Self Build Structural factors

Not all building materials are able to resist compressive and tensional forces equally.

Brickwork, concrete and masonry are very strong under compression but have little ability to resist tensional stress. This means that they are very useful at supporting dead loads, provided the line of action of the force does not pass outside the column or wall which the material forms; otherwise tensional forces will be set up, which the material is unable to resist. For this reason, brickwork, blockwork and masonry are unable to resist bending or – eccentric – forces.

Steel and timber, on the other hand, are equally strong under conditions of compressive or tensional stress and thus suitable for use to resist bending stresses. They are therefore widely used for spanning functions.

It is often economical to combine the compressive strength of concrete with the tensional strength of steel in the form of reinforced beams, lintels and fire-resisting floors.

The structure of a house makes it stable when carrying vertical loadings. It is also able to withstand horizontal loadings, such as wind pressure when these forces are applied from the outside. However, the structure has little or no ability to resist spreading pressure from inside the building because there is, in most cases, no external buttressing to the building.

It is most important that no spreading loads are allowed to build up inside the structure at right angles to the faces of the walls. This can usually be prevented by incorporating timber or steel ties in the structure or by buttressing between the cause of the spreading force and the wall.

Common causes of spreading forces are pitched roof structures and arches. These elements must be properly constructed and sited to eliminate the risk of eventual structural collapse.

Roof structure

There are four types of roof commonly found:

Pitched roof.

The roof structure consists of sets of triangular frames.

With pitched or double-pitch the greatest structural danger is that is a tendency for the feet of the rafters spread outwards and thus overturn the external walls of the building. In practice the rafters are tied together either ‘A collars at high level or with ceiling joists, or both. The simple collar type of roof is only suitable for small spans and is frequently found in double-pitch roofs.


If the roof is hipped —that is, if the gable ends are also pitched —. the wall plates are half-lap jointed where they cross to help counteract the spreading tendency. It is vital that these tie members are sound and not cut during alteration works, as without the ties the weight of the tiles or slates will certainly spread the roof and severely damage the external walls. It is particularly important that the tie joists of trusses should not be cut.

Inclined-joist roof.

This type of roof, is common to 19th century terrace houses, is characterised by a valley gutter running from the front to the back of the house centrally over the unit, with apexes at the party walls which usually stand up to form a fire-break between the units of the terrace.

The structure of this roof is in the form of inclined joists which are notched over timber plates so that the down thrust of the roof structure acts vertically on the walls and on to a central beam at the valley gutter. This beam is usually supported at mid span by the central partition wall, which should not be removed unless it is replaced with a suitable beam.

Lean-to roof.

The structure of a lean-to-roof is similar to half of an inclined-joist roof. However, as the joists are often supported at their upper ends by spiking to a wall plate, there is a marked tendency to “spread” at the feet of the rafters because there is little resistance to prevent the wall plate from subsiding.

To counteract this, the structure must be triangulated — that is constructed from frames consisting of an inclined rafter, a vertical strut and a horizontal tie joist. On no account should the horizontal ties be cut. Flat roof. Roofs of this type are inherently stable, as the loading at the support bearings is always vertical. The joist spacing is generally greater than in a floor structure. If any additional loading is to be applied to a flat roof, it may be necessary to strengthen the structure in case the joists deflect excessively, which will result in cracking of the ceiling below.

Upper floors

Loading from upper floors will generally be transmitted vertically into the walls and columns supporting a floor. If alterations are to be made, it is important that a bearing of at least 4 in. (or 10 cm) is maintained at the joist ends; otherwise the joist may become over-stressed. Joisted floors should be -strutted- to prevent the joists twisting or buckling under stress or humidity changes.

Ground floors

If the ground floor is over a basement, its structure will be similar to That of an upper floor.

In other cases the ground floor is not, strictly speaking, a structural element: it does not assist the stability of the building as a whole.

Service layouts

Whether you are carrying out alterations to your house in one upsurge or whether they are to be phased over a period, it is most important to plan all the circuits and distribution runs for electricity, gas, water, heating and drainage before embarking on any major programme of alterations. If the services are well planned from the outset, they will be trouble free, conveniently placed and accessible, tidy, concealed and economical to install.

Gas, water, electricity

Service entry. These services will, in most cases, enter the building below the ground. The exception may be electricity which, in older houses and in rural districts, may be supplied via an overhead line.

There is a clear demarcation of ownership of the service main between the consumer and the service authority or company. The consumer is not permitted to alter any part of the incoming service which is the service authority’s property but must apply to the authority, which makes any alterations required and charges the consumer according to a fixed scale of fees for alteration work.

