Loft Boarding Calculator

How Many Loft Boards Do I Need?

The usual answer is the area divided by the board size, plus 10%. For a 3.6m × 2.4m area on 1220 × 320mm loft panels that sum gives 25 boards. It ignores three things. Every board end has to land on a joist, so boards are trimmed to whole joist gaps. The last row is usually ripped narrower. And short pieces cut for the ends of rows can come out of offcuts.

So this calculator lays the boards out. Boards run across the joists, in rows one board wide, with the end joints staggered from row to row. All the pieces are then packed into boards, longest first, so offcuts get reused. For the same area that comes to 24 boards in the plan, 26 with 5% spare, which is 9 packs of 3. On 2400 × 600mm sheets it is 8.

What This Calculator Is For, and What It Is Not

It is for storage boarding laid over ceiling joists. It is not for a habitable floor, and it is not a structural check. Loft Leg's own guidance is storage of up to 25kg per square metre plus the weight of an adult moving around, and it says the loading limit of the existing ceiling structure should be confirmed by a structural engineer. We do not give load or Building Regulations advice. If you are unsure, ask a structural engineer or your local building control.

How the Layout Works

• Boards run across the joists. Loft Leg says boards should be oriented perpendicular to the joists. A board's length runs across the joists and its width runs along them.
• Every board end sits on a joist. So each piece is cut to a whole number of joist gaps. A 1220mm board on 400mm joists reaches three gaps (1200mm), so 20mm comes off each full board. On 600mm joists it reaches two gaps, also 1200mm.
• End joints are staggered. Each row starts with a different part-length piece, so no joint sits on the same joist as its neighbour's.
• The far edge is taken to the nearest joist. If your length is not a whole number of joist gaps, boarding runs to the joist beyond it so the last board ends are supported, and the results say how far. A span up to 50mm past a joist is treated as ending on it, because Loft Leg's own drawings do the same: 1220mm boards over "2440mm" are drawn across 6 gaps of 400mm.
• The last row is ripped if the width is not a whole number of boards. It still uses a full board per piece.
• Offcuts are reused. All pieces are packed into boards together.

How Many Loft Legs, and Where

Loft Leg's rule is one leg on each joist the boards cover, then legs along each joist at the outer edges of the platform and "at every point where the loft boards meet". Legs go no more than 600mm centre to centre. So the count is the number of joists times the number of board edge lines, and the diagram above shows every one of them.

Checked against Loft Leg's own drawings. We counted the leg symbols in their drawing for 1220 × 320mm boards on 600mm joists over 4800 × 2440mm (11.72 m²): 80 legs, one at every crossing of a joist and a board edge, and no others. The calculator gives 80 for the same area. Their 400mm drawing (an L-shape, 9.36 m²) has 94 legs, and the same rule gives 94 when the L is split into rectangles. Their other 600mm L-shape has 74, and every crossing in it has a leg as well.

That comes out as a grid. One 2400 × 600mm board across joists at 400mm centres has 14 legs, and each board added beside it adds 7. At 600mm centres it is 10, then 5 more each. Narrower boards mean more edge lines, so the same area on 320mm-wide loft panels needs more legs than on 600mm sheets: 90 against 50 for the 3.6m × 2.4m example.

Fixings. Loft Leg says to allow approximately 6 screws per leg, 4 into the joist and more into the board, and to use 4.0 x 30mm countersunk multi-purpose screws, not nails. Legs are sold in boxes of 12. Without legs we give no screw count, because we found no sourced rule for it.

Boards and Legs We Use, With Sources

Every board and leg in the calculator has a UK merchant product page, and no product page means no entry. Stock varies by merchant, so check before ordering. Use the ranking under the results to compare sizes for your own loft.

Leg heights and the insulation each allows are from Loft Leg's own questions page.

Will the Boards Fit Through the Hatch?

The most common loft hatch is 562 × 726mm, according to Insulation Superstore's hatch guide, which also lists 450 × 600, 542 × 645, 600 × 700 and 700 × 900mm. The calculator compares each board's width with your hatch. A board goes through flat if its width fits the hatch's shorter side, and on edge if its width fits the longer side. It does not model room to swing a long board round above the hatch, which is what stops most 2400mm sheets in practice.

