Where solid floors are to be constructed, the floor area is covered with hardcore consisting of broken bricks and stone, coarse gravel or broken concrete, to within about 75 mm of the floor level. This hardcore is well tamped and consolidated and a layer of sand is spread over it to form a base for a polythene damp proof membrane. This should be laid in one piece and can be carried over the inner leaf of the cavity wall to form the damp proof course or it can be turned on to the wall and a felt or other type of damp proof material can be laid on the brickwork.
The concrete floor is then laid and tamped with a long board which will reach from wall to wall. There are two ways in which this type of floor can be finished. One is to trowel the concrete smooth and level to form a finished surface on which plastic tiles or other flooring material can be laid. This smoothing is often carried out with a power float which consists of a large rotating metal disc. Without any walls in the way the disc can be moved over the whole of the surface right into corners that would not be accessible if the walls had been erected.
The other method is to lay the base concrete to about 38 mm below the finished level of the floor. Then, when the building is complete and the roof is on and even the glazing completed so that the building is weather tight, a screed of fine concrete is laid and trowelled off by hand.
The mix used for the floor base concrete is the same as that used for the foundation’s. The mix used for a finishing screed has no coarse aggregate. It is important in both cases that only enough water to make the mix workable is used, because not only will excessive water make the concrete weak, it will also cause the finished surface to be permanently dusty.
Load-bearing partition walls are built up from the foundations at the same time as the external walls. They are usually brick or blocks and are bonded into the outer walls. Where suspended timber floors are to be installed no hardcore is needed but the ground under the floors is covered with about 50 mm of concrete to prevent the growth of vegetation and to prevent the sour smell of damp soil or rotting vegetation arising. This concrete does not have a damp proof membrane under it.
Where suspended wooden floors are constructed, the partition walls are honey-combed below floor level, that is to say 57 mm spaces are left between the bricks to allow a free passage of air under the floors. Dwarf walls also have to be constructed in the same way to support the joists which are only 127 mm deep and 50 mm thick. These dwarf walls are built at 1828 mm centres. Air bricks are also inserted in the exterior walls to allow fresh air to blow under the floor so that the timber will be protected against rot. Little ducts made of slate or asbestos should be made between the outer leaf of the cavity wall and the inner leaf, because the cavity should be filled with still air.
These little ducts should also be formed where air bricks have to be inserted in walls to ventilate larders. They are useful too if at a later date the cavity is filled with insulating foam, because they will prevent the ventilation points becoming clogged.
Services for the house should not be forgotten and provision must be made for the entry of water pipes, gas pipes and electricity cables. For this purpose holes are left in the brickwork below ground at the level at which the service will be laid and a small wooden box is constructed inside the building to provide a hole through the hardcore and concrete.
This also applies to the drains, and similar provision is made for the soil pipe. Water services should be brought to the building at least 457 mm below the surface of the ground so that they will be below the general frost level.
It is not usual to lay drains until after the heavy foundation work has been completed, because they could be disrupted by heavy lorries carrying the concrete or the hardcore, or the sand and gravel needed for making the concrete on site. In particular, lorries carrying ready mixed concrete should be given a clear and firm access to the site so that they can get right up to the foundation to discharge their load.