Timber Deck Bearer Spacing Over Soft Ground
You are building a detached timber deck, 5.4 m by 4.2 m, attached to the house on one long side and free-standing on the other three. The ground is reactive clay that moves with moisture, and the deck is only 400 mm above grade, so there is limited room for bearers and joists. The client wants a low-maintenance finish and has chosen a composite decking board. The bearer and joist sizes have been specified by an engineer, but the engineer's span table assumes bearers are supported at 1800 mm centres. The digger has hit a buried service trench along one edge, so one bearer line cannot be founded where the plan shows it.
1. How would you resolve the bearer that cannot be founded where planned, without changing the engineer's spans?
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The engineer's span table is only valid if the supports are where the table assumes. Moving a bearer line or leaving a longer unsupported span invalidates the design, so the first rule is: do not simply shift the bearer and hope.
Options, in order of preference:
- Re-route the bearer around the trench with a doubled bearer or a deeper section, then have the engineer confirm the revised span. This keeps the support spacing but changes the member, which needs sign-off.
- Bridge the trench with a concrete strip footing or a galvanised steel beam founded either side of the trench, so the bearer line stays where the plan shows.
- Add an intermediate support (a pier) between the existing lines so the effective span reduces, which is conservative but needs the engineer to confirm the pier is founded below the zone of influence of the trench.
What you must not do is cantilever the bearer over the trench or pack it up on loose fill. On reactive clay, a pier founded in disturbed trench fill will settle differentially, and the deck will rack or the composite boards will pop. The judgement is to treat the trench as a founding problem, not a layout problem, and to get the revised detail from the engineer in writing before proceeding.
2. The composite decking has a different expansion behaviour from timber. How does that affect the joist spacing and the fixing detail?
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Composite boards move more with temperature than timber and move differently along the board than across it. Two consequences matter here.
First, joist spacing: composite manufacturers publish a maximum joist spacing, often 400-450 mm for a board laid across the joists, and sometimes tighter for boards laid at 45 degrees. The engineer's joist spacing was set for structural load, not for the decking's deflection limit, so the tighter of the two governs. If the engineer's spacing is wider than the decking allows, add joists rather than argue.
Second, fixing and gaps: composite boards need a gap at butt joints and at the perimeter to allow expansion, and the gap depends on the board length and the temperature at installation. Fixings should be the manufacturer's clips or screws in slotted or oversize holes so the board can move. Do not butt boards tight against the house or against a fascia, and do not use a fixing that clamps the board.
The judgement is that the decking is a separate design layer from the structure. The engineer's spans govern the frame; the manufacturer's instructions govern the board. Both must be satisfied, and the tighter one wins.