Water entry. The water undertaking runs its pipe to a point close to the site boundary, and installs an external stopcock in a small pit. From this point the water service is in the consumer’s ownership; in addition to the external stopcock there should be a stopcock at the point of entry inside the building for the consumer’s own control. Gas entry. The gas undertaking runs its service into the building and as far as the meter. The meter is owned by the undertaking. The consumer owns that part of the service beyond the outlet of the meter. The installation cost of a new service includes all the pipe from the company’s main to the meter outlet position.

Electricity entry. The ownership of the electricity service is as for the gas service. It is common practice, but not essential, to mount the consumer’s distribution board adjacent to the meter. One reason for this is the relatively high cost of the large-capacity cable which runs from meter to distribution board.

Grouping entry points. It will be seen that gas, water and electricity services all involve some sort of control or metering at the input position. As these services will probably all enter the building near- one another, it is good practice to group all these entry points together, preferably in a cupboard or enclosure where they are readily accessible.

Heating services

Here the main planning problems are in siting the heating unit and the hot water storage cylinder or its equivalent.

The hot water draw-off points must be as close as possible to the source of the hot Water, whatever that source is. This will cut down expensive pipe-loss of heat and reduce the time taken for the hot water to reach the taps.

Internal distribution

Horizontal. If the construction of the ground floor consists of timber flooring on joists, the distribution is very simple, as the pipes and wires may be run between joists. With a solid floor, such as concrete, the best form of distribution is in chases and ducts cut into the floor, preferably with a removable cover plate. However, cutting chases and ducts in a concrete floor can be hard work. If you do decide to carry out this operation, it would pay to hire a rotary electric percussion hammer.

Small pipes can be accommodated in chases cut in the finishing screed. In this case it would not be necessary to break through the damp-proof membrane in the floor, which otherwise would have to be repaired and bonded to the new DPM below the duct. Electric wiring should always be installed in conduit.

New concrete floor. If you are proposing to lay a new concrete floor, plan the ducts required and lay timber formwork when you cast the concrete. As the formwork will tend to float out of the concrete, it will need to be held down. Conduit tubing and junction

boxes for electric wiring can be fixed to the Surface of the oversite concrete, so that the finishing screed covers all the installations except the junction box cover plates.

Ceiling ducts. Pipes may be run in horizontal ducts either at skirting or ceiling level. Ceiling ducts are, in many ways, preferable as they are not dust traps and do not get in the way so much, but the service will need to be run down to floor level where required and, in addition, the length of the service may be greater.

If you decide to run the service at high level, consider running it in the floor above rather than in a duct — particularly if the service is running in the same direction as the joists.

Upper floors. If the house has timber suspended floors, the services can readily be run in the floor void. There is obviously no problem in running the service along the direction of the joists but, where the service runs across the joists, it is vital that the following points are observed:

1 Never notch the joists for pipes to cross.

2 It is inadvisable to notch the joists even for wires to cross, as the wires could be damaged by nails knocked into the floorboards. However, small pipes may be set in shallow notches in standard joists, but not -proud- of these. Care should be taken not to puncture pipes when replacing flooring. Do not notch the joist nearer than 1 ft 4 in. (or 40 cm) to the bearing of the joist or within 4 ft (or 1.2 m) of the centre of the joist span. If the joists are springy or undersized, as they frequently are in older buildings, do not notch the joists.

2 It is always preferable to run services through holes drilled at a point midway between the floorboards and the ceiling below. This is because the stress in the joist is least midway between its top and bottom surfaces. Do not drill such holes nearer than 1 ft 4 in. (or 40 cm) to the bearing of the joist and no nearer than 4 ft (or 1.2 m) to the centre of the joist span. Always ensure that pipes are not compressed and have a slight clearance round them.

Vertical. Gas, water and electrical services can be run vertically in ducts, chases or inside studwork partitions. These are all straightforward techniques. But it generally happens that pipes and wires in chases run exactly where you want to knock in a nail to hang a picture. So they should always be protected with a suitable cover strip. Electric wiring can be run in purpose-made plaster-depth conduit.

Points to watch

1 Always test all pipework and wiring before covering up.

2 Lag all cold water pipes, whatever their position, as this will prevent condensation forming inside the duct or floor.

3 The water installation must be designed to avoid the forming of air locks and, where there is the chance of an air lock, a bleed valve should be fitted at that point.

4 Access panels must be fitted for all stopcocks, bleed valves and drain stopcocks.

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