Summary: Learn how to lay loft boarding.
The first task is to inspect your loft to calculate how much insulation (assuming it is not already insulated) and chipboard you will need. Usually access into the loft is limited so make sure you have sufficient room to get your chipboard in before starting. If need be you might have to make the loft access bigger.
You should aim to buy 8'x 2' (2400mm x 600mm) lengths of water resistant floor grade chipboard. This is usually green in colour.
When boarding a loft in this fashion you may want to consider cutting an access hole in one or more of the boards to allow easy access to cabling or piping.
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