Cladding Calculator

How Many Cladding Boards Do I Need?

The usual answer is the wall area divided by the area of a board, plus 10%. For a 3.0m × 2.2m wall with a window on 119mm Wickes treated shiplap that sum gives 24 boards. It forgets three things. A board covers 110mm, not 119mm, so it covers 0.26 m² rather than 0.29. A window interrupts every course it meets. And short pieces are only worth having if they come out of offcuts.

So this calculator lays each wall out. It works out the courses from the cover width, cuts each course round the openings, staggers the joints, and packs every piece into boards, longest first. For that wall it comes to 23 boards in the plan, 25 with 5% spare, 7 packs of 4.

How the Layout Works

• Cover width, not board width. Shiplap overlaps. Howarth Timber says a minimum of 10mm overlap with 2mm gaps for movement. The calculator uses the surface cover the merchant states for each board.
• Courses round openings. Each course is cut into segments either side of any window or door it meets. Boards above and below run on.
• Joints staggered. A course longer than a board is split into pieces, with no joint within 300mm of a joint in the course below.
• No tiny pieces. No piece is shorter than 300mm unless the gap itself is shorter, for example beside a window.
• Offcuts are reused. All pieces from all walls are packed into boards together, longest first. A board can be cut into two halves, each within a saw cut of half a board.
• Either direction. Horizontal boards run the length of the wall in courses. Vertical boards run floor to eave, in columns.

What it does not do: it does not lay gables or sloping edges, corner trims, counter-battens, membrane or fixings. Enter a gable as a full rectangle and raise the waste allowance. Felting the shed roof too? Our shed roof felt calculator lays out the strips, overhangs and ridge strip.

Battens Behind the Cladding

Horizontal boards go on vertical battens. Howarth Timber's installation guide says support battens should be attached at 600mm intervals, with a minimum size of 38 × 50mm, and that screw fixings should be at least 40mm, or half the board width, whichever is greater, from the end of a board. Vastern Timber recommends 400 to 500mm centres depending on the profile, and for vertical cladding, horizontal battens at the same centres. The two guides differ, so the dropdown offers 400, 500 and 600mm.

Board ends need a batten too. Howarth Timber: "butt joints should always meet on a suitable batten support width". Kalsi Plastics' guide says to support board ends and trim bases around every opening (it is written for PVC shiplap, but the same fixing point applies to timber), and Ruby's guide says to add battens round windows, doors and other openings. Every course that meets a window is cut off at the frame, so those cut ends need something to fix into. The calculator adds a batten at each side of every opening for horizontal boards, and above and below for vertical boards. It leaves the extra out where a regular batten already falls within 50mm of the edge. That 50mm is our own choice, not a supplier figure. The window in the default wall gets 2 extra battens on top of 6 regular ones. Extras are counted at full height, and regular battens that fall inside an opening are still counted, so the total is slightly generous. The diagram shows regular battens as grey dotted lines and the extras as purple dashed lines.

Cladding Boards We Use, With Sources

Every board in the dropdown has a Wickes product page that states its surface cover, pack quantity and pack coverage, and no product page means no dropdown entry. Stock varies by merchant, so check before ordering, and use Custom for anything else, such as the longer lengths that timber merchants sell.

Common Wall Sizes: Boards Needed

Generated from the calculator above. Each wall is a plain rectangle with no openings, boards run horizontally, and waste is 5%. The last column is the usual area-plus-10% sum on the treated board, for comparison.

Use the calculator for your own walls. A window, a door or a different board changes the answer more than you would expect.

Frequently Asked Questions

How many shiplap boards do I need?

The usual sum is the wall area divided by the area one board covers, plus 10%. The catch is that a board covers less than its width, because shiplap overlaps. Wickes states a 110mm surface cover for its 119mm treated shiplap. For a 3.0m × 2.2m wall with a 900 × 1000mm window that sum gives 24 boards. This calculator lays every course out, around the window, with staggered joints and offcuts reused. The same wall needs 23 boards in the plan, 25 with 5% spare, which is 7 packs of 4.

What is the difference between board width and cover width?

Cover width is the width of wall one board actually covers once it is fitted. A shiplap board has a rebate, so each board tucks under the next. The 119mm Wickes treated board covers 110mm, which is 9mm less than its width. Howarth Timber's installation guide says shiplap should have a minimum of 10mm overlap, with 2mm gaps between the upstands for movement, and profiles differ. Always use the merchant's stated cover, not the board width.

Should cladding run horizontally or vertically?

Horizontal is the usual choice for shiplap, with vertical battens behind. Vertical boards need horizontal battens and a counter-batten system so water can drain and air can move behind. The calculator does both. For the same wall, vertical needs 27 boards against 25, because each board is cut to the wall height and the joints between boards go.

How far apart should the battens be?

Howarth Timber says support battens should be attached at 600mm intervals, and recommends a minimum batten size of 38 × 50mm. Vastern Timber recommends 400 to 500mm centres, depending on the profile. So the dropdown offers 400, 500 and 600mm. This wall needs 6 vertical battens at 600mm centres, plus 2 more at the window edges (see below), 8 in all and 17.6m before you frame the window.

Do I need extra battens round windows and doors?

Yes, for the board ends. Where a course meets a window, the boards stop at the frame, and each cut end needs a batten to fix into. Howarth Timber says butt joints should always meet on a suitable batten support width. Kalsi Plastics' installation guide says to support board ends and trim bases around every opening (their guide is for PVC shiplap, but the fixing point is the same for timber). Ruby's guide says to add battens around windows, doors and other openings. So with horizontal boards this calculator adds a vertical batten at each side of every opening, and with vertical boards one above and one below. It skips the extra where a regular batten already falls within 50mm of the edge. That 50mm is our own rule of thumb, not a supplier figure. On this wall it adds 2 battens to the 6 regular ones.

How do you cut cladding round a window?

Each course is cut into segments either side of the opening, and the boards above and below it run on. The calculator works this out course by course, so a window takes out a different length of each course it meets. The short pieces left beside the window are cut from offcuts of longer pieces where they fit. Without the window the same wall needs 25 boards in the plan, with it 23.

Why are there joints at all?

Boards are sold in lengths, and a wall is usually longer than one board. Where a course is longer than a board, the joint is staggered from the course below, by at least 300mm, so no joints line up. No piece is shorter than 300mm unless the gap itself is shorter, for example beside a window. Screws or nails should be at least 40mm, or half the board width, whichever is greater, from the end of a board (Howarth Timber).

Can I use this for a shed gable or a sloping wall?

Not exactly. The calculator lays rectangular walls. For a gable, enter it as a rectangle to its full height and raise the waste allowance, because the triangle of offcuts will not all be reusable. Corner trims, counter-battens, breather membrane and fixings are not counted.