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Unit: Inspecting and testing subfloors

LMFFL2004A: Moisture test timber and concrete floors
LMFFL3101A: Inspect sub-floors

Section 1: Subfloor systems

Concrete slab subfloors

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Concrete slabs are very popular in project homes and commercial buildings, especially in ground floor construction.

This drawing shows a cross section of a typical on-ground slab for a brick veneer building.

You can see that there's an external skin of brickwork and an internal wall frame in timber or steel.

Because brickwork is porous and absorbs moisture, there is a cavity between the external and internal skins to stop moisture from seeping through and reaching the inside wall.

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Near the bottom of the cavity is a damp proof course (DPC), also called flashing, which runs around the full perimeter of the building.

Any moisture that does seep in through the bricks is caught by the DPC and channelled back outside through weep holes.

The brickwork sits on a rebate in the concrete, which helps to protect the inside floor from moisture penetration.

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Underneath the slab is a waterproof membrane, also called a vapour barrier.

Its purpose is to stop moisture in the soil from being drawn into the underside or sides of the slab.

If moisture was allowed to enter, it would slowly rise to the top of the slab and cause moisture problems in the floor.

In residential construction, on-ground slabs are generally 100 mm thick, with increased thickness at the edges and wherever extra strength is required internally.

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All structural slabs are strengthened with reinforcing steel, often called 'reo' by concreters.

The steel is a combination of square mesh across the body of the slab and trench mesh in the footings and beams.

They are joined together with tie wire.

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One of the most common designs for an on-ground slab is the stiffened raft slab.

It is 'stiffened' by the beams, and is a 'raft' in the sense that it floats directly on the soil, without the need for piers or other deep footings.

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Suspended slabs are often used in upper floors of buildings.

Because of their weight, the internal supporting wall needs to be stronger than a timber frame, so they're generally supported by brickwork.

This drawing shows a cavity brick or double brick wall, with the slab protected from moisture by the cavity between the two 'leaves'.

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Sometimes in commercial buildings or multi-storey flats, the suspended slab is supported by both leaves of the double brick wall.

In this case, flashing is used to collect any moisture that runs down the inside of the external wall.

There is also a 'V' in the underside of the slab to stop water from running across to the internal wall.

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Learning activity

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Flashing is sold in rolls and comes in a variety of materials. It's not only used at floor level in a wall, but is built into the structure under window sills, on roofs (especially around chimneys, skylights and vents) and in various other places where joints might allow water to penetrate.

Do you know what materials flashing is made from? See if you can name at least three different types of materials. Which one is most common around the base of an external brick wall?

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