Preparation and setting out for construction work

Although the majority of us would claim that we know very little about architecture, we are all very interested in buildings, because we have to live and work in them. We get our first introduction to building when as children we are given wooden blocks to play with, or the patent clip-together construction sets. We learn the crude basics of proportion when we find that our out-of-proportion buildings tend to fall down. We teach ourselves to put lintels over openings and we get an idea of bonding the blocks.

Later, when we are the proud owners of our own houses, the urge to build returns; we want to lay bricks, tile walls, build extensions and garages. This is the point at which building begins – when the need for a structure to meet a special purpose is first recognised.

The architect’s drawing board is the next stage, when the plans are drawn for official approval and the structure is designed to meet both the requirements of the user and the Building Regulations, which are intended to ensure that the finished structure will be safe and sound.

For the craftsman and the handyman with an urge to build, to create his own environment, building starts with the practical work on the site. Whether the structure is to be a block of offices or a small dwelling house, the first step is to clear and level the site. All vegetation and top soil are removed from the area where the structure is to stand. This can be done by hand for an extension to the living room of a small house, but for a building the size of a house, or larger, mechanical aid is needed. Ground preparation for a dwelling consists usually of digging out the foundations to the specified depth and clearing the earth from the floor area. Even if the ground floor is to be suspended timber, the soil still has to be removed and the site covered with concrete.

At one time, only those trees that would actually interfere with building operations were removed, but since the severe drought of 1976 there has been a lot of panic over the possible effect of trees on the stability of the surrounding ground and, therefore, on the foundations of buildings in close proximity to trees.

Unfortunately, all damage of whatever nature which occurred to buildings during what was probably the driest summer for 200 years, was blamed on trees. Building Research Establishment papers first published in 1947 and based on shallow foundations only 425 mm deep were used as a guide to the positioning of trees around buildings some thirty years later in 1977 when shallow foundations were 1 m deep.

In 1953 it was suggested that the height of a tree was a rough guide to the spread of its roots and that a house should be kept away from trees a distance equal to the mature height of the tree. Where there were groups of trees the distance was increased to one and a half times the height of the trees. This extra distance was applied to single trees of popular, elm and willow, and in this case, where there were groups of trees, it was suggested that the distance should be two and a quarter times the mature height of the trees.

As shrinkable clay covers a large proportion of the country it is important that the tree problem is kept in perspective. Although a mature tree can take 20 000 gallons of water from the ground in a year, there is evidence to show that much of the damage that occurred during the hot, dry summer of 1976, could have happened even without the presence of trees and there were, in fact, cases of damage where there were no trees at all.

When clearing the site for building it is necessary to remember that if a mature tree is removed, the water table will rise and this will itself bring about some movement of the ground caused by the clay swelling as it takes up the water, and damage can also be caused by this clay heave.

It would seem sensible, therefore, to leave mature trees as they are, if they do not get in the way of the proposed building. They will have already produced a stable ground condition and it will be necessary only to take the foundations down perhaps a little deeper than the usual 1 m depth. Where new trees are planted on the site, it should be borne in mind that they will not reach maturity for 60 to 80 years and by that time the dwelling will be reaching the end of its useful life.

When trees are removed from the building site, it would seem prudent, therefore, to cut them down and clear out the stumps as far in advance of building operations as possible. In this way the ground water can stabilise at a new level and the clay can swell without the pressure displacing the building foundations or the solid floors.

The principles involved in setting out buildings are quite simple and few implements are necessary to carry out the work. A large wooden square, plenty of string and a number of stout wooden stakes about 50 mm x 50 mm, a long level and a long straight edge with parallel sides will be needed. Accurate setting out of large buildings is done by means of a dumpy level which is basically a small telescope that can be swivelled in a horizontal plane. The object lens has a hair-line cross on it so that a level reading can be taken on a surveyor’s staff which is marked in feet and inches and placed some distance away. A theodolite can also be used as it is a similar instrument, the telescope of which can also be moved in a vertical plane, but these are not essential for small works like garages and extensions. It is quite possible to set out the average dwelling house without using any sophisticated equipment.

