Timber

Subfloor framing timber was typically boric-treated pinus radiata, although heart rimu was used in some areas for bearers. The introduction of NZS 3604 in 1978 saw the requirements for bearer and joist size, span and spacing clearly specified.

Back to top

Bearers

All subfloor framing was separated from the concrete (or concrete blocks depending on the foundation wall construction) by a bitumen-impregnated fabric damp proof course (DPC), to protect the timber from moisture. Timber used for subfloor framing was commonly wet or green.

Bearers were typically 100 x 75 mm or 100 x 100 mm and were laid on edge over DPC and piles. A bearer or a 100 x 50 mm wall plate was laid on DPC along the top of the foundation wall in the direction parallel to the bearers. Bearers at right angles to the wall were end supported on the wall. Some early houses may have used the 1950s/1960s detail where bearer ends were supported on a pier or half-pile cast with the foundation wall. In the body of the floor area they were either supported directly by the piles or by jackframing (Figure 1).

Figure 1: Jack framing on piles.

Joins to bearers were made over piles using a halved or a splayed joint. Bearers were secured to piles with No. 8 standard wire gauge (SWG), galvanised mild steel wire ties that were embedded in or threaded through the piles and folded over and stapled to the bearer. Bearers or plates were generally fixed to foundation walls with 9.5 mm diameter mild steel dowels or bolts that were cast into the top of the wall.

Towards the end of the 1970s, with the introduction of NZS 3604, pile to bearer connections were bolted with 12 mm galvanised bolts or fixed using the newly developed 6 and 12 kN subfloor connectors.

Back to top

Floor joists

Floor joists were typically 150 x 50 mm or (200 x 50 mm, but the size could range between 100 x 50 mm and 250 x 50 mm according to span). They were generally spaced at 450 mm centres across the bearers and supported at their ends on the foundation wall plate or bearer fixed to the top of the concrete wall. Long span joists were often propped until the timber was sufficiently dry to prevent sagging.

Perimeter joists were doubled with packing between to give a separation of 25-50 mm to provide support for the wall and floor framing (where a timber strip floor was laid inside the wall framing). Where internal loadbearing walls were parallel to the floor joists, joists were also doubled (Figure 2). 

Alternatively, 100 x 50 mm soldiers or short jack studs spaced at 450 mm centres and supporting the bottom plate of the wall framing, were sometimes used. Joist ends could be trimmed by a continuous boundary joist or have solid blocking between joists.

Figure 2: Floor framing under loadbearing wall.

As floor framing was constructed from green or wet timber, shrinkage was an issue as the timber dried, particularly with joists deeper than 150 mm, and this often resulted in squeaking floors, or, where joists were not propped until dry, sagging.

If joists required stiffening (this depended on the live load of the floor and the span of the joists), solid blocking was typically installed at midspan. Solid blocking replaced the herringbone strutting of earlier periods as it was easier to install, and consisted of 50 mm wide blocking that was the full depth of the joists.

An issue that arose with solid blocking was that where green timber was used, the longitudinal shrinkage often resulted in small gaps between the end of the blocking and the floor joists, and this contributed to floor squeaks. Herringbone strutting could always be tightened up later if accessible.

Joints to floor joists were made over bearers, either by lapping the joist at least 300 mm, or by butt jointing with a flitch plate.