What's inside
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A string line is a simple way to carry a straight reference across a run that is too long for a level or straightedge. For setting posts, aligning a wall plate or checking a long timber edge, the line itself is cheap; the useful differences are how it resists sag, how easily it tensions, and whether you can see it against the work.
Choose a line that suits the span
For short, sheltered work, ordinary braided mason’s line is usually enough. It is inexpensive, easy to knot and available in bright colors. A 0.8–1 mm line is a practical general-purpose size: fine enough to give a clear reference, but less fiddly than very thin line. The drawback is stretch and sag. Pulling harder may reduce sag, but it can also bow slender posts or pull a stake out of position.
For repeated layout or longer runs, look at low-stretch braided line sold for construction layout. It tends to hold a setting more consistently, but it still sags under its own weight and can be harder to see if it is very fine. A low-stretch construction layout line is worth considering if you set out posts often. For a one-off, short run, basic mason’s line is usually the better value.
| Line type | Best for | Trade-off |
|---|---|---|
| Basic braided mason’s line | Short runs and occasional layout | Can stretch, fray and sag more noticeably |
| Low-stretch braided layout line | Repeated work and longer references | Costs more; still needs a sag check |
| Fine monofilament line | Situations where a narrow visual reference helps | Hard to see, prone to tangles and uncomfortable to handle under tension |
Length, color and visibility
Buy enough line to cover the run with spare length for tying off. A 50 m spool covers most small building and garden layouts without constant joins. Fluorescent orange, yellow or green is easier to pick out against timber and soil than white line. If you work in bright sun, check visibility in the actual setting: glare can make a pale line disappear, while a dark line may blend into shadowed timber.
Very fine line can make a neat reference but is easy to snag and difficult to find after a break. Thick line is more visible, yet catches more wind and can obscure a small gap. For exposed work, do not treat color as a substitute for checking the line against a fixed mark.
Setting posts on a string line
Establish the line from two firm end points, such as stakes or temporary batter boards set outside the excavation area. Keep the anchors clear of the post holes so digging and concrete work do not disturb them. Pull the line taut enough to remove slack, not so hard that the anchors shift or the line stretches.
For posts that must share a face, position the string as a reference for that face and bring each post close without pushing it into the line. A small gap—around 2–3 mm—is useful: direct contact can deflect the line and make a correctly placed post appear out of line. Check each post in both directions with a level, then recheck the line after placing or adjusting several posts. A line that has been bumped may still look taut while giving a false reference.
On a long run, sag is the main trap. As a rough check, sight along the line at the middle or measure its clearance from a straight reference at several points. If the line dips enough to change the required position, divide the run into shorter sections using intermediate references, or use a long straightedge or laser to verify it. Do not solve sag by increasing tension indefinitely: thin stakes can lean, and flexible posts can bow.
Aligning long timber runs
To check a timber edge, tie the line beyond both ends and set it parallel to the intended edge. Use small, equal spacers at the end points to create a consistent offset, then compare the timber with the line at several places. The offset keeps the timber from touching and deflecting the reference. A 10–20 mm spacer is often easier to work around than a tiny gap, provided both ends use the same thickness.
Timber is rarely perfectly straight. A string line shows deviation from the chosen reference, but it does not tell you whether the piece should be forced straight, replaced or simply oriented with its better edge outward. Check for bow before fixing, and avoid pulling a badly bowed length into position with fasteners if that would leave stored stress in the assembly.
Tensioning, winding and care
A simple line reel makes payout and storage faster, especially when the line is used often. A basic hand reel is convenient but not essential for occasional work; a spool and a cleat or knot will do. Consider a mason’s line reel if you regularly set out long runs. Avoid winding dirty, wet line tightly onto a reel: grit wears the braid, and trapped moisture can leave it unpleasant to handle.
Use a secure knot that you can untie after loading, and keep the line away from sharp corners and fresh concrete. If it is frayed, kinked or visibly stretched, replace it rather than trusting it for a critical alignment. A small amount of care matters more than a premium spool: sound anchors, a clear offset and repeat checks are what make the line useful.
What to buy
For occasional carpentry, a bright 0.8–1 mm braided mason’s line and a modest spool are a sensible starting point. Spend more on low-stretch line if you lay out long runs repeatedly or need consistent references across multiple jobs. Choose a reel for convenience, not accuracy; it cannot compensate for sag, loose anchors or a line that has been knocked out of place.