The Self Build Self Help Site Questions answered

A one-stop shop of information for people interested in self build - whether self building a complete home or undertaking an extension, renovation or modification.

Questions answered
Should I use trench fill or strip foundations?
This really depends on how deep you have to go when digging the trench. If you have to go deep, then trench fill should be considered, amd may well be the cheaper option.

So why may you have to go deep? The usual reasons are:-

  • the ground is disturbed or soft, and the trench gets dug deeper until the ground at the bottom is very firm. If you can easily push a rod into the ground, you probably still need to go deeper.
  • there may have been trees on the ground, the roots of which will render the ground disturbed
  • sloping ground can be unstable, requiring deep trenches until firm ground is found
  • there are foundations of another building nearby but lower. The 45 degree rule comes into effect - the bottom of your foundations should not be at an angle greater than 45 degrees above the other foundation. Hence you may have to dig deeper.

Now, if you have a deep trench, it may be cheaper to fill with concrete - you may wish to calculate the comparative costs of concrete versus laying blocks. You will be able to calculate the cost of concrete from the trench dimensions and costs from the ready mix company.

To calculate the costs of blockwork, assume a cost of £2 per block for the brickies. (Lets make it easy and assume that this figure will also cover the sand and cement for laying the blocks and cement & aggregate for the cavity fill.)

The brickies will prefer trench fill - it is not easy standing in a deep trench laying blocks - especially if you have large feet!

Photo of window with slate sill

This is obviously an example of a trench prepared for trench fill. The depth to the top of the batten is 34 inches!
Still a lot of concrete required to get to the top of the marker sticks.

Photo of window with slate sill

Photo of window with slate sill

At the bottom of the trench can be seen a thin seam of coal. Coal can produce an acid with rain water, which will weaken the concrete - a stronger mix to cover this seam was asked for from the Readymix company
These foundations are on a sloping site, which necessitated step foundations - the concrete is stepped by securing form work into the trench.

Photo of window with slate sill

(Click on pictures for a larger view)

When should I use sulfacrete cement?
Sulfacrete is ‘Sulphate resisting portland cement’ and, as its name implies, it combats the effects of sulphates. Sulphates are naturally occurring compounds which can be contained in blocks and bricks during the manufacturing process. With normal cement, sulphates will tend to be gradually washed out by rain, forming a white deposit on the face of the blocks/bricks. Apart from being unsightly, especially on fair faced work, the cement will tend to be weakened slightly. The surface of bricks may also break up as the sulphates react with cement to create other compounds which expand and physically damage the bricks. This can often be seen in very old houses.

Walls that are to be rendered and painted will not normally be affected. What is most affected is brick work, especially where the work is in contact with the soil, ground water, exposed to salt air near the sea, and sea water.

So, to minimise the risk, use a sulfacrete cement. Also, keep the bricks dry before they are used as wet bricks will tend to release the sulphates quicker during the time when the cement is setting.

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