Last edited 13 Sep 2016

How to layout a building


[edit] Introduction

The process of laying out (or setting out) a building or structure is an important part of surveying, as it enables the works to proceed on site exactly according to the prepared designs.

Accurate setting out is fundamental part of the construction works, and errors can be very expensive and time consuming to correct. It should only be undertaken by competent persons, and all work should be thoroughly checked, preferably by different personnel.

Setting out is usually undertaken once the site has been subject to a condition survey and desk study, and has been cleared of any debris or obstructions. Works necessary to create required levels may also have been completed before the layout process begins.

The position and orientation of the structure is generally described in architect's or engineer’s drawings and defining precisely how the layout should be arranged.

Controlling dimensions and references on the plans will determine the positioning of the building, and in particular its foundations. These include; overall length and width, distances to road centre-lines and to other structures, internal structural measurements, approaches and rights-of-way and so on.

The controlling points of the structure can then be marked so that the construction team is able to easily identify them. This usually consists of marking the building’s corners, horizontal and vertical positions, using stakes, batter boards with string lines, drill holes, cut-and-fill notations, and other methods.

[edit] Temporary Bench Mark (TBM)

The TBM is a fixed point on a site to which all levels are related and should be established at an early stage. Where possible the TBM should relate to an ordnance bench mark. On the site, it could relate to any permanent fixture, such as a manhole cover or firmly-driven post.

[edit] Baseline

Typically the first layout task is establishing a baseline to which all the setting out can be related. The baseline is a straight reference line in respect to which the building’s corners are located on the ground. It often coincides with the ‘building line’, which is the boundary of the area, or the outer boundary of a road or curb, often demarcated by the local authority.

[edit] Horizontal controls

These are the points that have known coordinates with respect to a specific point. Other points such as layout corners can then be located. Plenty of control points should be used so that each point of the plan can be precisely located on the ground.

[edit] Vertical controls

These enable design points to be positioned at their correct levels. The vertical control points are established relative to specified vertical datum – often a timber post set in concrete.

Horizontal and vertical controls are generally established during the levelling phase using a theodolite or similar instrument. For more information, see Surveying instruments.

[edit] Building layout

For a simple building layout, such as a rectangle, the outline of the building is marked by cord fixed to corner posts. A theodolite, site square or builder’s square is used to turn off 90-degree angles for the remaining corners. Ranging rods may be required to establish a straight line between corner posts.

Corner posts are usually 50 x 50 mm timber posts driven firmly into the ground, with a nail in the post’s centre. The outline may be marked on the ground with dry lime or similar powder. Timber profile boards can be used at the corners. Profile boards are typically between 0.6-1 m in height and comprise two 50 x 50 mm posts driven at least 600 mm into the ground, with a 150 x 38 mm crossboard.

Where the outline of a building is more complex than a simple rectangle, it may be necessary to establish a range of points in the same way as for laying out a simple rectangle. However, great care is required, as small errors are more likely to be introduced as more points are positioned. Often the easiest way of laying out an irregular building shape is to first lay out a large rectangle which will enclose the entire building or the greater part of it. Once this is done, deductions and alterations can be made to obtain the precise layout required.

[edit] Trenches

The layout of trenches establishes the excavation size, shape and direction, as well as the width and position of walls. Trenches are excavated once the building outline has been set out. The width is often marked with a line of dots of dry lime powder for accurate excavation by hand, whereas the centre line is marked for accurate machine excavation.

Outline profile boards are often used to control trench positioning, width and depth. In order that they do not obstruct the excavation work, profile boards should be set up at least 2 m clear of the trench positions. The level of the profile crossboard should be related to the site datum and fixed at a convenient height above ground level, often with cords strung between two profiles at either end of the trench. Bands can be painted on the crossboard for identification purposes.

Pegs are often driven into the bottom of the trench to mark the top of the concrete strip that is subsequently poured.

The corners of walls are transferred from intersecting cord lines to mortar spots on the concrete foundations using a spirit level for accuracy.

[edit] Reduced level excavations

The overall outline of a reduced level area can be set out working from a baseline. Corner posts are fixed to the outline of the excavation area and the outline marked with dry sand or similar material. To control the depth of the excavation, sight rails are set up at a convenient height and at positions which will enable a traveller to be used.

A traveller is a profile board with a fixed height, used for controlling excavated levels between profile boards. By placing the traveller in the sightline between two level boards, it is possible to see whether or not the excavation has been carried out to correct levels. The height of the traveller is the desired level of the sight rail minus the formation level of the excavated area.

[edit] Framed building

Framed buildings are usually related to a grid, often set out from a baseline. The intersections of the grid lines mark the centre points for isolated or pad foundations.

The layout of the grid is established using a theodolite and the grid intersections marked using pegs. Once the grid has been set out, offset pegs or profiles can be fixed clear of any subsequent excavation work. Control of excavation depth can be by means of a traveller sighted between sight rails or by level and staff related to a site datum.

[edit] Find out more

[edit] Related articles on Designing Buildings Wiki

[edit] External resources

  • CivilEBlog - Building layout
  • 'Building Construction Handbook' (6th ed.). CHUDLEY, R., GREENO, R., Butterworth & Heinemann (2007)