Fastening Lap Siding So It Still Moves

The split started at a nail and ran to the butt

Strip a failed lap elevation and the evidence sits in the same place every time. The boards are sound, the coating is tired but continuous, nothing has rotted and nothing struck the wall. What you find is a hairline on most boards that begins at a fastener head and travels with the grain to the nearest free edge, usually downward to the butt. On a south or west wall it will be on nearly every course; on the sheltered elevation of the same building, on almost none. That distribution is the diagnosis. A board that changed size against a fastener which would not let it move relieved the stress the only way it could.

Which makes a fastener schedule for lap cladding a movement document before it is a strength document. Holding power is the straightforward half of the problem and the half most crews solve without thinking about it — a ring-shank nail of the right length into a stud will hold far more than the wind will ever ask of one board. The difficult half is deciding, for every board on the building, which single point is not allowed to move and how much clearance the rest of the board gets. Add one well-intentioned extra fastener and you have not made the wall stronger; you have converted a hanging board into a restrained one and moved the failure from a joint you would never notice into a split you cannot repair.

Everything below is organised around that one decision and its consequences: where the pin goes, how many pins the elevation actually needs, how far each has to reach into framing to be worth counting, and which cladding materials change the answer because they change size for an entirely different reason.

One pin per board, and everything after it is clearance

A lapped board is not a panel bolted to a frame. It is a plank suspended from a single line of points, resting on the top edge of the course beneath it, and the contact between the two is a sliding joint that has to stay sliding. Put the pin line high on the board — up near the top edge, where the course above will hide it — and the board hangs. It can grow downward, shrink upward, and cup very slightly without any of that being resisted, because the only thing holding it is a row of small pins along one horizontal line and there is nothing above them to push against.

Move that line down toward the butt and the geometry inverts. Now the widest-moving part of the board is the part that is pinned, and the free edge is the thin top edge tucked under the next course where it can do nothing useful with its freedom. The board still moves the same amount; it just does it against a restraint. Add a second line of fasteners at a different height on the same board and there is no ambiguity left at all — the timber between the two lines is a member in tension every dry spell and in compression every wet one, and the weaker of the two fixings eventually tears out a strip of wood on its way to relieving the load.

The rule that comes out of all this is short and it survives translation across materials: one fastening point per board per framing line, positioned where the manufacturer says, and never a fastener that passes through two courses at once. That last clause is the one that gets broken quietly. A nail driven at the butt of a course, into the top edge of the course beneath, stitches the two boards into one member twice as tall as either — and because the head is hidden under the lap, nobody sees it until a board splits at a fastener nobody remembers driving. Where an evaluation report calls for face nailing, and plenty do at higher design pressures or on wider boards, it is still a single line per stud, positioned to clear the plank below unless the report explicitly says otherwise.

What is actually moving, and what is driving it

The five common lap claddings do not share a mechanism. Two of them respond to moisture, two to temperature, and one to both in a way that is small enough to ignore right up until it is restrained. Confusing the mechanisms is how a crew ends up applying a habit learned on cedar to a vinyl wall, or a fibre cement fastening pattern to engineered wood, and both of those substitutions produce a failure that looks like poor workmanship rather than a category error.

The practical consequence is that the schedule's clearances belong to the material and not to the trade. A gap at a butt joint that is correct for one product is a visible defect on another and a split waiting to happen on a third. Everything in the table below is the mechanism and the direction of the movement, not a figure to build from; the figure comes from the document that governs the product, which for proprietary claddings means the installation instructions and the evaluation report and nothing else.

Where the schedule tightens, and where it is allowed to relax

Wind pressure on cladding is not uniform across an elevation, and the fastener schedule is where that non-uniformity gets paid for. ASCE/SEI 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, treats siding as a component and cladding element and divides a wall into an interior zone and a narrower strip along each corner where the design pressure is higher, with the width of that strip derived from the building's least horizontal dimension and its mean roof height rather than being a fixed distance. AS/NZS 1170.2, Structural design actions — Wind actions, reaches the same place by its own route for Australian and New Zealand work. The product's evaluation report then converts a design pressure into a fastening pattern, and it is that pairing — pressure from the wind standard, pattern from the report — that decides whether an elevation is blind nailed, face nailed, or both.

