Profile Boards (Batter Boards): How to Set Out an Extension Footprint

The groundworker turns up on day one, knocks a few timber stakes and crossboards into your garden, runs string between them, and starts marking the ground. To most homeowners it looks like the least important part of the job. It is in fact the most expensive thing to get wrong. Every wall and trench is positioned off those strings. Pour the concrete out of square, or 200mm too close to the boundary, and the fix is breaking out set concrete. The timber that holds the strings costs a few pounds. It is the cheapest insurance on the build.

What profile boards are and when you need them

A profile board (builders call them batter boards, builder's profiles, or just profiles) is a simple timber frame that holds your setting-out string lines clear of the ground you are about to dig. Each one is two pointed stakes driven into the ground with a horizontal board, the crossboard, fixed across the top. You set one up a little way beyond each corner of the building, run string between the crossboards, and those strings mark the lines of the walls and trenches.

The whole point of them, the thing almost no guide explains clearly, is this: the string lines have to come off so the digger can excavate the trenches, then go back in exactly the same place afterward. If you marked the footprint with pegs driven into the ground inside the dig, the excavator destroys them the moment it starts. Profile boards stand back from the trench, untouched by the dig. You lift the strings off, excavate, then drop the strings back onto the same nails or notches and the setting out is exactly where it was. That is why they exist.

You need them (or, more accurately, you need your groundworker to set them up properly) on any job where you are digging foundation trenches to a marked-out plan. For a domestic extension that means setting out the footprint before the first spade goes in. It is the first task on site, and everything downstream depends on it being right.

Set them up, not you

On almost every domestic extension the groundworker or builder erects the profiles as part of their day rate, not you. Your job as the person managing the project is to understand what you are looking at and check it before anyone digs. Setting out is rarely a separate line on the quote because it is bundled into the groundwork.

The parts of a profile board

A profile board has only three parts, and the dimensions are not fussy. What matters is that the stakes are solid and plumb and the crossboard is level.

There are three ways the boards get arranged, depending on the job:

• A single profile is one board controlling the lines for one wall. You set one at each end of a wall and run the string between them.
• A corner profile is an L-shaped pair, one board on each face meeting at a corner. This is the common arrangement for an extension: a profile pair just beyond each corner of the footprint lets you string both walls that meet there.
• Continuous profiles run a board right along each side of the building. On a larger or more complex footprint this lets you string every wall at once, which is the real advantage trades talk about: you set up lines for the whole building in one go and can see the entire plan strung out before anything is dug.

How they get set up

The sequence below is what a good groundworker does, and what you should expect to see. Understanding it lets you spot when a corner has been cut.

Getting the footprint square: 3-4-5 and diagonals

This is where the forums argue and where homeowners get confused, so here is the plain version. There are two separate jobs: squaring a corner, and proving the whole rectangle. They are not the same thing.

The 3-4-5 method squares a single corner. It comes from Pythagoras: a triangle with sides of 3, 4 and 5 units has a perfect right angle between the 3 and the 4. Measure 3 units along one line, 4 units along the line meeting it, and if the diagonal between those two points reads exactly 5 units, the corner is square. Use multiples for accuracy: 3 metres, 4 metres and 5 metres, or better still 3000, 4000 and 5000mm. The longer the triangle, the smaller the error.

The catch, and the thing the trade is right about, is that 3-4-5 on a short template does not set out a whole building. If your template is a millimetre out, that error multiplies down a long wall and you can finish 100mm out at the far end. So 3-4-5 is a check on a corner, not a method for the whole footprint.

To prove the whole rectangle is true, you check the diagonals. A rectangle is only square if its two diagonals are exactly equal. Measure corner to opposite corner one way, then the other way; if the two figures match, the footprint is square. You can work out what the diagonal should be before you start: it is the square root of (length squared plus width squared). For an 8.2m by 3.6m extension that is the square root of (8.2 x 8.2 plus 3.6 x 3.6), which comes to about 8.95m. Set your corners off the building line, then adjust until the measured diagonals match the figure.

Equal diagonals = square

For any rectangle, equal diagonals prove it is square. Tape opposite corners both ways. If the two diagonal measurements match, the footprint is true. If they differ, one corner needs to move.

The four checks to make before anyone digs

You do not need to set out the building yourself. You do need to walk the strung-out footprint before the excavator starts, because once the trench is dug and concrete is in, every one of these is expensive to fix. This half-hour is the highest-value time you will spend on the whole project.

Making your own versus buying a system

For a domestic extension, homemade timber profiles are the universal norm and there is no reason to buy anything fancier. Two pointed stakes and a crossboard off a length of rough-sawn carcassing, plus a reel of builder's line, is all the kit. A pointed 600mm site peg costs only a couple of pounds each, the crossboard is often an offcut, and a reel of line is a few pounds, so the timber for a whole four-corner extension comes to somewhere around 30 to 50 pounds of material. Against a six-figure build, that is the cheapest insurance you will ever buy.

You can buy reusable plastic and steel setting-out profile systems (the Cross-Bone plastic profiling system is one UK example, and adjustable batter-board kits exist). They go up faster than timber, can be lifted and re-set without losing accuracy, and pay off if you are setting out building after building. For a one-off extension they are not worth it. Timber does the same job for a few pounds and gets reused as pegs, props and firewood afterward.

External resource

Designing Buildings Wiki: How to lay out a building

The most technically complete free UK reference on setting-out profiles, with the standard timber dimensions, the 2m clearance rule and how levels are tied to the site datum. Written for construction students, so it assumes some background, but it is accurate and thorough.

designingbuildings.co.uk

When the existing house is not square

One reality that competitors gloss over: extensions start from the existing house, and the existing house is often not square. A wall that is 50mm wider at the rear than the front is common in older properties, and it makes the set-out a real headache, because the building line you are working off is itself out of true. There is no neat trick for this. The set-out has to reconcile the new build's geometry with a reference that is not perfect, and the right call is usually a judgement about where the discrepancy is least visible and least structurally awkward. If you find a discrepancy, report it to your designer rather than guessing. Do not let the groundworker simply "split the difference" without anyone deciding where the error should go.

Alternatives

There is no real substitute for a setting-out rig that holds the lines clear of the dig, which is what profiles do. The alternatives are mainly about how you control level rather than position. A rotary laser level can be used as a "virtual string line" with batter boards, projecting a level plane the boards are set to, which speeds up levelling on a larger site. An optical level or a laser level is how the crossboards get set to the datum in the first place. None of these replace the profiles; they work alongside them. For position and squareness, taut builder's line on solid timber profiles, checked with diagonals, is still how it is done on every domestic site.

Where you'll need this

• Setting Out - profile boards are the primary setting-out rig that holds the footprint lines clear of the dig, so the trenches can be excavated and the lines re-strung exactly afterward

Setting out is the first task on site for any extension or renovation project, and it is the cheapest stage to get right and the most expensive to get wrong. Errors here cascade into every later phase of the build, so the time spent checking the strung-out footprint before the dig is the best-value time on the whole project.