Line Pins: How Bricklayers Hold a String Straight (and the Green Mortar Mistake)

A self-taught builder lays eight courses of blockwork for a garden office footing, taking care to check each block with a spirit level. The wall looks fine until the render coat goes on and every undulation shows through. The inspector from building control eyes it, asks for a string to be run along the face, and the wall fails. Six metres of blockwork has bowed outward by 12mm in the middle. The problem is not the blocks or the mortar. The problem is that the bricklayer never set up a line properly, and nobody told him that the £6 – £8.50 pair of steel pins in the corner of every bricklayer's tool bag would have prevented the entire problem.

What they are and why you need them

Line pins are short steel spikes, typically 150-160mm long, with a sharpened point at one end and a broader flat blade or eye at the other. You drive them into the hardened mortar joints at each end of a wall, wrap a builder's line around them, and pull the line taut. The line becomes a straight reference that every brick or block in the course between the pins must sit to.

The pin does two jobs. It anchors the line against the tension you put into the string, and it holds the line out from the wall face at a consistent distance so you can read the line against the bricks you're laying. Without a pin, there is nothing to anchor a long line to mid-wall. With one in each end joint, you can span a 6-metre run, check the line against ten bricks at once, and keep the whole course dead straight.

Every bit of external brickwork and internal blockwork on a typical extension is built this way. External cavity walls, internal load-bearing partitions, garden walls, retaining walls, boundary walls. Any course over a metre long needs a line. And any line over a metre long needs pins or blocks to anchor it. They are basic kit, cheap, and easy to lose. Most bricklayers own three or four pairs because they vanish into mortar beds and skip bins with alarming regularity.

A quick vocabulary note before going further. The arris is the sharp top front edge of a brick or block (literally, the corner where the top face meets the front face). The perpend joint is the vertical mortar joint between two bricks in the same course. The bed joint is the horizontal mortar joint underneath a course. Gauge is the standard course height: 75mm for a standard UK metric brick (65mm brick + 10mm bed joint), and 225mm per three courses of standard 100mm blockwork. These terms matter because line pins go into perpend joints, not bed joints, and always at gauge height above the course just laid.

How a line pin is built

Understanding the construction helps you spot a good one. All of the quality pins on the UK market share the same anatomy.

The shaft is drop-forged or solid-forged carbon steel, usually 150-160mm long. Forging (heating steel and shaping it under a hammer) gives the pin resistance to bending when tension is applied along the line. Cast pins (poured into a mould) bend in hard joints. Cheaper pins are stamped from flat sheet steel, which works but flexes more under load. Premium pins like the Sheffield-made Footprint and the Marshalltown LP62 are one-piece forged.

The tip is tapered to a point and heat-treated so it does not mushroom when you tap it in. A mushroomed tip will not enter a hardened mortar joint cleanly. The Marshalltown "leaf-point" design is a bit broader than a simple spike, which lets the same pin double as a ground stake for garden setting-out.

The head or eye at the top is where the line wraps. The geometry varies by brand. Footprint's pattern has a wide flat head with a convex blade. OX Pro and Marshalltown have a rounded circular head. Ragni uses a bulbous centre section. The details do not matter much. What matters is that the line wraps two or three times around the head without slipping down the shaft under tension.

The finish is usually chrome plating on the premium brands and zinc or galvanised on the budget ones. Chrome resists rust better when pins are left out on a damp site overnight. Rusty pins stain mortar and can snap tips when tapped into hard joints. Keep pins in a tool roll or a small zip pouch, not loose in a bucket with wet gear.

Click to enlarge

How to use them properly

This is the section every product page skips and where every beginner goes wrong. Buying pins is easy. Using them correctly takes five minutes to learn and is the difference between a wall that passes building control and one that bows.

Build your corner leads first

You never set up a line on an empty foundation and start laying. The proper workflow is to build the corners (called leads) up by four or five courses first, racking each course back by half a brick so the leads form a staircase. Check each corner with a spirit level for plumb and level, and with a gauge rod (a length of timber marked at 75mm intervals for brick or 225mm intervals for blockwork) for correct course height.

The leads are your truth. The line between them just transfers that truth across the wall face. If your leads are wrong, your line will be wrong, and your whole wall will be wrong no matter how carefully you use the pins.

On some jobs, speed profiles or corner poles are used instead of traditional leads. Profiles are vertical metal posts clamped to each corner with gauge marks pre-set. The line ties off to a clip at the correct course height. For most homeowner-supervised extensions, your bricklayer will build traditional leads; profiles are more common on new-build estates.