For ordering purposes a tightened corner zone can be averaged over the elevation as a few extra fixings per course, which is close enough to buy against and nowhere near good enough to build to. The zone has a real width and a real boundary, and a crew working from an averaged figure will spread the extras evenly across the wall, leaving the corner exactly as it was. Mark the zone on the elevation drawing, price the average, and set out the actual pattern on the wall.

1. Establish the design wind pressure for this building's components and cladding from the governing wind standard, not from what the last house on the street used.
2. Read the fastening pattern that the product's evaluation report pairs with that pressure — blind, face, or both, and at what framing spacing.
3. Mark the corner strip on each elevation drawing at its real width, with the tighter spacing written inside it.
4. Convert the tightening into an average per-course figure for the order, and keep the drawing as the thing the crew works from.
5. Check that the framing behind the tightened zone can take the extra points without splitting a stud edge, particularly where two elevations meet at a single corner post.

Ends, joints and the ways a correct schedule still splits a board

The last fastener at each end of a board is the one most likely to split it, and for reasons that have nothing to do with movement. It is close to a cut end, which is where checks and shakes live; it is often near a knot the setter-out did not see; and it is being driven into the shortest possible length of fibre. BS EN 1995-1-1 sets minimum end and edge distances for nails and gives the conditions under which pre-drilling is required, characteristic density being the main trigger — which is why the dense hardwoods get drilled as a matter of routine and construction softwood usually does not. Pre-drill the end fixings regardless of what the species table says; it costs seconds and it is the fastener with the least margin on the whole board.

Butt joints are the other repeat offender, and each material handles them differently enough that carrying a habit across is dangerous. Wood and engineered wood lap wants its butts landed on framing so both ends are fixed, staggered course to course so no vertical line of joints develops, and gapped where the manufacturer publishes a gap. Fibre cement wants a joint flashing behind the butt so that the joint itself is not relied on, and it wants the fasteners kept back from the end by whatever the instructions state. What none of them wants is a butt joint fixed by driving one nail through both boards into the same stud, which is a splice made out of two end-grain-adjacent fixings and fails at both.

Then there is the gun. A pneumatic nailer set for framing will over-drive siding all day: on wood it dimples the face and breaks the coating film at the exact point where water will sit, and on fibre cement it crushes the board around the head so the fastener is bearing on a crater rather than on sound material — which is why product instructions specify a flush head, neither proud nor countersunk, and why the first thing to do on a new elevation is set the depth on an offcut. Countersunk heads on fibre cement are one of the most common findings on a warranty inspection, and they are also one of the few that can be fixed on the day by turning the regulator down.

1. Set the nailer depth on a piece of the actual board before the first course, and reset it whenever the compressor, the hose length or the temperature changes.
2. Pre-drill the end fixing on every board, and every fixing in a dense species or near a visible check.
3. Land butt joints on framing for wood and engineered wood; flash them behind for fibre cement; stagger them either way.
4. Walk the first three courses and confirm no fastener has passed through two boards at once, before the fourth course hides them.
5. Keep one offcut with a correctly driven head on it as the reference the crew can compare against for the rest of the elevation.

What to write on the elevation before the scaffold moves

Four things belong on the drawing rather than in somebody's memory: the pattern the report called for, the fastener specification including its material and shank type, the penetration into framing the stack-up produced, and the corner zone width with its tighter spacing. All four are decisions taken once, at the start, on evidence that is available at the start — and all four are impossible to verify afterwards, because the whole schedule is buried under the course above it before the day is out.

The reason to bother is the next trade on this wall, which will be a re-clad or a repair in a decade or two, by someone who will read the fastener pattern off the failures rather than off a document. A wall whose boards are pinned once, deep enough, and clear of each other is one that comes off in pieces and goes back on. A wall that was stitched together with good intentions comes off in splinters, and the sheathing behind it comes off with it.