Self-Build Housing - The Complete Works by Brian Adams

Author:Brian Adams [Adams, Brian]
Language: eng
Format: epub
Publisher: Brian Adams
Published: 2015-03-10T22:00:00+00:00

3.2 SUBSTRUCTURE

3.2.1 FOUNDATIONS

Whilst, with a relatively light timber-frame building, foundation loads are not high, foundation concrete nevertheless has to be of a good, consistent quality. Many builders – both amateur and professional – nowadays use ‘ready-mixed concrete’ in preference to mixing it themselves on site, the saving in time and effort more than offsetting the increased cost. This assumes that a lorry can get reasonably close to the trenches, otherwise a lot of hurried wheel-barrowing will be required. (Concrete can be pumped across a site, but the plant needed is unlikely to be affordable by the self-builder). The concrete’s strength (expressed as a ‘C’ number – a figure related to its ability to withstand crushing) as stated in the drawing or specification – is given to the ready-mix supplier with the order so the strength will be guaranteed. The quantity to be supplied is measured in cubic metres (cu.m) – easily calculated from foundation width(s) x depth(s) x length (with a maximum of 7cu.m per vehicle load). Note however that timing is very important - the time from the introduction of water into the sand/cement/stone mix (carried out whilst the vehicle is on its way to your site) to placement – should not exceed 30 minutes, otherwise the concrete will start to set and should be rejected on arrival. This therefore means that you must be ready (with helpers, wheelbarrows, shovels, etc) as soon as the vehicle arrives. Get the concrete roughly into the trenches first, and worry about compaction and levelling later. The lorry driver will not want to hang about either – concrete setting in his drum is seriously bad news! As quickly as possible thereafter, spread the concrete throughout the trenches, to a thickness of at least 200mm; if too much has been poured, thicken the foundation evenly with the excess. Compact it by ‘tamping’ it down using a short plank with ropes or poles at either end (so you don’t have step down into the concrete) to remove any voids. Mechanical vibration is very unlikely to be required, but do not over-do the tamping as the solids and water will start to separate, creating a weaker mix; the concrete surface does not have to look pretty. If more than one load is required, succeeding lorries should be timed to arrive at around an hour intervals, to give you time to spread it as described.

To avoid unforeseen problems with varying soil conditions and careless workmanship, it is common to include steel reinforcing rods in the foundation strip. These will again be specified by the architect or engineer, and can be ordered from most builders merchants or steel fabricators. They are simply laid overlapped along the trench and supported some 50mm off the bottom (with clean stones or brick under) and the concrete is poured around and over, taking care not to disturb them. Once the concrete is in place, they can be forgotten about.

Steps in foundations to accommodate sloping ground, etc, should not exceed 200mm high (or

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