Common Loft Sizes: Boards and Legs Needed

Generated from the calculator above. Each area has joists along the shorter side at 400mm centres, 5% waste, and legs for 270mm of insulation.

Use the calculator for your own loft. Which way the joists run, and the joist spacing, change the answer more than you would expect.

Frequently Asked Questions

How many loft boards do I need?

Most calculators divide the area by the board size and add 10%. That misses how the boards actually land. Boards run across the joists, every end has to sit on a joist so each one is trimmed to whole joist gaps, and the last row is usually ripped narrower. This calculator lays every board out. A 3.6m × 2.4m area on 1220 × 320mm loft panels at 400mm joists needs 24 boards in the plan, 26 with 5% spare, which is 9 packs of 3. The area-over-board-size sum with 10% on top gives 25.

Which way should the boards run?

Across the joists, not along them. Loft Leg's own guide says boards should be oriented perpendicular to the joists. Every board end then lands on a joist, and the end joints are staggered from row to row so no two neighbouring rows join over the same joist. Tell the calculator which way your joists run and it does the rest.

How many loft legs do I need?

There is one leg wherever a joist crosses the edge of a board, including the two outer edges. Loft Leg's rule is a leg on every joist the boards cover, and legs along each joist "at every point where the loft boards meet", never more than 600mm centre to centre. The calculator counts those crossings from your layout, not from a legs-per-square-metre guess. For one 2400 × 600mm board across joists at 400mm centres that is 14 legs, and each board added beside it adds 7. At 600mm centres it is 10 legs, then 5 for each extra board.

Why does the calculator give more legs than 3 per square metre?

Because the rule of thumb is for wide boards. Loft Leg puts a leg at every point a joist meets a board edge, so the count follows the grid your boards and joists make. With 600mm-wide boards on 600mm joists that is a 600 × 600mm grid, about 2.8 legs per m². With 320mm-wide loft panels on 400mm joists it is a 320 × 400mm grid, about 7.8 per m² before the edges. Loft Leg's own drawing for 1220 × 320mm boards on 600mm joists over 4800 × 2440mm shows 80 legs, which is 6.8 per m², and the calculator gives 80 for the same area. Your 3.6m × 2.4m example comes to 90 legs, 10.4 per m². Wider boards need far fewer legs.

Which loft leg height do I need?

It depends on the depth of your insulation. Loft Leg's standard 175mm leg is for up to 270mm of insulation (joist plus quilt), and the XL 300mm leg is for up to 400mm. Both are sold in boxes of 12. Enter your insulation depth and the calculator picks the leg. Beyond 400mm neither leg works and it says so.

How many screws do I need for loft legs?

Loft Leg says to allow approximately 6 screws per leg: 4 to fix the base to the joist, and more to fix the board to the top platform. Use 4.0 x 30mm countersunk multi-purpose screws. The calculator multiplies that by your leg count. If you board straight onto the joists without legs we give no screw count, because we found no sourced rule for it.

Will the boards go through my loft hatch?

Pick your hatch size and the calculator checks each board's width against it. The most common hatch is 562 × 726mm, according to Insulation Superstore. A 320mm-wide loft panel goes through flat. A 600mm-wide sheet does not go through flat and only fits turned on edge, and that is before you have to swing a 2400mm board round above the hatch, which we do not model.

Can I use 18mm boards on 600mm joists?

Check the merchant's own guidance. Wickes says its 18mm P5 chipboard flooring is suitable for joists up to 400mm apart, and its 22mm board is for joists up to 600mm apart. The calculator warns you when your joist spacing is wider than the board's stated limit.

Is this safe to use as a floor?

This is for light storage boarding over ceiling joists, not a habitable floor. Loft Leg says its legs are for storage of up to 25kg per square metre plus the weight of an adult moving around, and that the loading limit of the existing ceiling structure should be confirmed by a structural engineer. We give no load or Building Regulations advice. If you are unsure, ask a structural engineer or your local building control.