Timber Decking & Handrails

Timber deck fixing guide

A deck and its supporting structure are generally exposed to the weather, so it's essential to choose suitable timber for both the decking boards and the supporting timbers.

Timbers in contact with the ground should have a natural durability rating of 1 or 2 (in-ground contact) or be preservative treated to H4. Timbers above ground should have a natural durability rating of 1 or 2 (outside, above ground) or be preservative treated to H3.

Suitable commercially available species include Blackbutt, Spotted Gum, Ironbark, Turpentine, Tallowwood, River Red Gum, Blue Gum, Merbau, treated pine and Cypress.

Timber posts

Where possible, place timber posts on stirrups rather than in the ground. Stirrups provide the required termite barrier, protecting the deck and stopping it becoming a bridge for termites into the house. A termite barrier is recommended even when preservative-treated posts are used.

Decking profiles

Common decking profiles are plain, pencil round and reeded. Some decking has a reeded finish on one face. Despite the urban myths (which always sound very logical), it doesn't matter whether you fix the reeded face up or down. The choice is yours. It will greatly affect the look of the deck, but not its life span.

Decking spacing

Leave an appropriate gap between decking boards. Boards exposed to the weather, treated or untreated, take up and lose moisture as conditions change through the seasons, even when sealed or coated. The gap must be wide enough to allow for the boards expanding in width.

The right spacing depends on the board width, whether it's hardwood or softwood, and whether it was seasoned or unseasoned when fixed. Recommended spacings for boards under 100 mm wide:

Fixing decking boards to timber joists

For a weather-exposed deck, all fixings such as bolts and nails should be hot-dip galvanised, stainless steel or similar.

The two nails in each board at a joist shouldn't line up with the nails in the other boards on that joist. Stagger them so there are two lines of nails along the joist. This lessens the chance of a single line of nails splitting the top of the joist over time. Drive each of the two nails at an opposing angle to increase holding power.

Some decking nails are designed to sit proud of the board surface in an attractive way. Others are punched below the surface, leaving a void that should be filled with a suitable filler. The filler reduces moisture collecting in the void and the staining that can follow.

To reduce splitting, keep nails away from board edges, and pre-drill when nailing board ends to joists. Most decks have some butt joints. Space them randomly so butt joints in neighbouring boards don't sit next to each other.

There are two ways to protect the top surface of the joists: brush on a timber preservative before fixing the boards, or fix a purpose-made protective strip to the top of the joists.

AS 1684 Residential Timber-Framed Construction sets the minimum requirements for the spacing, size and grade of bearers and joists, and for the thickness, span, grade and fastenings of decking boards.

Fastenings for decking boards

* Hot-dip galvanised or stainless steel, 2 nails per board at each joist crossing.

Decking thickness & span

Fixing decking boards to steel joists

This isn't always successful, because the screws normally supplied aren't suited to fixing timber to steel. In humid periods the expanding timber can make the screws fail in shear. We recommend nailing or screwing the boards to a timber batten fixed above or beside the steel joist.

Finishing the decking

As a minimum, the industry recommends applying a protective coating to all surfaces of each board, including freshly cut ends, preferably before fixing. This includes penetrating finishes and film-forming coatings. The coating slows the rate at which the timber takes up or loses moisture, which considerably reduces checking. It should contain a fungicide to stop mould growing on any sugars or starches in the coating.

Tannin bleed

Most timbers contain water-soluble extractives that give colour and some decay resistance. These leach to the surface whenever moisture leaves the timber and can be washed onto other surfaces, leaving stains that are hard to remove from brickwork and concrete. To reduce staining, use seasoned timber and coat all surfaces, including freshly cut ends.

Avoid tools that may leave fine iron filings on the timber; filings left on the surface can cause unsightly black stains. Angle grinders cutting bricks throw off iron filings from the metal mesh in the cutting disc.

Resin bleed

Some softwoods such as radiata pine and slash pine can be prone to resin bleed, although only some boards are affected. If a board shows obvious signs of resin bleed, don't use it in the deck, or fix it where the resin won't be a problem. Sometimes it only shows up after a spell of hot weather; the resin can then be cleaned up or the board replaced.

Bushfire-prone areas

Building a deck in a bushfire-prone area? Specific species and construction requirements apply. Contact us for advice on suitable timbers.