Wait for the joints to go off

Here is the rule that no beginner guide mentions and that causes more bowed walls than any other mistake.

In practice, this means you lay your leads, give the mortar in the leads a few hours to set (or leave them overnight), and then drive pins into joints within those leads. If you try to drive a pin into the same bed you just laid ten minutes ago, the brick below will wobble, the pin will rotate under tension, and the line will go out of true. A whole course laid to a line anchored in green mortar can end up 10mm out.

The fix is patience. Build the leads, have a cup of tea, drive the pins into the hardened joints near the top of the leads, then infill. Or, on long jobs, lay your leads at the end of one day and infill the next morning.

Drive the pin into a perpend joint at gauge height

The pin goes into a vertical mortar joint (the perpend), not a horizontal bed joint. Two reasons. First, the vertical joint is wider (typically a full 10mm head to foot), which gives the pin shaft more material to grip. Second, the line tension pulls the pin along the wall face, and a perpend joint resists that pull better than a bed joint, which would let the pin slip downward.

Drive the pin in just below the top front arris of the course you are about to lay. With the pin head tilted slightly towards you, tap it firmly with the butt of a brick trowel handle or a small hammer. Three or four taps will sink it 30-50mm into the joint. You want enough depth that the pin resists being pulled out by the line tension, but not so deep that the head is flush with the wall face. The head needs to sit proud of the face so you can wrap the line without the line rubbing on the bricks.

Wrap and tension the line

Wrap the builder's line around the pin head two or three times, not once. One wrap slips under tension. Two or three wraps bite on themselves and hold. Pull the line to the opposite end of the wall and repeat at the second pin.

The line tension should be firm enough that the string "pings" when you pluck it. Trade slang is "tune it to an E-flat." Too slack and the line will sag in the middle. Too tight and the pin will pull out of the joint, especially if the joint is not fully hardened.

The line must sit at the top front arris of the course about to be laid. Not above, not below, not behind the face. Right at the top front edge where the next brick's top front corner will be.

The 1mm rule: never let the brick touch the line

This is the single most important technique in bricklaying and the one that no UK beginner guide spells out clearly.

Different traditions phrase this rule differently. UK bricklayers say "the top front arris kisses the line." US masons say "leave a 1/32 inch gap." Both mean the same thing. The top front corner of each brick should sit level with the line, with a sliver of daylight between the brick's face and the string. The brick uses the line as a visual reference, not as a physical stop.

Sight along the line from one end after laying every three or four bricks. Every brick face should sit the same distance behind the string, in a flat plane. If you see the line bulging outward, find the offending brick and tap it back with the trowel handle before the mortar sets.

Watch for sag on anything over 4 metres

On runs over roughly 4 metres, even a taut nylon line sags under its own weight. A 6-metre span will dip by several millimetres in the middle no matter how hard you pull it. Lay blockwork to that sagged line and the middle of your wall will sag with it.

The fix is a tingle. Lay a brick at the midpoint of the run, gauged off the leads (use a spirit level across from the lead to check its level, or pull a separate pair of pins for just that section). Then place a thin metal plate, called a tingle plate, on top of the tingle brick, with the builder's line resting on top of the plate. A second brick weight on top of the plate holds everything in place. The tingle plate lifts the line back to its correct height, straightening the span.

You can buy purpose-made tingle plates for a couple of pounds, or improvise one by folding a piece of cement-bag paper and weighting it with a spare brick. On long walls, most bricklayers use two tingles to split a 12-metre run into three 4-metre sections.

Move up and repeat

Once the course is laid, move the pins up one gauge mark to the next course height. Pull each pin out, drive it into the perpend joint one course higher, reseat the line, and start the next course. With two people working, one pulls pins and the other wraps line, and the changeover takes less than a minute per course.

Line pins vs corner blocks: when to use which

Both tools hold a builder's line, but they work differently and the choice matters.

The clear decision rule most guides miss: use pins for the initial datum course before leads exist, or for any scenario where there is no accessible square corner. Use blocks once the leads are built and you just need to reposition the line quickly between courses on a standard straight wall. Most experienced bricklayers carry both and switch between them as the job demands.

A third option worth knowing: runners or timber profiles. These are short timber battens screwed to a wall that provide a tie-off point for the line when neither a corner nor a hardenable joint is available. They are used on garden wall extensions where one end ties back to an existing house wall. Not a primary tool, but a useful fallback.

How to check a pin is still straight

Steel pins bend. Usually slowly, through repeated driving into hard joints or from being stepped on in a tool bag. A bent pin pulls your line out of square and you will not spot it unless you check.

