Foundation walls

Foundation walls were typically concrete or concrete block, and may have had either ventilation grilles or concealed ventilation.

A continuous perimeter foundation wall could be:

  • an in-situ, reinforced concrete wall with ventilation grilles
  • an in-situ reinforced concrete wall where the floor joists are cantilevered past the line of the wall to allow air flow into the subfloor between the joists
  • a reinforced concrete block wall with pre-cast concrete ventilation grilles (or cantilevered joists as above).


In-situ reinforced concrete walls with ventilation grilles

In-situ reinforced concrete walls were typically 125 mm thick for single-storey construction and 150 mm thick for two-storey construction. If the height of the foundation wall was more than 1.8 m above ground level, it was considered as the lower of two storeys, and therefore 150 mm thick.

Foundation wall footings had a minimum depth into the ground of 300 mm and were a minimum height of 300 mm above ground level (Figure 1).

A wall less than 150 mm wide had reinforcing typically consisting of 12 mm diameter deformed mild steel reinforcing bar in the footing and 10 mm diameter deformed mild steel reinforcing bar at the top of the wall.

A 150 mm wide wall was typically reinforced with 2 x 10 mm diameter deformed mild steel reinforcing bars in the footing and a 12 mm diameter deformed mild steel reinforcing bar at the top of the wall. If the wall height was more than 900 mm above the footing, additional 10 mm diameter reinforcing bars were installed at 375 mm centres both ways.

The top and bottom of the foundation walls were required to be horizontal and so on sloping sites the foundation walls were generally stepped and often extended in height with timber jack framing. The length of the concrete overlap of the step was equal to the height of the step or a minimum of 600 mm and had continuous reinforcing.

Bolts or steel dowels were embedded at 1400 mm centres into the top of the concrete walls to provide fixing for the bearers or top plates. Steel dowels were simply mechanically bent over the top plate after it was installed (recent BRANZ testing under load conditions has shown that this method of anchoring bottom plates has limited capacity when compared to a bolted-down bottom plate).

Precast concrete ventilation grilles, which were fitted into openings cast into the foundation wall, provided subfloor ventilation. They were spaced at 1.8 m centres maximum around the perimeter walls and a maximum of 750 mm from the corners. This is the same as current ventilation requirements - see Foundation and subfloor- subfloor moisture and ventilation.

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In-situ reinforced concrete walls with concealed ventilation (no grilles)

Instead of inserting ventilation grilles into in-situ concrete foundation walls, concealed ventilation was sometimes used. The method of construction and requirements for reinforcing foundation walls or construction of jack framing were the same as when ventilation grilles were installed.

Ventilation was provided in two ways:

  • the bearer or plate was laid along the top of the foundation wall (over a damp proof course or DPC) with a 12 mm overhang. The ends of the bearers were also laid with a 12 mm overhang and 75 x 50 mm packing was cut between them. The floor joists and timber framing extended beyond the sides and ends of the bearers by 25 mm and 12 mm packing pieces were fixed at 450 mm centres to the bearers and plate creating a 12 mm gap through which air could still pass and enter under the building after the cladding had been fixed
  • the bearer or plate was installed to the top of the foundation wall, and the floor joists (and bearer when joists at right angles to the foundation wall) were cantilevered 200-300 mm past the line of the wall, allowing air to flow between the framing into the subfloor space - see subfloor framing.


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Reinforced concrete block foundation walls

Reinforced concrete block foundation and/or basement walls constructed from 200 mm wide hollow concrete blocks became common during the 1970s, both for low foundation walls and where a basement garage was required (Figure 2).

The concrete blocks were laid on an engineer-designed in-situ reinforced concrete footing in a stretcher bond pattern.

Horizontal and vertical reinforcing was generally likely to be installed, although R.J. Willson in the 1974 metric edition of Timber framed construction noted that blocks could be unreinforced for a low foundation wall for single-storey construction (although a reinforced concrete footing was required).

The block cells that contained reinforcing were filled with grout (cement and fine aggregate), and bolts or steel dowels were embedded in the bond beam at the top of the wall to provide fixing for the bearers or plates. Some walls may have had all cells grout filled, but this was not always done. Unless ventilation grilles or gaps were allowed for (Figure 3), or the joists were cantilevered to provide ventilation over the top of the wall, the subfloor spaces where concrete block foundation walls have been used may be inadequately ventilated.


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Brick foundation walls

The 1970 imperial version of Timber framed construction also gave details for brick foundation walls with integral brick piers, although these would have been relatively uncommon where a weatherboard cladding was used.