Levelling has to be carried out from some fixed point called the datum. It is usually marked by a wooden peg driven into the ground until the top is at least 150 mm above the highest point of the building area, which is of course the damp proof course level. The datum peg can also be set to a given height above the invert of a drain. The invert is the bottom of the drain and is measured in the drain channel where it passes through an adjacent inspection chamber.

The datum peg must be clear of all building work and is cemented into place to prevent it being moved during building operations.

The large wooden square which is used to set the corners of the building at right angles can be made out of 50 mm x 25 mm batten, using the 3:4:5 formula well-known to all builders. Any triangle which has sides that bear the same length ratio as the numbers 3, 4 and 5 will have one right angle. Any unit of length can be used, I.e. centimetres, metres, feet or yards. A handy size of square is 3 ft x 4 ft x 5 ft. The three pieces of timber can be cut off to these lengths, or the sides can be made a little longer, provided that the third piece is fixed in the correct place.

A halved joint is made at one end of each side batten and these are then screwed together temporarily using one screw. The hypotenuse piece is then placed on top of the two side battens so that its outer corner is exactly in line with the outer corner of the two side pieces. If the sides have been cut longer than the required unit length, then they must have marks made at the required ratio length and the hypotenuse outer corners placed on them. The joints can then be marked at each end and the halvings cut out. Alternatively, the batten can simply be screwed into place on the top of the side pieces and the second screw driven into the right angle halving.

For a strong job the timber is taken apart and the joints cut and glued with a waterproof adhesive so that the square will not be affected by the weather and will withstand rough usage.

Another simple piece of equipment that can be made is a set of boning rods. These are formed out of two pieces of batten 50 mm x 25 mm, or for a stronger job 75 mm x 25 mm, which are fixed together in a tee-shape. They may be 1 m or 1.2 m high with the top cross piece 457 mm to 600 mm long. A set comprises three rods all exactly the same size. They are used for levelling by standing one on a fixed point such as a datum peg. A second rod is then held on a distant point, level with the datum peg. This point will have been fixed by sighting with a dumpy level or by using a spirit level and straight edge. The third boning rod is then used to produce as many other points between these two extremes as required, simply by holding it in place and sighting over the tops of the boning rods. Boning rods are also useful for lining up drains to the correct falls. A variation is to level a peg, using a straight edge and long spirit level, as far from the datum as possible. Then a boning rod is stood on each of the pegs and, by sighting over them to the third rod, a third peg can be levelled at almost any distance from them.

A long straight edge is another item that can be made to aid setting out. A 2440 mm straight edge has to be at least 100 mm wide and if the length is 3 m or more, it will have to be 150 mm wide, otherwise it will have a natural sag that will prevent an accurate level being taken.

The wood must be about 25 mm thick, straight grained and free from knots or heartwood that would make it twist under site conditions. The edges are planed square and straight. It is tested by using it to draw a straight line on a sheet of plywood, plasterboard or on the floor. The straight edge is then turned over and brought up to the line from the other side. If it is accurate the edge and the line will match. If the straight edge is hollow it will show up by meeting the line at the ends but not in the middle. If the straight edge is bowed then it will cover the line in the middle and a new line is drawn to show how much bow there is. The straight edge can also be reversed end-to-end to get it even more accurate.

Setting out

Setting out begins by fixing the datum peg from which all vertical measurements are taken. This peg must, obviously, be driven into the ground at some point where it will not get in the way of subsequent building operations. Its height is established from some permanent level which should be marked on the plan. It may be a surveyor’s bench mark which is chiselled into a wall or on a kerb stone. Another place for obtaining a level is the invert of a drain. This will entail lifting the cover of an inspection chamber and lowering a surveying staff on to the open channel or invert, then taking a reading with a dumpy level.

The dumpy level is basically a small telescope mounted on a tripod. The object glass has crossed hair lines for sighting on to the surveying staff. The level must be set up carefully and levelled in all directions to get a correct reading. It has a spirit bubble and adjusting screws to enable this to be done.

