How do I snap a chalk line straight?

A chalk line marks a perfectly straight line between two points over any distance, instantly, on any surface a pencil would be useless on. It is how flooring, tiling, roofing, plasterboard and stud walls are all set out, and there is very little to it beyond two habits: mark the ends properly, and lift straight up.

What you'll need

• Chalk line reel and chalk refill
• Two accurately marked end points
• A helper or a hook, nail or screw for the far end (optional)

Step by step

1. Mark both end points accurately first.The chalk line only joins two points; it does not find them. Both should be marked from a single datum rather than one from the other, and both should be pencil crosses rather than lines so there is no ambiguity about where the line goes.
2. Fill the case and shake it.Powdered chalk, not too little. Shake the reel so the chalk works into the line, and wind the line in and out once or twice on a fresh refill. A dry line snaps a faint dotted mark that is useless on a rough surface.
3. Anchor the far end.The hook on the end of the line catches over an edge, a nail or a screw. On a floor slab, a screw or a nail in a joint works. Failing that, a second person holds it on the mark — and holds it still, on the surface.
4. Pull it taut and set both ends on the marks.Tension matters. A slack line snaps a wavy mark. Pull it firm, then press both ends down onto their marks with a finger or a thumb so the line touches the surface exactly where it should.
5. Lift straight up, not sideways.Pinch the line somewhere near the middle, lift it perpendicular to the surface by roughly a hand's width, and release cleanly. Any sideways component throws the line off in that direction and produces a curve. This is the whole technique.
6. For a long line, snap it in two halves.Beyond four or five metres, a line sags in the middle and snaps thin or curved. Hold the line down at the midpoint with a finger, snap one half, then the other. Every half is a straight line between two points that are already on the true line.
7. Choose the chalk colour deliberately.Blue and red are the common ones. Blue is easier to remove and is right for anything that will be seen or decorated. Red is more permanent and is for work that will be covered. On a floor about to be tiled, either is fine; on a finished wall, blue and lightly.
8. Mark critical points with a pencil as well.Chalk brushes off with a sleeve, rain or a passing foot. Where the line matters, put a pencil cross on it at each end and at intervals so it can be re-snapped in exactly the same place.

Tips

• Most reels double as a plumb bob — the case is weighted and the line hangs from a hole in the top. That is why the case is that shape.
• On a dusty floor, sweep first. Chalk on dust marks the dust, not the floor, and the line disappears at the first footstep.
• Red chalk on porous brick, block or concrete is close to permanent. Do not use it anywhere you will later want a clean surface.
• Do not snap over an edge or a corner. The line will not touch the surface in the middle, and you will get two half-lines and a gap.

Common mistakes

• Lifting the line sideways as you snap it — The line lands where it was released from, not where it was stretched. Any sideways lift bows the mark by exactly that displacement.
• Snapping a slack line — A slack line whips and lands in a wave. Tension is what makes the line straight in the first place.
• Snapping the full length across a long room — Sag in the middle produces a mark that curves and fades. Pin the midpoint and snap two halves.
• Using red chalk on a surface that will be seen — It is designed not to come off. It will show through emulsion and it is very difficult to remove from block, brick and unsealed concrete.

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