Quick answer: Don't lay boards directly on top of loft insulation. Squashing mineral wool reduces how well it insulates, and 270mm of insulation sits well above the joists. The fix is to raise the boards on loft legs fixed to the joists, so the insulation keeps its full depth underneath and there is a small ventilation gap between the insulation and the boards.
Loft boarding is one of the jobs we see done wrong most often. A homeowner, or sometimes a handyman, screws chipboard straight onto the joists, flattening whatever insulation was there. The loft becomes a handy storage space and the house quietly gets colder. Here is how to avoid that.
Why squashing insulation matters
Mineral wool insulates because of the air trapped between its fibres. Compress it and you squeeze that air out, so it no longer slows heat in the same way. The Energy Saving Trust is clear on this: if you are using mineral wool, don't squash it, because that will reduce its effectiveness as an insulator.
The problem is simple maths. The recommended depth for mineral wool is 270mm, but the ceiling joists in most lofts are far shallower than that. Boards screwed straight onto the joists leave only the depth of the joist for insulation, and anything above it gets crushed. You end up paying to heat a loft full of storage boxes.
Raised loft legs: how they work
Loft legs (sometimes called loft stilts) are rigid supports that screw onto the top of the joists. They lift the boarding platform clear of the insulation so it can sit at its full depth underneath.
- Insulation first. We make sure the insulation underneath is at the right depth, usually a layer between the joists and a second cross-laid layer on top, before any boards go down.
- Legs fixed to the joists. The legs are screwed to the joists at regular spacing, so the load still goes down into the timber rather than onto the ceiling.
- Battens or bearers across the legs, where the system uses them, give a level frame for the boards.
- Boards fixed on top to create a solid storage platform above the insulation.
The leg height needs to suit the insulation depth. If the legs are too short, you are back to squashing the insulation, just a little less.
Leave a ventilation gap
A raised floor isn't only about depth. The Energy Saving Trust advises leaving a small gap for ventilation between the insulation and the boards to prevent condensation.
Warm, moist air from the house can work its way up through the ceiling and insulation. If the boards sit tight on top, that moisture can collect on the underside of the boards. A small air gap lets it disperse into the ventilated loft rather than condensing. The same thinking applies at the eaves: boarding should never block the airflow paths that keep a cold loft dry.
What storage loads are sensible?
Boarding gives you a surface to store things on, but it doesn't make the ceiling joists any stronger. In most homes the ceiling joists were sized to hold up a plasterboard or lath-and-plaster ceiling, not a loft full of heavy items. Raised legs transfer the weight into the joists; they don't add capacity.
Sensible loft storage looks like this:
- Suitcases, bags of clothes and bedding
- Christmas decorations and seasonal items
- Light plastic storage boxes
- Paperwork in moderation
Things we would advise against storing in a standard loft:
- Heavy furniture, gym equipment, or large stacks of books
- Paint, chemicals or anything that dislikes temperature swings
- Anything you would be upset to lose if a roof leak or condensation occurred
Spread weight out rather than piling it in one spot, and keep heavier items closer to the supporting walls below rather than in the middle of a span. If you want to use the loft as a room or for heavy storage, that is a structural question for a builder or structural engineer, not a boarding job.
Common DIY mistakes
- Boarding straight onto the joists and squashing the insulation underneath. This is the big one.
- Removing insulation to make the boards sit flat. We still find lofts where insulation has been pulled out from under the boarded area altogether, leaving a large cold patch over the landing.
- Standing on the ceiling between the joists while working, which is how feet come through ceilings.
- Burying cables and downlights. Electrical cables and recessed lights under boards and insulation need appropriate clearance and protection.
- Blocking the eaves with boards or stored items, cutting off ventilation.
- Boarding over damp, mould or droppings. Covering a problem doesn't fix it. If you see signs of condensation or mice before you start, deal with that first. Our guide to loft condensation and mould explains what to look for.
- Boarding the whole loft when you only need storage near the hatch. Boarding everything adds cost and can make future access to cables, pipes and the roof harder.
How much of the loft should you board?
Most people only need a section near the hatch, enough for the items they actually store. A boarded area around the hatch also makes the loft safer to get in and out of. We usually recommend boarding what you will use and leaving the rest as full-depth insulation, which keeps the cost down and the insulation working.
Older Cardiff lofts: things to check first
Many terraces and semis around Cardiff have older roofs, sometimes with lath-and-plaster ceilings below and joists that are not perfectly level or evenly spaced. That affects boarding. Uneven joists mean the legs need setting carefully so the platform is flat, and older ceilings can crack if someone kneels or stands between the joists. Older lofts are also more likely to have old wiring, water tanks or signs of past roof leaks, all of which we check before a single board goes down.
Boarding and insulation together
If your insulation is thin, it makes sense to top it up at the same time as boarding, so the insulation goes under the raised floor at the right depth from the start. See our guide on how much loft insulation you need. If you want a warmer space rather than just somewhere to store boxes, insulating the rafters with SuperFOIL can be an alternative worth discussing.
It is also worth thinking about getting up there. A proper metal loft ladder makes a boarded loft far more usable than balancing on a stepladder.
Getting it done properly
Our loft boarding service starts from £500, with boards raised on loft legs so the insulation underneath isn't squashed. We check the insulation depth, ventilation and wiring first, and we can top up insulation or fit a ladder on the same visit. Surveys are free and quotes are usually back within 24 hours. Request your free quote.