Boarding a loft is one of the most popular small home improvements in the UK. A few sheets of chipboard turn a dark, awkward roof space into somewhere to keep suitcases, decorations and the things that do not fit anywhere else.
The catch is what happens underneath. In many homes the ceiling joists are only around 100mm deep, and the recommended depth of loft insulation is 270mm. Fix boards straight onto the joists and something has to give, and it is always the insulation.
Why Boarding Straight Onto the Joists Is a Problem
Loft insulation works by trapping still air between its fibres. Crush it and you squeeze that air out. A 270mm layer forced down to the 100mm depth of the joists holds a fraction of the air it was designed to, and insulates about as well as a 100mm layer would.
The result is a loft that looks fully insulated, with all 270mm still physically there, but performs like one that was never topped up.
In heat-loss terms, crushing 270mm down to 100mm means roughly two and a half times as much heat flowing up through the boarded part of the ceiling. Board most of the loft that way and you have undone most of the benefit of the insulation.
The Options for Boarding a Loft
| Approach | Insulation underneath | Good for | Watch out for |
|---|---|---|---|
| Boards straight onto the joists | Crushed to joist depth | Lofts with very little insulation and no plans to add more | Loses most of the benefit of a 270mm top-up |
| Raised boarding on loft legs | Full depth kept | Most storage areas in typical lofts | Leg spacing, board fixing, and not overloading the joists |
| Raised timber frame or deeper battens across the joists | Full depth kept | Larger platforms, or where legs are awkward | Extra weight and the height lost under a low roof |
| Rigid insulation boards under the deck | Thinner, higher-performance layer | Where height is very limited | Cost, and keeping the layers continuous |
How Raised Loft Boarding Works
The most common solution is a set of loft legs, sometimes called loft stilts: plastic or metal supports that fix to the top of the joists and stand tall enough for the insulation to sit underneath at its full depth. Battens or the boards themselves are then fixed to the tops of the legs.
- Get the height right. For 270mm of mineral wool on joists around 100mm deep, the top of the legs needs to sit roughly 175mm or more above the joists so the insulation is not touched.
- Leave a small gap. A little air space between the top of the insulation and the underside of the boards helps avoid condensation forming under the deck.
- Lay the insulation first. Get the full 270mm down, cross-layered, then fit the legs through it and board over.
- Leave access routes. Keep space to reach junction boxes, tanks and pipework later.
Mistakes to Avoid When Boarding
- Boarding right into the eaves. Keep the boarded area well back from the edges so the airflow at the eaves stays open. See should loft insulation go into the eaves.
- Trapping cables under insulation and boards. Cables that end up buried run warmer. Lift them to sit above the insulation where you can, and get an electrician's advice on circuits that are heavily covered. Our guide to downlights, cables and water tanks explains why.
- Boarding over damp or soiled insulation. Once boards go down, you will not see the insulation again for years. If it is damp, mouldy or has had rodents in it, deal with that first.
- Treating it as a floor. Ceiling joists are sized to carry a ceiling, not a bedroom. Keep storage light, spread the load, put heavy items near supporting walls and do not use the loft as a room without a proper conversion.
- Boarding over the cold water tank area. Leave the area under the tank uninsulated and accessible; it relies on warmth from below to avoid freezing.
What to Do With Old Insulation Before Boarding
Boarding is a natural moment to look at what is already in the loft, because once the deck is down it is hard to get back to. If the existing insulation is clean, dry and simply needs topping up, lay the extra layer and fit raised boarding over it. If it is flattened, dusty, damp or soiled, clearing it first and starting again with a clean 270mm layer is usually the better long-term answer.
Our loft insulation removal service clears the joists ready for boarding, and our insulation replacement service lays new insulation to 270mm so it is ready for raised boarding on top.
Frequently Asked Questions
Can you put loft boards on top of insulation?
Not directly on top of thick insulation. If you lay boards on the joists over 270mm of insulation, they crush it. Use raised boarding on loft legs or a raised frame so the boards sit above the full depth of insulation.
How high should loft boarding be raised?
High enough that the full depth of insulation fits underneath without being compressed, ideally with a small air gap above it. For 270mm of mineral wool on joists of around 100mm, that usually means the boards sit roughly 175mm or more above the top of the joists.
Does compressed loft insulation still work?
Partly. Insulation works by trapping air, so squashing it reduces its insulating value roughly in proportion to how much thinner it becomes. 270mm crushed to 100mm performs about like 100mm.
How much weight can a boarded loft take?
Ceiling joists are designed to carry a ceiling, not heavy loads. Light storage spread over boarding is normally fine, but heavy items should go near the walls or over supporting walls, and a loft should not be used as a room without a proper conversion. If in doubt, ask a structural engineer.
Should I remove the old insulation before boarding?
If it is clean, dry and in good condition, it can usually stay, with raised boarding over it. If it is dusty, flattened, damp or soiled, it is easiest to clear it first, lay new insulation to 270mm, then fit the raised boarding.
Related Reading
Need a Hand With Your Loft?
Planning to board your loft? If the old insulation needs to come out first, send a couple of photos from the hatch and we will give you a written price to clear it and lay new to 270mm.
Or email info@loftrestore.co.uk. Based in Cheshire, working across the UK.