What decking can I use and how far apart do the joists go?

Thickness sets the spacing

The rule is about what the board can carry between supports, so it follows thickness rather than material.

The ceiling is 600 mm (23 9/16 in) whatever the board, the way the Edmonton guide writes it.

Almost everyone builds at 16 in anyway, because it suits the widest range of decking, it stiffens the feel underfoot, and it leaves the option of changing the board later without reframing.

What a diagonal really costs

Turn a board 45 degrees and it no longer crosses the joist spacing. It crosses the diagonal, which is the spacing multiplied by about 1.41.

• Joists at 16 in, boards on the diagonal: the board spans about 22 1/2 in.
• Joists at 24 in, boards on the diagonal: about 34 in, which no deck board is supported for.
• To keep a diagonal board inside a 16 in support limit, the joists come to about 11 in, so 12 in is the practical answer.

Herringbone is worse again, because it needs blocking under every change of direction, not just closer joists. This is the calculation that gets skipped when somebody decides on the pattern after the frame is already built, and it is why a beautiful diagonal deck can feel bouncy.

Manufactured decking brings its own rules

Composite, PVC, fibre reinforced products, liquid membrane coatings and sheet goods are not covered by the lumber tables. What the permit office wants is documentation: a CCMC evaluation report listing the product uses and limitations, a testing report, or Alberta engineer stamped drawings or specifications showing code compliance. You submit a copy with the application.

After that, you follow the manufacturer's joist spacing, floor construction and preparation steps, not the lumber numbers. Two things that come up often:

• Many composites call for 12 in centres for a diagonal lay, and 16 in for a square lay.
• Stair treads are usually tighter still, because a tread carries a point load at its middle.

Picture frame borders and blocking

A picture frame border, the board run around the perimeter so no cut ends show at the edge, is the detail that makes a deck look finished. It also needs blocking under the mitre line all the way round to support both the field board and the border board. On a diagonal deck it needs blocking at every place a board lands on the border.

That is framing work, and it is decided before the decking is ordered. It is a good example of why picking the surface early saves money.

The quantities that fall out of the geometry

A materials list is more than arithmetic, and this site does not pretend to produce one. But a few numbers come straight out of the shape, and the demonstration above shows them: deck area, lineal feet of decking with a waste allowance, guard length, and an approximate guard post count.

Waste defaults to 10 percent. That is right for a straight lay on a rectangle and low for a diagonal, where 15 percent is closer, and low again for a picture frame with mitred corners. Treat the number as a check on a quote, not as an order.

Gaps, joints and fasteners

• Leave about 1/8 in between boards so water and debris fall through. Soaking wet treated lumber will open its own gaps as it dries.
• Stagger the butt joints. Joints lined up across the deck are both a weak line and a visible one.
• Land each board on enough joists to hold it. A short piece bearing on two joists at the end of a run will move.
• Use fasteners compatible with the treatment, and follow the composite manufacturer on hidden fastener systems, which often set the gap for you.
• Consider joist tape over the top of each joist. It is cheap, it goes on in an hour, and it stops the one place water sits.