Repairing Concrete Floors

Filed Under: Do it yourself, Home repair, Remodeling    by: ITC

A solid floor often consists of concrete rag that has been laid on top of a layer of hardcore in the ground. This is then covered by a damp-proof membrane, which may in turn be protected by a thin layer of mortar called a screed which is leveled out arid topped by the flooring. But not all solid floors are laid directly on top of the ground.

You may find solid floors in flats and houses, at first floor level and above, as an alternative to framed floors which are supported by beams and structural walls and are usually floorboards mounted on joists.

How are solid floors constructed? You’ll only find the concrete floor surface laid directly on the soil in the oldest houses. And even then you’re more likely to find a heavy concrete slab beneath the surface flooring. If your floor is old, it will probably consist of hardcore with up to 150mm (6in) of concrete on top.

Surface finish, such as a 25mm (1 in) terrazzo screed, quarry tiles, floorboards on slim joists or wooden block flooring like parquet. Was then laid on top.

With this older style of construction little consideration was given to rising damp. Parquet blocks were sometimes bedded in bitumen, but only if the surface of the floor came below soil level would any systematic damp-proofing be used. In that eventuality, the structural slab was covered with asphalt, which was in turn covered with a further 75mm (3in) of concrete.

The design of a modern solid floor slabs allows for a much more rigorous approach to damp proofing. A 100mm (4in) structural slab is laid over a damp-proof membrane that protects the walls as well in order to provide complete protection. On top of the structural slab may come a mortar screed suitable for carpet or vinyl laying or a direct surface finish? Damp proofing is vital because although well-laid concrete. Basically water proof, it still may allow damaging water vapor to pass through without the membrane.

Cracks are the most common fault in solid floors – and are usually due to drying shrinkage or thermal movement. But, provided the cracks are no wider than 6mm (I/4in), you can repair them quite easily yourself. Minor surface damage such as roughness and unevenness is also simply repaired.

If, however: the cracks are wider than 6rnm (‘/4in), that means that something more serious is wrong with your floor – probably a crack going all the way through the slab as well as the screening – so you should call in a builder or surveyor to advise you. A still more worrying problem, especially in a direct-to-earth floor, is rising damp. If a damp-proof membrane already exists and there is still evidence of damp, the damp is likely to be localized: the result of a small break in the membrane.

This may show up only after the old porous floor surface has been replaced with some more modern flooring. The original surface would have let damp through, so avoiding damage, but as the modern ones don’t let water vapor evaporate. It tends to build up underneath and attack both the adhesive and the flooring.

So. If you are going to fit new flooring on an old solid floor, first of all check that it’s damp proof (see Ready Reference). If you do have damp. Then you can put up with your existing flooring: replace it with a new flooring of the same

All you do is slightly widen the crack by using a club hammer and cold chisel, going down to a depth of 12mm (1/2in), undercutting the surrounding floor a little. This is very important as it allows the mortar to get a much better grip on the surface than it would if you were attempting to fill a crack with smooth, feathered edges.

Clean out any dust or debris before brushing on a coat of PVA adhesive, smoothing in the mortar with a small trowel and finishing it off level with the surrounding floor. The chances are that the crack will open up again at some stage. Especially if the floor on one side of it has pushed either up above or down below the rest. The only really permanent solution is to have the floor completely replaced before you lay your new floor surface.

Inserting a damp-proof membrane the simplest way to do this is to remove the skirting boards and strip off the plaster on the walls to a point 50mm (2in) above the wall’s damp-proof course. If at this stage you find your walls don’t have a damp-proof course. It is advisable to get one put in before going any further.

Various forms of damp-proof membrane are used, ranging from thick polythene (often known as ‘1000 – gauge’ or ‘sheet 1000′), PVC, butyl rubber or polyisobutylene for large scale work (where the sheets are loose laid or bonded to the structural slab with water-resistant adhesive), to bitumen emulsion for smaller scale jobs.

These sealants are sprayed or painted on the slab in two coats. With the second one being brushed at right angles to the first to prevent thin spots and pin holes where damp could come through. To protect your room completely against rising damp you should link the membrane into the damp-proof course.

This is easy if the dip is above the floor, but it is possible that it’ll be below the level of your floor. If that’s the case then you will have to dig a small channel round the floor so you can gain access – Then all you do is paint bitumen. Either up or down the wall or fit the poly-sheets with more water resistant aches.

Some solid floors are not laid direct. Some of them are cast in situ around reinforcing steel and some are precast concrete floorboards that are locked together_ others, which are made of hollow terracotta blocks that are linked together with Meta Rods and that have the gaps filled in with concrete, are also technically known as solid floors.

You can create a damp-proof membrane by using two coats of bitumen emulsion and applying them over the concrete slab and the exposed brickwork. The second coat should be brushed on at right angles to the first.

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