Decking Installation Guide

Substructure Installation

Substructure: For decking installation we recommend 6” x 2” pressure treated softwood available from most DIY or timber merchants (very inexpensive option).
Maximum Joist Spacing (centre to centre): Residential use must not exceed 455mm (330mm if at 45° angle).
Airflow requirements: Always allow for adequate ventilation under the decking boards, use larger gaps and air vents if the deck is to be sunken or enclosed (contact us for more detail).
Board Spacing: Minimum 5mm (increase this to 10mm for hot, humid, damp or poorly ventilated areas).

Butt Joint Installations

Butt Joint Gaps: 5mm in-between boards and where a board meets a wall. If area is often hot, humid, damp or poorly ventilated, a 10mm gap to the wall must be left.
Recommended Fixing Method: Only use approved composite deck screws, preferably stainless steel. Screw holes must be countersunk with a SMART-BIT® or 3.5mm combi drill bit (available from UK Composite Decking®). Fit decking over at least 3 joists with 2 screws at every joist point and decking must be screwed down 20-40mm in from the cut end of the board.

Drill & Countersink: UK Composite Decking® can supply a SMART-BIT® that will drill & countersink in one quick operation, designed specifically for composite decking installation.
Fascia Boards: When attaching fascia boards to the side of a deck, it is important to make sure they are screwed down every 200mm (with 2 screws) along their length.
Using UK Composite Decking® Heavy Duty Posts: If you intend to use the UK Composite Decking® Heavy Duty Post  at any length over 1.4m, then you must use a 2” scaffold pole down the centre to reinforce it (pole must come up to within 500mm of the top of the post).

 UK Composite Decking® Post Caps: These are designed to be a push fit but the legs can be trimmed to suit if necessary.


Lay the boards directly onto a non-self draining surface without consulting with our technical department first.
Use hidden fixings or clips.
Butt boards up end to end closer than the specified amount.