Cover, not width
Weatherboards lap over each other, so each row only shows part of the board. That visible part is the cover, and it is the only width that matters for quantities. A 180 mm board lapped 30 mm covers 150 mm. Divide the net wall area by the cover in metres and you have lineal metres; divide the wall height by the cover and you have rows.
lineal metres = wall area ÷ cover (m) rows = wall height ÷ cover, rounded up
| Board | Cover | lm per m² | 100 m² wall, +10% |
|---|---|---|---|
| Linea 150 / 150 mm plank | 120 mm | 8.33 | 917 lm |
| Linea 180 / 180 mm splayed timber | 150 mm | 6.67 | 733 lm |
| 190 mm timber weatherboard | 160 mm | 6.25 | 688 lm |
| PrimeLine 230 / 230 mm plank | 200 mm | 5.00 | 550 lm |
| 140 mm timber weatherboard | 115 mm | 8.70 | 957 lm |
Covers are typical figures. The data sheet for the product you buy takes precedence, and the calculator has a field for it.
Setting out the rows
Rows rarely divide evenly into the wall height, and a sliver of board under the eave looks wrong. Cladders set out from the bottom so that a full board, or the top of a board, lands at the window heads and the eave, adjusting the lap by a few millimetres per row across the height to make it work. The lap can usually move 5 mm either way without breaking the product's minimum. Do that arithmetic on a story rod before the first board goes on, and mark every row on the corner studs.
Joins, corners and fixings
Boards join on a stud with the joints staggered row to row. Fibre cement planks such as Linea use a joiner or an off-stud joint per the manufacturer's detail; timber boards are butted on a stud with the end grain primed, or scarfed. External corners take a corner mould or a box corner, internal corners a stop bead or a scribed board. Fixings are one nail or screw per board at every stud for most profiles, driven above the lap line so the head is hidden by the next board; some wide planks specify two. Over 450 mm studs that is about 2.3 fixings per lineal metre. Behind everything goes the sarking or wrap and, for fibre cement in a cavity system, the battens.
Timber or fibre cement
Fibre cement planks come pre-primed, do not rot, and take paint for years between recoats; the trade-off is dust when cutting and a dead flat look that some people miss. Timber weatherboards look the way weatherboards have always looked and can be cut, scribed and repaired with ordinary tools, at the cost of priming every cut end and painting every five to ten years. Both come in the same sort of stock lengths and both are quantified the same way.
Worked example
A weatherboard extension with 40 m of wall at 2.7 m, 12 m² of windows and doors, in a 180 mm board with 150 mm cover. Net area 96 m², which is 640 lineal metres, 704 with 10 percent waste. In 4.2 m stock that is 168 boards. Eighteen rows up the wall. Fixings at 450 mm studs, about 1,640.
Common questions
How many lineal metres per m²?
One divided by the cover in metres: 6.67 for a 150 mm cover, 8.33 for 120, 5.0 for 200.
What is the cover?
The visible part of each board after the lap. From the data sheet.
How much waste?
10 percent plain, 15 percent with lots of openings and gables.
Where do boards join?
On a stud, staggered, with a joiner for fibre cement or primed ends for timber.
Quantities only. Fixing schedules, cavity requirements, flashings and minimum laps come from the manufacturer's installation manual and the NCC.