I've been looking into why cladding estimates go wrong, and it usually isn't the arithmetic. You measure a wall, then you buy the width printed on the pack. Cover is how far each row actually climbs. Once you've got those two, the estimator will count the boards.
This page is how to measure one wall, pick the shape you are actually buying, and read the numbers the tool gives back.
Cover is how far each row of boards climbs the wall. It is not the width printed on the pack — and the estimator only works once you have measured one real wall.
What you'll learn
How to put a tape on one wall, in millimetres, and when to run the tool twice
Why a 125 mm board does not cover 125 mm of wall
The four shapes the estimator knows — and where waney, loglap and secret-fix sit
When a gap is added to the board, and when you should type the cover from the pack
How waste and one window come off the count
How to read the shopping list, and what those numbers are not
Measure the wall
The tool counts one rectangle, then repeats it.
Take a tape to the wall you are cladding. Write two numbers, in millimetres:
Width — across the face, left to right
Height — from where the first board will sit, up to the top of the last row
Then count how many walls are that same size. Four identical shed sides is four. A long wall and a pointed gable are two jobs. Run the tool once for each.
If two walls differ, do not average them and hope. The tool will not invent a different wall for each side.
A few notes from using it:
Boards are assumed to run across the wall — left to right. If you are hanging them up and down, swap width and height in your head and run it again.
Leave the batten spacing at 600 mm unless a drawing, or the person who designed the job, asked for closer. Closer battens mean more battens and more nails. They do not change how much each board covers.
One window or door can be taken off the whole job. If you have several, add their widths and heights together first — or leave those fields at 0 and subtract on paper.
You do not need a drawing. You need a tape, two numbers, and an honest count of how many walls match.
Pick a profile — cover is not the pack width
The width on the pack is the board as sold. Cover is how far each row actually climbs the wall. They are not the same number.
You hang a board. The next one overlaps it, or sits in a groove, or leaves a gap. The bit of wall that one board truly covers is smaller than the board — or, with an open joint, a little larger. That second number is the one that fills the wall.
The estimator has four buttons. That is the whole list.
If the ticket says waney, use feather-edge
If it says loglap, use shiplap
If it says secret-fix, use tongue and groove
A shadow gap is not a fifth button. It is a field on square-edge
Feather edge. Sawn weatherboard — the classic shed look, thicker on one edge, thinner on the other. Cover is the board width minus the overlap you leave. The tool starts you on a 125 mm board with a 25 mm overlap. That is 100 mm of cover. A starting point on the page, not a rule for every mill.
Shiplap. A board with a step cut into the edge, so one board sits over the next. Loglap is the same family — a rounder face, same idea. Cover is the bit you see once the boards are lapped, not the width on the pack. Type the figure on the pack if you have it.
Tongue and groove. A tongue on one edge, a groove on the other. TGV and secret-fix sit here. Cover is the visible face — the bit you see once the tongue is hidden. Secret-fix changes the nails you see, not the boards you buy.
Square edge / open joint. Straight-sided boards, either butted tight or with a gap between them. Here the useful number is pitch: the board width plus the joint you leave. Set the gap to 0 if you butt them.
A 125 mm square-edge board with a 10 mm open joint occupies 135 mm, not 115. The gap is added, not subtracted. That is the mistake the tool is there to stop.
If you only remember one thing before you open the estimator: ask what you are actually buying, then ask what each row covers. The second number is the one that fills the wall.
Open joint versus pack cover
Two habits, not two extra profiles.
Open joint is for square-edge only. The field is labelled “Joint / shadow gap”. It is added to the board width to get pitch. The page treats 10 mm as a normal UK open joint. It does not belong on shiplap or tongue and groove.
Pack cover is for the machined boards. For shiplap and tongue-and-groove, if the pack prints a cover width, type that figure. The tool uses it as-is. If you leave the field blank, you still get a count — then the page tells you to confirm the cover on the pack you actually buy.
Mills do not all cut the same step into the edge. The ticket on the pack is the number.
Waney and feather-edge
If the ticket says waney — or green oak with a live edge — treat it as feather-edge. Do not look for a fifth button. Do not pretend waney has one exact cover.
