How do I store things in a loft safely?

What survives a loft and what does not, boarding without ruining the insulation, and the weight question nobody asks.

Difficulty: intermediate
Time: 2 hr
Read: 2 min
Safety: warning

Short answer

Board above the insulation on raised legs, never directly on the joists — laying boards on the joists squashes the insulation and wastes it. And check the joists can take the weight; many were never designed for storage.

A loft looks like free storage and comes with three constraints: the structure may not be designed for load, the insulation is easily ruined, and the environment destroys a lot of what people put up there.

Safety

Never stand between joists — you will go through the ceiling. Use boards, work in good light, and do not work alone. Do not compress loft insulation with boarding laid straight onto the joists; it destroys its value. Check the ceiling joists can take the load before storing anything heavy — many are sized only to hold a ceiling.

Step by step

Check what the joists can carry.Many ceiling joists are sized to hold a ceiling, not a load. If you plan anything substantial, get advice — sagging ceilings and cracked plaster below are the warning signs.
Board above the insulation, not on the joists.Modern insulation is deeper than the joists. Boards laid directly on top compress it and destroy much of its value. Raised loft legs lift the boarding clear and keep the insulation lofted.
Leave the eaves ventilation clear.Do not board or stack right to the edges. The roof void needs airflow at the eaves or moisture condenses on the timbers.
Do not cover downlights or bury cables.Recessed lights need clearance or proper fire-rated covers, and cables buried under insulation or boarding can overheat. Keep both accessible.
Know what will not survive.Lofts swing from freezing to very hot and are often humid. Photographs, documents, vinyl records, candles, electronics, anything with batteries, and paint all degrade or are damaged.
Use sealed plastic boxes, not cardboard.Cardboard absorbs moisture, collapses and attracts pests. Clear sealed boxes let you see the contents and keep dust and insects out.
Label everything and keep a list.On the box and on a note by the hatch. Loft storage becomes a place things go rather than a place things are kept otherwise.
Keep weight near the walls and spread it out.Loads are best carried near the supporting walls rather than in the middle of a span. Never concentrate heavy items in one spot.

Tips

Store the heaviest things near the hatch as well as near the walls. Carrying weight across joists is where accidents happen.
A proper loft ladder and a light transform how usable a loft is, and both are inexpensive relative to the space gained.
Photographs and documents belong in the house, not the loft. Temperature swings and humidity destroy them within a few years.

Common mistakes

Boarding straight onto the joists — It compresses insulation that is deeper than the joists, losing much of its value and creating a cold spot above the boarded area.
Storing anything paper or photographic — Loft humidity and temperature swings destroy them. Documents and photographs need to live in the house.

Questions people ask

Can I board over loft insulation?

Not directly onto the joists if the insulation is deeper than they are — it compresses it. Use raised loft legs to lift the boarding above the insulation.

What should I not store in a loft?

Photographs, documents, electronics, batteries, candles, vinyl and paint. The temperature swings and humidity damage all of them.