Having taken a reading from the invert, the dumpy level is turned on its pivot and used to level the datum peg which is driven to a given height above the invert. When it has been levelled accurately the datum peg is concreted around to ensure that it will not get moved during the construction work.

Additions and extensions to existing buildings usually employ the damp proof course of the main buildings as the datum point which enables the heights to be matched for floors and window sills. But it is often more convenient to set up the datum peg well clear of the structure than to try to level from the d.p.c. for every measurement that has to be taken later.

The building line is established next. This is a line fixed by the local authority and marks the position of the front main walls of buildings. Only small projections such as porches and bay windows are allowed in front of this line. In built-up areas the position of the building line can be seen by the position of the existing buildings, though in old streets the authority may have moved the line back in order to provide space for future road widening. This must be established before planning the project, otherwise a situation could arise where some or all of the new building would have to be demolished.

In the case of a detached or isolated building, great care must be taken to ensure that the positioning is correct as it may not be possible to line up the new building with existing ones. The position must therefore be checked by measurement from the road, or from other fixed points which are marked on the plans.

Setting out these stages in building is critical, as mistakes would reflect throughout the rest of the operations and it would be disastrous if, because of failure to check with the local authority, it was discovered at some later stage that the building was out of position.

When the datum peg and the building line have been established, the profile boards are set up. These boards mark the corners of the building and are set up well clear of the foundation trenches so that they will not interfere with the building operations. This is especially important if a mechanical digger is to be used.

The boards must be long enough to span the whole width of the foundation trench because on them will be marked the width of the foundation trench and the face of the brickwork and for cavity walls the inside face of the inner wall will be marked also.

The boards can be set up separately or they can be joined together on one of the pegs so that they form a right angle. Minimum sizes are 50 mm x 50 mm for the pegs and 100 mm x 25 mm for the board itself.

First, the front line of the building is marked by a string line and then a pair of pegs is driven to mark the front corners of the building. These are used as guides for positioning the profile board. From the front wall line, measurements are taken for the rear wall of the building and a parallel line is set up.

On this line the rear corner pegs are driven. Lines are then set up to mark the side walls. These must be at right angles to the front and back walls and are set out by using the large wooden square. A final check is made by measuring the diagonals of the building. These must be the same, and if they are not, the measurements must be checked to ensure that the front and back lines are parallel and that the sides are also parallel.

The rear profiles are then fixed and the positions of the brickwork and the foundation trenches are marked on the top edge of the horizontal board. The foundation concrete is marked separately if it is different to the trench width. When the accuracy of all the lines and angles has been checked, the marks are cut into the top of the profile boards with a fine toothed saw, so that they cannot be rubbed out or otherwise obliterated.

This completes the setting out for the main walls of the building. Intermediate walls are set out in the same manner except that they will most likely be half-brick in thickness and only one line marking the face of the brickwork will be needed.

Non-load-bearing partition walls made of lightweight blocks do not need to have deep foundations; they can be built from the concrete floor slab.

At this stage the foundations for the projections such as porches and bay windows are set out. Centre lines are essential for this work if the setting out is to be accurate. Curved walls can be set out with a trammel, which is simply a batten pivoted on a wooden peg driven into the ground at the striking point of the curve. An alternative is to make a template out of timber. The curves are cut out of wide boards about 25 mm thick. These are joined together and braced with 50 mm or 75 mm wide struts. A template can only provide the outer curve pattern for checking and adjusting the brickwork. A trammel can have all the setting out lines marked on it – the trench width, concrete width if different from the trench width, outer face of a cavity wall and the inner face of the cavity wall.

A trammel is only as accurate as the positioning of the axis peg, which must be perfectly plumb. The higher the peg the more movement will take place making the shape of the curve vary. A trammel is therefore best used in the early stages for setting out the ground and guiding the brickwork up to damp proof course level. From that point it would be better to use a wooden template to ensure the correct shape of the curve.

Similar Posts