Waney means the board still has the rounded outer surface of the log, sometimes with bark. Widths wander along the board and across the pack. Start the overlap around 50 mm, average the widths in the pack, or run the job twice.
Fifty millimetres is a start, not a certificate. A waney pack that looks like 150 mm on the fat end may cover a good deal less once you have lapped it and discarded the barky edge you will not show. If the widths jump around, two passes — one on the narrow average, one on the wide — will tell you more than a single invented cover.
Waste and openings
Waste is applied once, after the running board count. It is not added on top of every row.
The tool starts at 10%. That is a fair shed or garden-room start. Go higher around windows, doors, and barges — the boards that finish a gable. Corners, angled cuts, and a fussy first row eat boards the same way.
One opening — width × height of one window or door — is taken off the whole job. Several openings: add them, or leave the fields at 0 and deduct on paper.
Board length is a separate decision. The estimator offers 1800, 2100, 2400, 3000 and 3600 mm. Longer packs mean fewer joins across the wall. They do not change the cover. Order lengths that suit the wall you measured, not the first pack you see.
Use the estimator
Open the cladding estimator. It is a shopping estimate, not a certificate.
Enter one wall in millimetres, how many walls match, and the profile you are actually buying. Type the board width as sold. Then the number that is not the pack width: overlap on feather-edge or waney; pack cover on shiplap or T&G if it is printed; joint on square-edge, or 0 if you butt. Leave battens at 600 mm unless you have been told otherwise.
Read the big number. That is what to order. Next to it sits cover or pitch, so you can see why feather-edge, T&G and open joint do not buy the same.
If you leave the page on its own example — four walls, each 4000 × 2400 mm, 125 mm feather-edge, 25 mm overlap, 2400 mm boards, 10% waste — you get 176 boards, 100 mm cover, 38.40 m², 422.4 linear metres, 32 battens at 600 mm, and about 845 nails or screws. That is the tool showing its working: 24 rows × 1.67 boards per row × 4 walls + 10% waste.
Copy the list, share the job, or print a sheet for the merchant. Buy the last pack unopened if you are hovering near a full bundle. Easier to take back than to scramble mid-job.
Trust the working line: rows × boards per row × walls, plus waste. If that line does not match the wall you measured, the profile or the cover is wrong — not the arithmetic.
What the numbers are not
The estimator counts boards, cover, battens and a nail allowance. It does not choose a species. It does not say how long the timber will last. It does not size a cavity (the air gap behind the boards), a membrane, a flashing, or a wind load.
What to check at the rack is a different job from counting boards. Start with Buying timber before you build.
Movement is also not in the count. A clad wall still has to let each board move. The estimator will not leave you a gap at the ends. For that, read Designing for wood movement. For the whole outdoor build — membrane, battens, the air gap, trims — Cladding a Garden Room is the place, not this how-to.
If you need the rating system for how long a species lasts outdoors, Wood Durability Classes is there. This page will not retell it.
Nails and screws on the results sheet are a shopping number: one or two per board at each batten, plus a little spare, depending on the profile you picked. Feather-edge is counted as one nail through the thick edge, just clear of the board below. That is usual practice on the tool, not a structural specification. Stainless outdoors, especially on cedar and anything near the coast — the tool says so; it still will not design the wall for you.
Take the list to a yard that will let you check the pack cover, the lengths, and the species on the ticket. The estimator gets you to the counter with a number. The shelf is where you find out whether that number was about the right boards.
Key points
Measure one wall in millimetres. Repeat it only for walls that really match.
Cover is not the width on the pack. Feather-edge subtracts overlap; shiplap and T&G use the figure on the pack; square-edge open joint adds the gap.
Four profiles only: feather-edge (waney sits here), shiplap (loglap sits here), tongue-and-groove (secret-fix sits here), square-edge / open joint.
If the pack prints a cover width, type it. Do not guess.
Waney has no single exact cover. Start around 50 mm of overlap and average the pack — or run the job twice.
Waste is applied once. One opening comes off the whole job. 10% is a start, not a rule for a wall full of windows.
The big number is a shopping estimate. It is not a species choice, a durability rating, or a fixing specification.
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