Subfloor framing to NZS 3604
How a timber floor on piles is framed and braced to NZS 3604, step by step, with why each step is there and the terms it uses.
- NZS 3604:2011
Design the subfloor in its tool
The subfloor tool sizes every joist, bearer, pile and footing on your own plan. This page explains how.
The steps
Know the floor
Its load: up to 2 kPa for a house and its decks, or 3 kPa for an office or communal kitchen, which reads Section 14's tables. What the piles hold up: the floor alone, or one, two or three storeys over it. And that the site is good ground.
Why Every table below is read by the load and by what stands on the floor, so the floor's use decides each size.
NZS 3604:2011 1.1.2, Table 1.2, 7.4.1.2, Section 14
Size the joists
From the joists' table, by the floor's load, their spacing and their span from bearer to bearer: the first size reaching the span. A joist running on over two or more spans may reach a little further.
Why A joist carries the floor to the bearers; closer joists, or deeper ones, reach further.
NZS 3604:2011 7.1, Table 7.1
Choose the flooring
The boards or sheets by the joists' spacing: the wider the spacing, the thicker the flooring. Decking has its own spacing and thickness.
Why The flooring spans between the joists, so its thickness follows how far apart they are.
NZS 3604:2011 7.2, 7.4, Tables 7.3 and 7.4
Size the bearers
From the bearers' table, by the floor's load, their span from pile to pile and the width of floor they carry, which is the joists' span.
Why A bearer gathers the joists' load and takes it to the piles; a longer span or a wider strip of floor needs a bigger bearer.
NZS 3604:2011 6.12, Table 6.4
Set the piles
Each pile stands at least 150 mm above finished ground, and no taller than 1.5 m for a concrete or masonry pile, 3 m for a timber one, 600 mm under a jack stud, 1.2 m for a cantilever pile, or 600 mm to an anchor pile's highest fixing. A straight concrete pile is at least 200 mm across, a tapered one 150 mm at the top and 200 mm at the bottom, a round timber pile 140 mm and a square one 125 mm; a concrete pile longer than 750 mm takes a central bar.
Why A pile carries the floor to its footing and, standing proud of the ground, keeps the timber dry; the taller it stands, the more it bends, so the limits keep it stiff.
NZS 3604:2011 6.4.1, 6.4.2, 6.4.4
The tallest each kind of pile may stand above cleared ground, drawn to one scale. Least widths, drawn wider than they are against the piles' length. Size the footings
From the footings' table, by the bearers' span, the joists' span and what the pile holds up. The footing's bottom is at least 200 mm below cleared ground, and it is at least 100 mm thick under a precast concrete pile, 200 mm under a timber one, 450 mm under a braced pile and 900 mm under an anchor pile.
Why A footing spreads the pile's load over enough ground to carry it, and an anchor or braced pile pulls as well as pushes, so it needs more weight and depth.
NZS 3604:2011 6.4.5, Table 6.1
The footing's underside lies at least its own thickness below cleared ground. Brace the subfloor
The subfloor's bracing demand is the subfloor level's, worked out as wall bracing's is. Lay subfloor bracing lines each way no more than 5 m apart, and brace them with anchor piles, braced pile systems or subfloor walls; a floor braced by piles alone has at least 4 of them each way, around its edge.
Why The piles hold the floor up but not still: wind and earthquake push it sideways, and the bracing stops it rocking off its piles.
NZS 3604:2011 5.5.2, 5.5.6
How bracing demand and lines are worked out
Subfloor bracing lines each way, braced by anchor piles and by braced pile systems, each two piles with one brace between them along the line. Detail the anchor and braced piles
An anchor pile goes at least 900 mm into the ground to its footing. A braced pile's brace slopes between 10° and 45° (6° to a bearer or joist), is 100 × 75 mm up to 3 m long or 100 × 100 mm up to 5 m, keeps its lower end 150 mm clear of the ground, and its lower bolt no more than 300 mm up; the pile stands at least 3 times the bolt's height.
Why A brace turns the push sideways into a pull down the pile and a push on the footing, so its slope, its section and where it is bolted decide how much it can take.
NZS 3604:2011 6.8, 6.9.2
An anchor pile's highest fixing above cleared ground, and its depth below it. A braced pile system, both piles on footings at least 450 mm thick. A brace's slope from level, and its length from bolt to bolt. Check a tall house
A two-storey house on a timber floor more than 1.7 times as tall as it is wide stands on a continuous foundation wall.
Why A tall, narrow house is more prone to tipping over, which a continuous foundation wall resists where piles alone would not.
NZS 3604:2011 5.5.3.2
Show it on the drawings
A foundation plan with every pile's kind, size and footing, the bearers and joists with their sizes and spans, the flooring, the subfloor bracing lines and each element on them, and a section through the floor.
Why It is what the building consent authority checks the floor against, and what the builder sets out from.
NZS 3604:2011 Sections 6 and 7
The terms
- Joist
- A member spanning between bearers that carries the floor. NZS 3604:2011 7.1
- Bearer
- A beam on the piles that carries the joists. NZS 3604:2011 6.12
- Ordinary pile
- A pile that carries the floor down to its footing and resists no sideways load. NZS 3604:2011 6.5
- Anchor pile
- A deep pile, fixed to the floor framing, that braces the floor on its own. NZS 3604:2011 6.9
- Braced pile system
- Two piles with a diagonal brace between them that braces the floor in its line. NZS 3604:2011 6.8
- Cleared ground
- The ground once topsoil and vegetation are removed, from which pile heights and footing depths are measured. NZS 3604:2011 1.3
- Finished ground
- The ground as it is left around the building. NZS 3604:2011 1.3
- Loaded dimension
- The width of floor a member carries; for a bearer, the joists' span. NZS 3604:2011 1.3
- Jack stud
- A short stud on a foundation wall carrying a bearer. NZS 3604:2011 Table 6.3
- Subfloor bracing line
- A line under the floor along which its bracing elements stand. NZS 3604:2011 5.5.2