How to Set Out a Foundation With a Rotary Laser

If you're starting a build — extension, garage, outbuilding, full house slab — getting the foundation levels right is the most important hour of work on the whole job. Get it wrong and every wall, every floor, every door reveal that follows inherits the error.

A rotary laser turns what used to be a two-person job with string lines, a spirit level and a lot of guesswork into something one person can do properly in twenty minutes. This is the practical, no-waffle guide to doing it on site.

What you need

A complete rotary laser kit — laser unit, tripod, levelling staff, receiver. We'll use the Grettz G60 as the worked example because it's what most UK trades reach for: 400 m range, IP54 rated, self-levelling, and £359.99 for the full kit. You can use any rotary kit and the steps are identical.

You'll also want:

• String line and pegs for marking the foundation perimeter once you've got the levels established
• A spirit level for cross-checking your laser is calibrated (more on this below)
• A spray paint can or chalk for marking the dig level on existing concrete or hardcore
• Tape measure — your 30m or 50m surveyor's tape

If you've not got a calibrated rotary kit yet, this is the moment to invest. Borrowing or hiring one for a single foundation job costs nearly as much per day as buying outright over a year of regular work.

Step 1 — Set the tripod in the right place

Pick a spot roughly central to your foundation footprint where the laser will have line of sight to every corner you need to level. Not at one corner — that means the receiver works at maximum range at the far corner and minimum at the near one, which can introduce small errors over distance.

Open the tripod legs wide. Press them firmly into the ground — if you're on soft ground or fresh hardcore, twist them in or pack stones around them. Movement of the tripod equals movement of your reference, and that ruins everything else.

Get the tripod head roughly level by eye. The laser's self-levelling motor will handle fine adjustment, but you want to be within its tilt range — typically ±5 degrees.

Step 2 — Mount the laser and let it level

Screw the laser onto the tripod head. Switch it on. The G60 takes about 15 seconds to self-level — you'll hear a soft motor sound, then a click when it's locked in. Most rotary lasers (including the G60) will flash a warning if you've placed them too far out of level to correct.

Once levelled, the laser projects a continuous 360-degree red beam at exactly the height of the spinning head. Anywhere within range, that beam is a perfect horizontal reference.

Quick site-test for calibration, especially if the laser's been bouncing around in the van:

1. Pick a point about 20m from the tripod
2. Hold the receiver against the staff, find the beam, mark the height
3. Move the tripod 180 degrees around the same point (so the laser is now on the other side, but the receiver stays put)
4. Re-find the beam, mark the new height
5. If those two marks are within 3mm of each other, your laser is calibrated. If they're significantly off, the laser needs recalibrating before you trust it for foundation work.

Step 3 — Establish your finished floor level (FFL) datum

Every foundation needs a fixed datum — a known, recorded level you can come back to. Usually this is your finished floor level (FFL), or a height-from-FFL like damp proof course (DPC) or top-of-slab.

Set the staff up at a known point — the threshold of an existing door, the level on a manhole cover, the surveyor's benchmark if there is one. Take the reading with the receiver. Write it down. Mark it on the staff with a paint pen or piece of tape. This is your "0 mm" reference for the rest of the job.

From here every other reading is relative. If your dig level needs to be 600mm below FFL, you're looking for 600mm above the reference reading on the staff. If your slab top needs to be 175mm below FFL, you're looking for 175mm above.

Step 4 — Take corner levels around the foundation

Walk the staff to each corner of your foundation. Place it on the existing ground or hardcore. Move the receiver up and down the staff until the beam locks (the receiver beeps when you've found dead-centre). Read the height.

Note each reading on a quick sketch with arrows showing which corner is which. Don't trust your memory — a 3mm-off corner is 3mm of taper across the entire wall above it.

What you're checking:

• Are all four corners on the same plane? Differences tell you where you need to dig deeper or build up
• Is the existing ground sloping? Plan your dig depths around it
• Where do you need fill? Where do you need to take off?

The G60's LR1 receiver shows the offset on a small LCD — "+12mm" or "-8mm" relative to the beam. Far quicker than reading off the staff for every shot.

Step 5 — Mark your dig level

Once you know where the finished slab needs to be, work backwards to your dig level. Mark each corner. Spray paint on hardcore or stake-and-paint on soft ground works.

Run a string line between corner marks for the edge of the foundation. The string isn't structural — it's just a visual reference for the digger driver or labourer when they're scraping out.

For trench foundations, the same process applies. Walk every metre or two along the trench, take a reading, mark the side. The digger should be working to those marks, not to "looks about right."

Common mistakes to avoid

1. Not checking the beam height against the staff zero. The staff has a zero point at its base. Your beam height is whatever it happens to be. People sometimes write the beam height down and then forget to take it off the staff reading, ending up 1300mm low. Always read the staff at the beam — never assume.

2. Trusting the laser without a witness mark. Mark a fixed point at the start of the day (a stake driven well into the ground, a paint mark on a wall). At the end of the day, go back and check the laser still reads the same height to that point. If it doesn't, either the laser drifted or the witness point moved — either way, your last few hours of work need re-checking.

3. Working at extreme range with no calibration check. A rotary laser rated to 400m doesn't mean it's accurate to ±1mm at 400m. The G60's spec is ±2mm at 30m — at 400m that's around ±26mm. For finer accuracy at distance, use a more sensitive receiver and move the tripod closer to the work.

4. Wind. On a windy day, even a heavy tripod can vibrate. The self-levelling head re-locks every time, which means your beam can hop up and down by a few mm.

Why a rotary laser beats string lines and spirit levels

A spirit level is theoretically accurate but practically slow — two people, a hose, a lot of fiddling. A string line tells you where things are relative to each other, not absolute heights. Both are fine for small jobs.

For a foundation, where you need a single accurate horizontal reference across 20m+ and you're working solo, the rotary laser is faster, more accurate, and means you don't need a labourer holding the other end of anything.

The kit we recommend

If you're setting out foundations regularly — extensions, conversions, full builds — the Grettz G60 Red Beam Rotary Laser Kit is the right tool. £359.99 gets you:

• Self-levelling rotary laser, 400m working range
• LR1 dual-sided receiver with LCD readout
• Aluminium tripod and telescopic staff
• Hard moulded carry case
• 3-year warranty
• Free UK next-day delivery

Shop the Grettz G60 Kit on grettz.com →

For larger plots or grading work, the G70 (long-range red) and G80 (dual-slope green for drainage) are worth a look.

The quick version:

1. Tripod somewhere central, pushed firmly in
2. Laser self-levels, take a calibration check if it's been moved
3. Set your FFL datum, write it down
4. Walk to each corner, take a reading, mark each one
5. Work backwards to dig levels, mark and run string
6. Witness-mark a fixed point and re-check at end of day

Twenty minutes done properly here saves the whole rest of the job. Most expensive part of any foundation is the bit you can't see — get it right at the start.