Checking For Damp In Floors

Solid floors

Solid floors, whether of concrete, tile or brick are often damp and may also sink and create an irregular surface. These may crack or disintegrate, especially at the edges. Correcting these defects may not be easy, because the basic problem may be that the floor was originally put down over too shallow and too wet a foundation. There will probably be no proper damp-proof course.

To find out whether a solid floor is damp use the following simple test.

Make a ring of putty or modelling clay roughly 3 ins, in diameter. Press the ring to the floor. Over this ring place a small piece of clean, dry glass. Press the glass firmly down onto the putty and it will form an airtight seal. Leave this glass and putty in position for 24 hours. If the floor is damp water that has risen through will be found condensed on the underside of the glass.

If on the other hand no condensation is found under the glass then the floor can be taken to be dry at that point. This test should be repeated on a number of points over the floor to make sure that any damp is not localised.


If your floor does show signs of damp damage, then you have two main alternatives. Firstly you may cover the floor as it is with a damp-proof coating, at the same time filling up any obvious depressions. This will give a satisfactory job provided the work is very carefully done and that the damp is not too severe.- Secondly, especially where sinkage is a real problem you will probably have to take up the entire floor and replace it with new concrete, including a damp-proof course.

Taking the simplest method first, provided the surface is relatively flat, you can use an ordinary bitumen paint. Two or three coats of this are particularly good over concrete which is showing only slight signs of damp. Before starting, fill any holes or depressions with cement made from 1 part of cement to 3 parts of sand. Do not make this too wet, and smooth it very carefully into place. Pay particular attention to the extreme edges of the floor and to entrances. Painting a floor, though, will certainly not serve where the depressions are deep and also where the surface is not of concrete but of bricks or tile. For these the best surfacing is given by a special waterproof screeding such as one made by Dunlop which we show in the pictures. This is simply mixed from a powder and a liquid, spread over the surface and allowed to dry. There is a certain amount of practise needed in getting an even, thin coating but to some extent the liquid is self-levelling and we can recommend it for use on floors with minor damp.

For really bad cases though, there will be no alternative but to lift the floor and to re-lay a new one.

At first glance this might seem to be a very difficult operation but in fact, if systematically tackled, there is nothing in the job that the ordinary handyman cannot face. Of course, there is a considerable amount of physical work involved both in removing the old floor and in mixing and laying the concrete of the new one. Technically, though, there are few problems.

First of all decide whether or not you have to take up the original floor at all.

If you can make space of three inches or so above the original floor, then you can lay a completely new floor on top of the old one. This naturally saves a great deal of work. Unfortunately, you will often find that doorways and other factors will not permit the raising of the floor level to this extent. You will then have to excavate the old floor to gain four to six inches depth.

One type of old floor that is sometimes found is the brick floor. In this case, removal of the bricks will automatically give you a four inch depth, nearly enough for the laying of concrete, and you will be surprised how easily a brick floor can be removed. Once you have got the first few out, the others can practically be lifted out with bare hands.

Quarry tiles used to be extremely popular and these too can be fairly readily removed once the first have come away. Often the cement in which they are bedded is fairly light mortar and it will be found to crumble away easily.

The sequence of events in removing a solid floor and replacing it with a new one is as follows. Remove the whole of the old floor to a flat depth of roughly 6 inches all over. Next, having prepared your foundation, lay in a 3 to 4 inch deep layer of concrete well rammed and levelled. Over this spread your damp-proof course. This is a sheet of heavy polythene which can be purchased in very large sizes indeed (enough to cover most modern floors in one unbroken piece). Cover the whole floor, allowing roughly a foot at every side to rise up the walls. Chisel a slot around the walls under the plaster about 6 inches above the final surface level of the floor. Tuck the plastic edges into this slot.

Finally a 11 to 2 in. layer of finer cement is spread over the plastic sheet and up the wall sides. This gives you a solid walking surface which will be perfectly dry at all times. To it you can glue tiles or sheet vinyl or indeed apply any sort of floor covering in complete safety. The cost of the job will be far less than that of having it done professionally and, given proper care, will be just as satisfactory.

A simple solution:, laying a surface of water-resistant plastic

1 The floor surface must be completely free of all traces of wax or dirt. Use a very strong solution of detergent to clean away traces of these, particularly of floor polishes. Old quarry tiles in particular will often have absorbed large quantities of polish and may take several washings and dryings to get them clean.

2 Semscreed is a commercial product supplied half in liquid and half in powder form. You will need a largish container in which to mix the material. First pour in the liquid.

3 Now stir in the powder part.

4 Continue stirring very thoroughly until you obtain a heavy plastic mass.

5 This is easy to spread over the surface of your tiles. The thickness required is only in. but of course you will have to level up any irregularities.

6 Allow the plastic to harden (which takes a very short time) and then, using plain water and a steel float, rub the entire surface to achieve a smooth surface. The wet plastic is practically self-levelling and this final work is by no means as difficult as it may appear. The result will be an almost dead flat, smooth and waterproof surface on which almost any floor covering can be satisfactorily laid.

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