courses to make up the difference. Anything derived from a triple-lap assumption is wrong for shakes, and the interlayment is a material line on the order that a shingle job simply does not have.

Two more things belong on the same page as the bundle count. Cedar wants to dry from both faces, which is why it is laid over spaced sheathing or battens rather than solid deck in most climates, and that decision changes the batten schedule you are about to set out as well as the fastener length. And the joints between shingles in one course must be offset from the joints in the courses below by the amount the CSSB manual states, with no joint aligning through consecutive courses — a rule that consumes shingles at the rakes and hips and which the bundle sum does not know about. Add the cutting allowance yourself, sized to how cut-up the roof is.

Transferring the gauge with a rod

A slater's gauge rod is a straight batten as long as the slope, marked once at the bench with every course position on it, top course and eave included. It goes up the roof, gets laid on the rafters, and each mark is transferred straight across. That is the entire method, and its virtue is that all the error lives in one act of marking done at waist height on a flat surface, where it can be checked, rather than in twenty repeated measurements taken on a slope with a tape hooked over a rafter. Make two rods on a big roof and check them against each other before either one is used.

The batten rows themselves count one higher than the number of gauge spacings, because a row is needed at both ends of the run: at the eave and at the top. The eave row is usually not a standard batten at all — it is thicker, or doubled, to tilt the first course up so the double eave course beds down onto the one beneath instead of standing proud on its own tail. The top row sits wherever the shortened ridge course needs it, which is a dimension taken from the ridge covering rather than from the gauge. Both of those are set out by hand and neither lands on a gauge multiple, so read the arithmetic below for what it is: the row total already carries one row at each end of the run and needs nothing added to it, but it says nothing about what section the eave row is or how far off the ridge the top row sits. Those two you still fix yourself.

Counter-battens change two numbers, not one. Everyone remembers that they hold the covering clear of the underlay; fewer remember that they add their own depth to every fastener on the roof. A slate nail has to pass through the slate, through the batten, through the counter-batten where there is one, and still take a proper bite into the rafter — the traditional sizing is twice the slate thickness plus the batten depth plus the penetration required, and the counter-batten quietly makes that a longer nail than the last job used. Batten section and grade themselves are a specification: BS 5534 sets them against rafter spacing in UK practice, the code in force and the covering manufacturer's literature do the same elsewhere, and a batten too shallow for the span between rafters will deflect under a slater's weight before it ever meets a storm.