Line Marking Spray Paint: Setting Out Trenches, Drains and Services on the Ground

A groundworker sprays a single white line down the side of a plot, the digger follows it, and the bucket clips a live water main nobody marked. The dig stops, the water board gets called, and a half-day job turns into a week of standpipes and arguments over who pays. The few pounds of spray paint that should have flagged that pipe in blue, after a proper location survey, never went down. Marking paint is the cheapest material on the whole groundwork stage and one of the few that can stop a five-figure mistake before the bucket moves.

What it is and what it's for

Line marking spray paint is an aerosol paint built to spray downward onto the ground, laying a crisp visible line as you walk along it. It is the material that turns the numbers on your approved drawings into marks the digger driver can actually follow. The building line, the trench positions, the drain runs, the gully and inspection-chamber locations, all of it gets sprayed onto the soil, grass, tarmac, or concrete before any excavation starts.

This is the working end of setting out. Setting out is the half-day at the start of groundwork where the groundworker transfers the dimensioned site plan onto your patch of ground: the precise position, size, and orientation of the extension footprint, plus the drainage layout. The string lines and profile boards fix the geometry. The spray paint makes that geometry visible at ground level, so that once the strings come off for digging, the driver still has a painted line to dig to and everyone on site can see where the trenches, pipes, and services are meant to go.

It is a consumable. One can marks a few hundred metres of line and then it is spent, the same way a tin of layout paint or a packet of marking chalk is spent. Nobody keeps a half-used can for the next job the way they keep a spirit level. You buy it by the can or by the box, use what the job needs, and bin the empties.

The paint is loud on purpose. High-visibility fluorescent colours, orange, pink, yellow, blue, red, green, white, are the norm, because a line you cannot see from the cab of a digger is a line that gets dug through. The bright colours also let you run a colour code, which is where marking paint earns its keep on a site with buried services. More on that below.

The inverted can: why a normal aerosol will not do

The defining feature of line marking spray is that the can sprays from the bottom, not the top. Hold it upside down, nozzle pointing at the ground, and it lays a line beneath you as you walk. This is the opposite of an ordinary aerosol, which is designed to spray upright and clogs almost immediately if you turn it over.

The reason is the dip tube inside the can. A normal spray can has a tube running from the valve down to the bottom of the can, so it draws liquid paint from the base while you hold it upright. Turn that can over and the tube is now in the gas space at the top, so it spits propellant and a few blobs of paint, then blocks. A line marking can has no dip tube, or an inverted valve, so it draws paint correctly when held nozzle-down. That is what lets you walk a long line at a steady pace with the can pointing straight at the floor.

That design is also why you should never reach for a tin of car touch-up paint or a leftover can of fence spray to mark out a trench. It will clog on the second pass, the line will be patchy and faint, and a faint line on bare soil is the line a digger driver guesses at. Buy the proper inverted product. It is not expensive and it is built for exactly this.

How you actually use it

There are two ways the paint goes down, and good groundworkers use both depending on what they are marking.

Against a guide, for anything that has to be straight and accurate. The string line strung between the profile boards, the edge of a profile board itself, a long straight timber batten, or a tape stretched along the ground all work as a guide. You hold the can a few inches off the floor, keep the nozzle just to one side of the string so you are not painting the string itself, and walk the line at a steady pace. Too slow and the paint pools and bleeds wide; too fast and the line breaks up. A consistent walking speed gives an even line about 25 to 75mm wide depending on the can and the nozzle height.

Freehand, for marks that only need to be roughly placed: a cross where a gully sits, a circle where an inspection chamber goes, an arrow showing the fall direction of a drain, a blob over a service that the location survey flagged. These do not need a straightedge. They need to be clear and in the right spot.

For long runs, a wheeled applicator earns its place. This is a wand or a small wheeled trolley that holds the can at the right height and angle and triggers it with a lever at the handle, so you push it along the line like a tiny lawn striper instead of walking bent double with the can in your hand. On a small extension you can mark the whole footprint freehand and against the strings without one. On a larger plot, a long driveway run, or a site with hundreds of metres of line, the applicator saves your back and gives a more consistent line because the can stays at a fixed height. The table below includes a typical applicator price.

Colour codes: read the line, not guess it

A site with one colour of paint is a site where every line means "something". A site with a colour code is a site where a line means a specific thing, and the digger driver can read it without asking. This matters most for buried services, where guessing wrong is dangerous.

UK safe-digging practice borrows the buried-utility colour convention used on plans and survey markings. The widely used shorthand is yellow for gas, blue for water, red for electricity, and green for telecoms or data, with separate colours often used for the building lines and trench positions so they do not get confused with services. This colour language sits behind the national safe-digging guidance and the move toward a single shared view of buried assets through the National Underground Asset Register. There is no single law that says "thou shalt spray gas in yellow", so treat it as strong, near-universal practice rather than a statutory colour standard, and confirm the convention your groundworker and any utility-location contractor are working to before the dig. The point is discipline: agree what each colour means on your site, mark to it, and tell everyone working the dig.

Mark after the service scan, never before

The single rule that turns marking paint from a tidy habit into a safety control is this: locate buried services before you paint, then paint what you found. The setting-out stage already calls for an underground-services search and a CAT and genny survey before anyone knocks a peg in. Marking paint is how the result of that survey gets recorded on the ground.

