There are two kinds of floor, suspended and slab on ground, each with its own set of problems. Suspended floors are fixed some distance above the ground. Slab on ground floors rest on the natural earth. Upper storey floors must obviously be suspended, but ground floors can be either suspended or slab on ground.

Suspended ground floors are typically constructed on a sub floor of brick or concrete piers or timber stumps, bearers and joists.

Although this was done in many older houses, suspended timber floors should not be placed too close to the ground. A clearance of at least 450 mm from the ground to the lowest part of a bearer is desirable. Anything less increases the chance of wood rot attacking the floor.

Upper floors are usually suspended on the load bearing external walls and they might also depend on internal walls for mid span support. Support at mid span reduces the depth of the joists, allowing more headroom.

Piers are usually spaced 1800 mm apart in both directions. Bearers of 100 mm x 75 mm hardwood are placed in parallel rows on top of the piers (after fitting termite shields where necessary). Joists of 100 mm x 50 mm timber are then nailed at right angles to the bearers at spacings of either 450 mm or 600 mm, depending on the flooring material. Finally, the floorboards are nailed at right angles to the joists.

Floors were once traditionally 'cut in' - that is, the walls were built on top of the joists and floorboards were cut to fit between the walls. This is still the case in full brick, stone, or concrete block houses. Nowadays, brick veneer and timber framed houses have 'platform floors' - where the flooring is fixed first and the house walls are built on top. This method makes building a lot easier and safer.

Although a platform floor can be made with regular floorboards, chipboard flooring is much easier and cheaper to install. A 'cut in' strip floor also can be replaced with a 'cut in' chipboard floor. Many houses built since about the mid 1970s have chipboard platform floors. Flooring grade chipboard comes in two thicknesses, 18 mm and 22 mm. The most common sheet sizes are 3600 mm x 900 mm and 1800 mm x 900 mm. Joists for 18 mm thick boards are spaced 450 mm apart and for 22 mm board, 600 mm.

The underside of ground floors, must be well ventilated to keep them dry, otherwise rot may form. Grilles or air bricks are placed into the walls below floor level, and gaps are left in intermediate walls so air can circulate. These openings should never be blocked up, as a draught proofing measure, for example.

The space beneath a suspended timber floor is ideal for running electric cables, water pipes and gas pipes. Whenever you are driving in nails look under the boards for pipes or cables that are in the way. Pipes and cables may also be embedded in a concrete floor. They should be sunk out of harm's way, but when making fixings use nails that are no longer than necessary.

The earliest slab on ground floors were laid down according to whatever the builder thought was best. Which often meant that no damp proof membrane was placed under the concrete to prevent moisture penetration.

Nowadays, there are strict rules about the construction of slab on ground floors.

The drawing shows how the various layers go together. The proper procedure must be followed.