Loft legs and raised loft boarding over insulation

Why compressing insulation matters

Mineral wool insulation works by trapping still air between its fibres. Squash it, and there is less air trapped, so it lets more heat through. Current guidance for lofts is around 270mm of mineral wool, but ceiling joists in most houses are only 100 to 150mm deep. If boards are screwed straight onto the joists, the insulation above the joists is flattened or has to be cut back, and that part of the ceiling loses much of its benefit.

Squashed insulation can also leave cold patches on the ceiling below, where condensation and mould are more likely. Raising the boards means the insulation keeps its full depth underneath.

What is raised loft boarding?

Raised loft boarding is a storage floor built on supports that stand on the joists. Loft legs are the most common type: each one screws to the top of a joist, and the boards are screwed onto the tops of the legs. Some systems use deeper timber battens fixed across the joists instead. Either way, a gap is left between the insulation and the underside of the boards, so the insulation is not touched.

• Plastic loft legs screwed to each joist
• Taller legs or stacked legs for deeper insulation
• Timber battens fixed across the joists
• Chipboard loft panels with tongue and groove edges
• A clear walkway from the hatch to the boarded area

How to fit raised loft boards

Fitting starts at the hatch. The joists are checked for size, rot, notches and any cables that would be trapped. Loft legs are screwed to the joists at the spacing set by the manufacturer, usually so every board edge lands on a leg. The insulation is laid back in full depth between and over the joists, then the boards are laid across the tops of the legs with joints staggered and screwed down. Leave access to junction boxes, the water tank and any lights.

Boards are screwed rather than nailed to joists or legs. Hammering in a loft shakes the ceiling below and can crack the plaster, and screws let a board be lifted later to reach a cable.

Can you put loft boards straight onto joists?

You can if the insulation fits entirely within the depth of the joists and you are happy with that level of insulation, which is uncommon now. Some older houses have only a thin layer, in which case boarding directly is physically possible but means you cannot add the extra insulation later without lifting the boards. In most lofts with a modern depth of insulation, boarding straight onto the joists squashes it, and raised boarding is the better approach.

Ventilation and condensation

The roof space above the insulation is meant to be cold and ventilated, usually through vents at the eaves or tiles. Boarding must not block those, so the boarded area stops short of the eaves and insulation is not pushed into them. Raised boarding also leaves an air gap above the insulation, which helps. Do not cover the full loft floor in boards and plastic sheeting, as trapping moisture can cause condensation on the timbers.

Checking the joists first

Ceiling joists are sized to carry a ceiling, not a heavy storage floor. Light storage, such as boxes of decorations and suitcases, is what most loft boarding is designed for. Heavy items, a water tank being moved, or using the space as a room are a different matter and fall under a loft conversion, which needs proper structural design and building control, and is not something CRF takes on. Joists that are damaged, undersized or have been cut in the past are pointed out before boarding goes down.

What raised loft boarding costs depend on

The cost of loft boarding and insulation depends on the area being boarded, the height of the legs needed for the insulation depth, whether new insulation is being added, how easy the loft is to get into, and whether a ladder or a bigger hatch is needed first. Raised boarding uses more materials and time than boarding directly onto joists. A cramped loft with a low pitch takes longer to work in than a tall, open one. The loft boarding cost guide explains how the boarding itself is priced.

Need it sorted? Book loft boarding

If you want storage without losing the benefit of your insulation, book loft boarding. Where the hatch is too small to get boards through, loft hatch installation can be done first in the same booking. Send a photo of the hatch, one inside the loft showing the insulation depth, and the area you want boarded through the contact page for a quote.

Frequently asked questions

Handyman ServicesCarpentry