A CAT (cable avoidance tool) and genny (signal generator) survey is the standard on-site method for finding buried cables and metallic pipes before digging. The contractor or groundworker sweeps the area, the tool beeps over a buried service, and the operator sprays a line directly above it. That sprayed line is the survey made visible. The digger driver then knows to hand-dig or stay clear along that line rather than driving a bucket blind through it.

Do it the other way round, paint the trenches and start digging, then find the services when the bucket hits them, and the paint has done nothing useful. Worse, an unmarked live service hit by a digger is one of the most dangerous events on any groundwork site. The order is fixed: survey, mark, then dig.

Surface, weather, and how long it lasts

Marking paint sticks to most things you find on a building plot. Bare soil, grass, gravel, hardcore, tarmac, and concrete all take a line. On loose soil or long grass the line is a little fuzzier and shorter-lived than on a hard surface, simply because the ground itself moves and the grass grows over it.

These paints are not meant to last. They are temporary by design, fading and wearing off over days to a few weeks depending on the surface, the traffic over them, and the weather. That is usually fine, because the line only needs to survive from the day it goes down to the day the trench is dug. The failure mode to watch for is a long gap between marking and digging. Mark out a footprint, then leave the plot for a fortnight of rain before the digger turns up, and the lines on soil may have washed faint or disappeared. If the dig is delayed, expect to re-mark.

Rain right after spraying is the enemy. Most line marking paints are touch-dry within a few minutes and rain-resistant once dry, but a line sprayed onto wet ground, or hit by a downpour before it has flashed off, beads up and washes away. Mark on a dry surface, give it a few minutes, and pick a dry window if you can.

There are water-based and solvent-based versions. Water-based marking paints are lower-odour, less hazardous to handle, and the common choice for general setting out. Solvent-based paints bite harder onto difficult or damp surfaces and tend to last a little longer outdoors, at the cost of stronger fumes and more care over storage and disposal. For most extension setting out, a standard water-based fluorescent line marking aerosol does everything you need.

Who uses it and where it fits

The groundworker is the main user. They mark the footprint and drains as the first physical task on site, usually as part of the setting-out price rather than a separate line on the quote. On a larger or more complex plot, a setting-out engineer or land surveyor establishes the control and may mark the key lines with paint as they go, especially where an optical or laser instrument is being used to position things precisely.

A utility-location contractor uses it too, spraying directly over the services their CAT and genny survey finds. On a self-managed build you will rarely spray it yourself, but you should recognise the marks when you walk the plot before the dig. A footprint in white, drains and fittings marked out, and any located services flagged in their safe-digging colours is what a properly set-out plot looks like the morning the digger arrives.

Cost and where to buy

Marking spray is cheap. A single can covers a few hundred metres of line, and a box of twelve covers an entire groundwork stage with cans to spare. You will find it at the trade counters and online ranges of the usual UK suppliers, usually listed under "line marking" or "survey marking" paint, with applicator wands sold alongside.

Brands you will see include Everbuild (their line marking aerosol is a common trade-counter pick), Soudal, Spectrum, and Sika, all selling much the same thing: an inverted fluorescent aerosol in 500ml cans. Buy a couple of colours rather than one, so you can run the safe-digging code and keep your building lines clearly separate from any service marks. For a single small extension, two or three cans plus the strings and profiles is plenty; there is little point in a box of twelve unless you have a big plot or several jobs lined up. Skip the applicator on a small footprint and add one only if you are marking long runs.

Common mistakes

Marking before locating buried services. The dangerous one. Painting the trenches and starting to dig before a CAT and genny survey means an existing gas, water, or electric service can be hit blind. Survey first, mark what you find, then dig.

Reaching for an ordinary spray can. A standard upright aerosol clogs the moment you turn it nozzle-down, giving a spitting, broken, faint line. A faint line on bare soil is the line a digger driver guesses at. Use a proper inverted line marking can.

No colour discipline. One colour for everything, or a different colour every time someone grabs a can, means nobody can read the lines. A gas line, a water line, and a trench line that are all the same colour are three lines that say nothing. Agree the colour code, mark to it, and tell the dig crew.

Letting the marks wash off before the dig. Lines on soil fade in days and wash out faster in heavy rain. Mark close to the dig, not weeks ahead, and re-mark if the job slips. A washed-out footprint on dig day is a footprint set out from memory.

Spraying onto wet ground. Paint on a wet or rain-hit surface beads and runs before it grips. Mark on dry ground, give it a few minutes to flash off, and pick a dry window where you can.

Where you'll need this

• Setting out - marking spray transfers the setting-out dimensions onto the ground so the trench and wall lines are visible to the digger driver once the strings come off
• Drainage - the drain runs, gully positions, and inspection-chamber locations are sprayed out on the ground before any excavation begins

Line marking spray is a groundwork-stage consumable that turns up at the very start of any extension, conversion, or new-build dig, wherever a footprint, a trench, a drain run, or a located service has to be made visible on the ground before the machine moves. It is the cheapest material on the stage and, used in the right order behind a proper services survey, one of the few that protects against a genuinely expensive and dangerous mistake.