P5 tongue-and-groove flooring boards are laid across the joists, with the long edge running at right angles to them and the short-end joints staggered. The board thickness must suit the joist centres and the exact board manufacturer's loading guidance.
18mm is commonly used up to 450mm centres; 22mm up to 600mm
Major UK P5 manufacturers such as EGGER and West Fraser publish 18mm boards for joist centres up to 450mm and 22mm boards for centres up to 600mm within their specified flooring systems. Check the instructions for the actual board being installed before treating those figures as the design limit.
Run the long edge across the joists
Lay T&G boards with the long dimension perpendicular to the joists. Stagger short-end joints in a brick-bond pattern and arrange the floor so board ends are properly supported as required by the board system.
Glue and mechanically fix to reduce movement and squeaks
Manufacturer systems commonly use adhesive at joists and tongue-and-groove joints together with mechanical fixings. Use the adhesive and fixing pattern specified for the board rather than copying a generic screw spacing.
Leave the specified perimeter expansion gap
EGGER and West Fraser both call for a minimum 10mm perimeter expansion provision, with additional movement allowance on larger floor areas. Keep that gap clear rather than packing it tight against walls.
Start with dry, correctly spaced joists
Do not try to use the flooring board to correct an unsuitable or badly uneven joist structure. Check the joists, strutting and centres before laying the deck.
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