String line fence posts UK: keep every post straight

The awkward moment comes after several holes have been dug: each post is only slightly out, but the complete fence bows across the garden. For string line fence posts UK projects, a taut cord gives you a simple, visible reference that does not depend on judging straightness by eye. Set the two ends properly, keep every intermediate post clear of the cord and measure from one consistent face. Those habits matter more than expensive equipment.

In short

Set and brace the first and last posts, stretch masonry line between matching reference points, then align each intermediate post to a fixed gap rather than letting it touch. Measure post centres from a known starting point, treat straightness and height as separate checks, and remove or barrier the cord whenever it could cross an access route.

Set the end posts first

Your string is only as trustworthy as the points supporting it. Set the first and last posts as sighting posts before excavating the whole run, on the correct side of the boundary. Check each one for plumb on two adjacent faces and brace it against movement. A straight fence in the wrong place is still a costly mistake.

Jacksons Fencing recommends using the posts at both ends as sighting posts in its post-and-rail instructions, and the same principle suits a panel fence. If the run changes direction, treat each straight section separately and install a firm corner post as the endpoint for both lines. Dig a few holes at a time rather than the whole run, because roots, old concrete and services can force a change. Before digging, check for buried services with the appropriate locating information and equipment. A piece of string cannot tell you what is underground.

Tie the cord to the same face and at the same height on both end posts. Masonry line is better than soft household twine because it stretches less and remains easier to tension. Pull it firm enough to remove obvious sag, but not so hard that it bends a stake or drags a newly set post out of plumb.

Choose a clear offset

A fence post should not support the alignment string. It should sit beside the reference with a repeatable gap. If one post touches and pushes the cord sideways by even a few millimetres, the next post can be aligned to the displaced cord, and small errors then propagate along the run.

There are two useful published approaches, and they should not be blended. Sheridan Fencing describes leaving about 2 mm between post and string, which is easy to see but easy to lose while you tamp or place concrete. Jacksons Fencing describes placing the string around 50 mm from the post edge, which gives more working clearance. Use one method for the whole run. For 50 mm, check each post with a rigid spacer without leaning on the line. For the close gap, look for daylight between cord and timber. Never pull a post into the line until it touches and call that aligned.

Choose one reference face and keep it. On uniform posts that may be the garden-facing edge. With rough timber, faces can bow, so use the centre or fixing line the rails or panels actually need. A string proves alignment only at its own height, so check each post for plumb with a string spirit level comparison or an ordinary spirit level.

Mark each post centre

Straight posts can still be unusable if the spacing is wrong. Measure the real components. Clips, brackets, gravel boards, post size and clearance all change the centre-to-centre distance, and a nominal panel width will not include them. Post-and-rail spacing follows the rail lengths and joints instead.

Lay out the first bay and establish the required centres from the manufacturer's installation details. Mark those positions from the same starting datum. A long tape measure comparison helps, but technique prevents cumulative error: measure several centres from the original datum rather than stepping the tape from one fresh mark to the next.

Check the complete calculated run before digging. If the last bay does not fit, solve it on paper or by dry positioning the components, not by squeezing the final posts together. Measure a sample of panels or rails, because timber can vary and an oversized panel is hard to force between posts that have already set.

Transfer each ground mark beyond the hole, because the mark itself will disappear during excavation. A labelled peg set clear of the disturbed ground works well. Keep the offset from centre mark to post face explicit, otherwise one helper may centre the post on the mark while another treats the same mark as its edge.

Keep the line true

Recheck the line whenever the weather changes, an end post is knocked or spoil piles up beneath it. A long cord sags under its own weight, and wet line, strong wind or vegetation can move it enough to spoil a close visual check. Sight along the cord from one end at intervals. A kink usually reveals contact with a post, peg or clump of grass.

Place each intermediate post at its marked centre, brace it loosely, check plumb in both directions, then confirm the offset. Add concrete or compacted backfill in stages and recheck after each stage, because a spade, tamper or the mix can rotate the post. Check plumb, centre and string clearance again before the material firms. If you fit panels as you go, do not let their weight pull a fresh post off line, and follow the manufacturer's curing instructions. Temporary battens can hold spacing. They do not replace a plumb check.

For attaching and reading the cord, see how to use a string spirit level. A hanging vial adds weight and can create a low point. Use it only when you are checking height, remove it before judging the plan line, and do not expect a long unsupported cord to behave like a rigid beam.

Deal with slopes and level tops

Straightness in plan and level in height are different jobs. The fence can follow one straight boundary even when the garden rises. Decide whether post tops follow the contour, panels step, or the whole top stays level before you cut posts or set their final depth.

On gently undulating ground, post-and-rail fencing often follows the contours. Panel fencing is commonly stepped, because rigid panels cannot bend to a slope. How you do that depends on the system, the fall and the gap you will accept under the fence. A string that follows the slope can guide a visual top line, but it is not level.

A small line level only shows whether the ends of a short string are at equal height. It does not make the posts straight, and sag, wind and the vial's weight make a long height transfer unreliable. Where repeated height marks matter, compare the limits in string spirit level versus laser level. Keep a written height datum rather than copying each post from the one before it. For stepped panels, calculate each step and confirm that rails, gravel boards and caps still fit. Stop if the planned height would leave an excessive ground gap or bury timber that is not meant for ground contact.

Finish without creating a hazard

If the string crosses a path, drive or shared access, it is a trip hazard because it is thin and taut. Do not leave it across a public pavement or boundary access. Barrier the work area, give a safe route, or take the cord down when you cannot supervise it. High-visibility masonry line is easier to see than dark twine, but colour is not protection. Keep children and pets away from open holes, wet concrete and stretched lines, and store loose coils at once.

Before taking the string down, inspect from both ends. No post should touch the line, each offset should match, every post should still be plumb and the centres should suit the components. Photograph the line if later work will cover your marks, then remove stakes, cord and markers rather than leaving synthetic line in the soil.

A straightforward garden run with known boundaries and clear ground is manageable for many people. Bring in a competent fencing contractor where the boundary is disputed, services are uncertain, retaining pressure is involved, public access is affected or the posts are too heavy to handle safely.