Guides · 14 September 2026 · 5 min
Laps, swages and seams: the allowances that make cladding fit
A pattern is only as good as what is added to it. The lap, where the holes go, the swage at every joint, and the one number to check before any pattern goes near a sheet.
The geometry of a fitting is the easy half. What decides whether it goes on the pipe and stays there is the set of allowances added round the edge: the lap along the seam, the holes that hold it, the swage at each joint. These are the numbers a tradesman carries in his head and a drawing office never asks about.
The lap
The lap is the overlap along the seam of a rolled piece. Enter it as one total and it is shared between the two edges: a 50 mm lap is 25 mm on each side of the seam line. The girth of the pattern is the true girth of the cladding, and the lap sits outside it. If you add the lap inside the girth, the rolled piece comes out small, and it will never sit on the lagging without a gap at the seam.
Where the holes go
Rivet and screw holes go on the seam lines, on both edges, not out in the lap. The reason is how the job is done. In the shop the metal is made up fully; on site the piece is offered up, lodged, and riveted through both holes at once. With the holes on the fold lines the rolled piece closes to the exact diameter every time, because the holes line up only when the seam is where the pattern says it is. Holes out in the lap let the piece close anywhere between too tight and too loose, and it will be whichever the fitter did not want.
The swage
Where two pieces meet, one reaches into the other. The swage is that reach: a male stage on one piece, a female on the next, so the joint locks and sheds water. Three millimetres is the working default and it is added at every joint on a bend, every mitre on a lobster, every collar on a tee. It is not added on a plain end that butts to straight cladding. Get the swage wrong and every piece in a run is out by the same amount, which is the kind of error that is invisible on one fitting and a full segment out by the end of a rack.
Worked to the outside of the lagging
Every dimension on a cladding pattern is on the outside of the insulation, not the pipe. A pipe size and an insulation thickness give a cladding diameter, and from that point the pipe does not exist. The place this bites is bend radius: the centre radius is called up on the pipe, but the throat and back of the cladding are a full insulation thickness further out and in, and a radius that is fine for the pipe can be impossible for the cladding.
The one number to check
A pattern printed at the wrong scale is a pattern for a pipe that does not exist, and a printer will scale to fit unless it is told not to. Print at 100 percent, never fit to page. Then put a rule on the calibration bar: it is 100 mm and it has to measure 100 mm. On a tiled A4 sheet the crosshairs in the corners line up sheet to sheet; if the bar is right and the crosshairs meet, the pattern is right. If either is off, do not cut.
TurboClad draws the lap, the holes on the seam lines, the swage at each joint and the 100 mm bar on every pattern, and prints them 1:1 on A4 or A3. The allowances are yours to set in Settings and they stay set. Offer the first template up to the metal before cutting a batch.