Mar 082013

Concrete Slab Step-by-Step

Whether you are casting concrete for foundation footings to support a simple timber structure, or for a solid slab that will form the floor of a garden building to be built from wood, the principles are exactly the same.

Of course, if the slab is to act as a foundation, it must be designed and constructed to carry the load of the brickwork, timber and any other materials used (including the roof structure). Also, the method of construction will affect the size and type of all foundations and footings.

This particular slab was cast to support a timber stud structure and a 75 mm-thick slab was laid on well-compacted fill. Low-strength concrete (cement, sand and stone mixed by volume in the ratio 1:4:4) is suitable for most garden buildings, and it was this mixture that was used.

The National Building Regulations state that a straightforward foundation like this one should be constructed using concrete that has a compressive strength at 28 days of at least 10 MPa. The ratio the regulations recommend is slightly weaker than the one we used, namely 1:4:5.




concrete slab punner

1. Build the foundation walls on concrete and allow the mortar to set. Fill with hardcore and sand, moisten and compact well with a punner or ramming tool, leaving about 75 mm of blockwork above the fill.

damproof sheeting

2. Spread polythene over the hardcore, overlapping all joins by 100 mm. This damp proof membrane forms a moisture barrier and is an essential measure to take against rising damp.

pour concrete

3. Working in batches, mix sufficient cement with sand and 19 mm stone in the ratio 1:4:4 (or 1:4:5) and enough water to make it workable but not wet and slushy. Pour the mixture over the plastic up to the top of the blockwork to form the stab.

level and smooth concrete

4. Use a straight-edged length of wood to compact and level the concrete so that it meets the top of the foundation walls. Otherwise, if you are going to be erecting panels as we did, they simply won’t fit.

insert strapping

5. While you are laying the slab, bend and insert galvanized hoop-iron strapping or steel strips in the concrete (or hollow blocks) at regular intervals around the perimeter, to coincide with the studs.





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