Decking screws

Which decking screws should you use?

Stainless steel. PTS, which supplies SPAX decking screws, recommends A2 stainless (equivalent to 304) for general decking and A4 (equivalent to 316) for coastal and salt-exposed sites, because A2 copes poorly with chlorides. It also advises pre-drilling in dense hardwood and within about 50 mm of a board end, where boards split.

What screws do composite boards need?

The maker’s. A composite warranty often names the fixing.

NeoTimber sells starter clips and 3 and 6 mm clips in packs of 100 with matching screws. Eva-Last says grooved boards take clips that give a 5 mm gap, and square-edge boards need spacers. An installer who face-screws even grooved boards says it ties the frame together better, using colour-matched screws and pilot holes with heads set flush; he admits hidden clips are quicker and cheaper.

How many screws does a deck need?

Count crossings: the number of rows times the number of joists. The decking calculator estimates two screws per crossing for a face-fixed deck and one clip per crossing for a clip system, and it prints the working. For a real build, one UK DIYer spent about £272 on fixings for a 23.5 m² deck, of which stainless deck screws were about £90.

What do screws cost?

Ultra Decking, a composite supplier, puts clips and screws at about £8.24 per m² of deck. One builder found the same screws about three times the price in a big DIY store compared with a specialist online. Both are anecdotes, not market prices.

What about screws into the frame?

Frame fixings are different from board fixings, and joist hangers and concrete screws are separate items. Joist tape goes on the joist tops under the boards; see joist tape.

How do you fasten composite decking to the frame?

By board type, from the makers:

• Trex’s UK site says to use two screws per joist, installed flush with the surface, never to countersink, and never to use ordinary exterior timber screws with composite boards, because they push the material up and cause a mushroom effect. Its post is written in imperial units from the US, so use your UK guide for sizes.
• Millboard’s fixings page says its Durafix A2 stainless screws have a six-point torx head with a T15 bit included, sit virtually hidden beneath the surface and need no pre-drilling or countersinking.
• Eva-Last’s square-edge boards take top-fix screws at 48 per m², according to its brochure.

Where the board is grooved, clips do the fixing instead; see clips.