Roll the pin across a flat surface like a concrete slab or a piece of plywood. A straight pin rolls smoothly. A bent pin wobbles or catches. Bent pins can sometimes be straightened by clamping the shaft in a vice and flexing gently, but the sensible answer is to buy a new pair for under a tenner and bin the bent ones.

Also check the tip. A mushroomed or chipped tip will not enter a hardened mortar joint cleanly. You will end up hammering too hard to drive the pin, which either blunts the tip further or splits the joint and disturbs the brick below. If the tip is visibly flattened, file it back to a point with a mill file or replace the pin.

What to buy

The budget tier gets you everything a homeowner needs for a one-off extension project. The premium tier is for regular use and people who like good tools.

Budget pair (Wickes or Magnusson option). Wickes own-brand drop-forged pins at £5, or the Magnusson 155mm pair at £5.99 from Screwfix. Steel is steel at this price point. Both are adequate for laying the blockwork on a typical extension. Zinc or galvanised finish means slightly more rust risk than chrome-plated options. You will bend or lose these before they wear out.

The sensible mid-range pick: Footprint 160mm pair. At £8.49 from Screwfix, Footprint pins are the Sheffield-made classic. Drop-forged carbon steel, chrome-plated, with a distinctive wide flat head. Bricklayers on UK forums call them "footies" and most have owned a pair for decades. Two-year manufacturer guarantee. The proper answer for a homeowner who plans to do more than one project.

Premium: Marshalltown leaf-point pair. At £9, it sits just above the budget and mid-range pairs, justified for a lifetime tool. One-piece forged hardened steel with a leaf-point tip that doubles as a ground stake, useful if you also need to peg out garden profiles. Marshalltown is the American brand most trusted by professional UK bricklayers. Overkill for a single extension but a decent lifetime tool.

Sets with builder's line included. If you do not already own a line, a combined set saves a couple of pounds versus buying separately. The Footprint 3-piece starter kit at £10 gives you pins, line, and a reel. The Silverline heavy-duty set at £4.9 is a budget alternative that includes a 10-metre line. Ragni sets with a 30-metre high-vis line at £13 – £16 are stocked at Toolstation.

You do not need to buy both pins and blocks on day one. Pick one anchoring system. For a homeowner supervising a builder, buy a pair of pins because that is what the builder will expect to find on the tool table. For a homeowner actually laying the blocks themselves, corner blocks are more forgiving on the first course. Buy both eventually. The combined cost is well under a tenner and change.

Alternatives

Corner blocks (line blocks) are the direct alternative. Covered above. They clip over a corner brick rather than driving into a joint. Quicker between courses but need an accessible square corner.

Speed profiles or corner poles are vertical metal posts clamped to each corner of a wall, with gauge marks and clip-on line attachments. Used on some new-build estates. Rental only for most homeowners; not worth buying for a single extension.

A laser level projects a visible horizontal line. Useful as a datum reference when setting out the first course above DPC, but not a replacement for a string line during active bricklaying. Laser lines disappear when you switch them off, and you cannot lay a brick "to" a beam of light the way you can to a physical string.

A timber runner (a short batten screwed or clamped to each end of a wall, with the line tied off at gauge marks) is the homemade fallback when neither pins nor blocks will work, usually on garden walls that tie back to an existing house wall with no clean corner.

Where you'll need this

Line pins appear at several stages of any extension or renovation project:

• Walls and blockwork - the primary use, guiding every course of blockwork in the superstructure
• Foundations and footings - setting the first course above DPC when corners have not yet been built
• Damp proof course - aligning the first block course bedded onto the DPC
• Snagging checklist - running a line along finished external walls to check for bows that failed inspection

Common mistakes

Driving pins into green mortar. Covered above. The single biggest beginner error. The pin rotates under tension, disturbs the brick below, and pushes the course out of level.

Line wrapped only once around the pin. One wrap slips under tension and your line goes slack mid-course. Two or three wraps bite on themselves and hold.

Letting bricks push the line outward. The 1mm arris rule. A single brick bulging the line out by 2mm turns into a 2mm bow across the whole wall. Sight the line from one end after every three bricks.

Ignoring sag on long runs. Anything over 4 metres needs a tingle. Without one, the middle of the wall will follow the line into a belly.

Reusing rusty or bent pins. Bent pins pull your line out of square. Rusty pins stain mortar and sometimes snap when driven. Both are cheap enough to replace; check your pins before each session.

Forgetting to move the line up. Laying two courses against the same line height leaves one course short and one course tall. A trivial mistake that happens more often than any bricklayer will admit to on a hot afternoon.