Bearers and joists for a deck

What are bearers and joists on a deck?

Bearers and joists are the two layers of the frame under the boards. Bearers rest on the posts or footings. Joists sit across the bearers, and the boards run across the joists. A builder’s tutorial puts it plainly: bearers run along the wall and span from post to post, and joists run at right angles to them. WoodSolutions’ guide also names a facing joist at the edge, a stringer on stairs and the bracing between posts.

How are bearers and joists sized?

From tables, not from a standard “90 by 45”. WoodSolutions’ Technical Design Guide 21 says its timber framing requirements are based on AS 1684 and points to the tables:

• Bearers: Table 49 where the deck floor is more than 1,000 mm above finished ground, Table 5 where it is 1,000 mm or less.
• Joists: Table 50 above 1,000 mm, Table 6 at or below.

Both sets assume a minimum end bearing of 50 mm times the bearer width, and 100 mm times the bearer width at intermediate bearings of continuous bearers. The guide says joists should be at least 45 mm wide in seasoned hardwood or treated softwood, or 50 mm wide in unseasoned hardwood, to avoid splitting. Joists 35 or 38 mm wide suit only proprietary fixings on the side of the joist.

Stress grades vary by timber: unseasoned hardwood is usually F11 or F14, seasoned hardwood F17 or F27, and treated softwood mainly F7. These sizes apply only to timber-board decks. A tiled deck or one with a spa needs a structural engineer.

What is the difference between joist spacing and joist span?

Spacing is the distance between joists, centre to centre. Span is how far one joist runs between supports. They answer different questions, and mixing them up is a common mistake.

• Spacing is set by the decking board. WoodSolutions’ Table 2 gives the maximum for timber boards (hardwood 19 mm at 500 mm, cypress 19 mm at 400 mm and 21 mm at 450 mm, treated softwood 19 mm at 400 mm and 22 mm at 450 mm).
• Span is set by the joist’s size, grade and the load, from the AS 1684 or maker’s tables.

The guide doesn’t spell the difference out in a single sentence, so this explanation is ours, built from how it uses the two terms.

How do you join bearers and joists?

Bearer joints must occur over supports. Joints in joists must be made over a bearer with at least 30 mm bearing for each joist. Both should be long continuous lengths spanning at least three supports. A builder in one video adds a few habits: join a long bearer directly over a post so the load goes down into the footing, bolt through the post offset from each other (he used two 10 mm galvanised bolts), brace the frame diagonally with strapping before adding joists, and sight down each joist and turn any bow upwards.

How do you protect the joists?

Put a 110 mm malthoid damp-proof course or a proprietary protection strip on top of each joist. WoodSolutions says it increases service life. Bretts sells a 50 mm by 20 m self-adhesive joist seal tape made for the same job.

What about aluminium bearers?

Aluminium bearers come with the maker’s span tables. Futurewood’s residential single-span bearer table lists 1,100 mm for a 50 by 45 mm profile at 600 to 1,500 mm bearer spacing (1,000 mm at 1,800 mm), and 1,800 mm for a 90 by 45 at 600 and 1,200 mm (1,700 and 1,600 mm at wider spacings). The table assumes a 2.0 kPa and 1.8 kN load, boards of no more than 27 kg per square metre and the maker’s own boards and brackets. See the span table guide.