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Under the boards: the piles, framing and fixings that keep a deck flat
Every deck looks good on the day it's finished. Whether it still sits flat and quiet years later is decided underneath, by the piles, the framing and the fixings. That part is the same on our decks whatever timber goes on top, so it's worth knowing what it is.
What sits under our decks
Piles: 125 × 125 H5, set in concrete.
Bearers and joists: H3.2, sized to the span, with extra framing wherever a board ends at a cut.
Fixings: stainless plates, brackets and screws, and on some decks the J Vice system from Glass Vice.
Boards: pine, Kwila or Vitex, fixed with stainless screws.
The treatment classes, in plain terms
Treated pine in New Zealand is graded by where it's used. BRANZ puts it simply: H3.2 is for timber outside and exposed to the weather but not in contact with the ground, such as deck joists, bearers and decking. H5 is for timber in ground contact, such as deck piles. That's exactly how our decks are built: H5 where the timber goes into the ground, H3.2 above it.
At Torbay the framing is H3.2 SG8, SG8 being the structural grade, on 125 × 125 H5 anchor piles set in 20 MPa concrete, with stainless plates, brackets and screws throughout. The deck is about 30 m² and sits 700 mm off the ground.
2 weeks on site
Framing for a curve: Meadowbank
A straight deck is forgiving; a curve isn't. At Meadowbank the deck wraps an existing pool, so boards end short all along the curve. The frame was built with extra bearers so a joist lands on each side of the curve, and every cut board is fully supported rather than left hanging. It's 190 joists on double 190 bearers, with a cantilevered edge and a double boundary joist throughout, and every structural timber connection uses J Vice stainless fixing systems from Glass Vice.
About 10 weeks on site
Fixings: Mission Bay
The Mission Bay deck is 140 mm smooth Kwila on 140 × 45 H3.2 SG8 joists and 190 × 45 H3.2 SG8 bearers, on 125 × 125 H5 concrete piles. The connections are stainless brackets, pile kits and CPC connections, with the J Vice system from Glass Vice, and the boards are fixed with 65 mm stainless Torx-head screws. Stainless is the standard on our decks because a fixing that rusts stains the timber and loosens over time.
3 weeks on site
When the frame is kept: Titirangi
Not every job needs a new frame. At Titirangi the old pine boards were rotting, but Kevin inspected the substructure properly and found it sound: no movement, no rot, good framing sizes, good fixings, posts in good condition. It needed only some blocking to strengthen it. The clients had a builder assess the frame before anything went down, and on Kevin's advice they added extra posts and framing under the spa themselves. The new surface went on top: porcelain on the Silca System, covered in tiling over an existing deck.
What actually decides it
Ground contact. Anything in or on the ground is H5. Anything above it is H3.2. Mixing them up is how a deck ends up failing at the base.
Support at every cut. Where a board ends, at a curve, a skimmer or a step, the frame needs a joist under it. That's extra framing, and it's the difference between a clean edge and one that sags.
The fixings. Stainless costs more than galvanised and won't rust out or stain the boards. We use it throughout.
The boards on top. Grip-tread pine can go grooves up for slip resistance or grooves down to help water run off under the boards. We prefer down.
Pine or hardwood doesn't change any of this. The piles, frame and fixings cost the same under pine as under Kwila. See choosing the timber for a deck.
Where we fit
We build the whole deck, frame first, and if you have an existing deck we'll look underneath before recommending anything, because sometimes the frame is worth keeping.