Weekend project: String theory

Setting a string profile of your construction project prevents tears down the track, writes Justin Newcombe.

Writing a whole column on setting a string line might seem like a bit of a waste of newsprint but so much time, effort and materials can be saved by getting this part of the building project right first time.

So I thought it was an opportunity I couldn't pass up. Setting a profile is something that can take a lot longer than you'd expect.

A good basic description of your string profile is to think of it as an "air drawing". A profile gives you something accurate to work to when you are building a deck, a fence or even a path or garden bed. They are good if you want to discuss something with your other half or test out a shape or size of a project before construction starts.

Setting up a fence profile, for example, means you can actually see the impact of the fence before it's built, how high it should be and what impact on sun, view or wind it may or may not be having. In a retaining wall situation, setting a profile up can let you gather all sorts of important information, like how much fill or drainage you'll need to bring on to site to back fill the proposed wall, how high the wall may need to be to create a flat area or how many retaining walls you'll need if you're going to terrace a steep slope.

I find profiles most useful on relatively flat ground, especially when excavations are required. A flat-looking site is almost never flat, the classic example being Lords cricket ground, which slopes a metre and a half from one end to the other.

An accurate profile gives you something to measure from and work to. The profile I'm using at the moment will be used for building a deck. It has to represent the final deck height as well as the position and height of the posts. It might only look like a piece of string but it's what you're going to be working from and to. A wonky string means a wonky deck.

Step 1

Create the outline of the deck so it can be seen in relationship to everything else on site. Start by selecting your first datum point - the point of reference from which everything else is measured. For a deck, usually this is the floor of the interior room the deck will open off. I set the height of the deck to below the doors opening off this room (as specified in the building regulations).

Step 2

Create a set of string lines to position the posts so they are correctly set relative to the final height of the deck. This is simply a matter of reduction (or deduction, Holmes). If my final deck height is 400mm above the ground then firstly I subtract the decking which is 20mm, then I subtract the joist which is 150mm and finally I subtract the bearers which are 200mm, which gives me a total of 370mm from that desired 400mm. This leaves only 30mm for posts, so posts are a waste of time. Instead, I'm going to box each hole and pour a small pile instead.

Step 3

Once the concrete is dry, set the bearer directly on to that.

Step 4

To get the corners at the right distance apart and at the right height, the best way is not by tying the string line on to pegs but by setting horizontal timbers on pegs. Set the pegs well back from the intended position of the structure. Using this method gives you a lot of control and flexibility, and better accuracy. Take your time to get this part right. Drive two pegs, 400mm or so outside the position of the intended structure and approximately 400mm apart. The line you want to erect should roughly bridge the gap between the pegs.

Step 5

Attach a horizontal baton (I've used fence palings) to the pegs using screws. The baton should be at the correct height (in my case the top of the deck) and be level. A quick grip is a big help when you are working alone. To set the correct height on the corners, use a string line and spirit level (or string level), mark each peg level with the original and screw in the horizontal baton. Repeat for each corner. You should end up with a set of rough timber corners at each end of the deck.

Step 6

Tap a nail into the top of the horizontal baton to tie the string line on to, then run the line to the opposite end and attach the line. To form a corner, attach the string to the baton perpendicular from the one the string line is already tied on to then run the string over the original line, forming a triangle. This should give you a corner made from string lines only, no pegs in the way. Repeat this until all four sides of your structure are described by the string.

Step 7

Get out the tape measure and level and check that everything is set to plan. To move the string simply tap a nail in at a new position on the horizontal baton and move the string over. To change a level, unscrew the horizontal baton, reposition it further up or down the pegs, check that it's level and then re-screw.