Patio String Line Leveling and Slab Laying Guide
Step-by-step guide made from Tidybricks patio restore's YouTube video.
Set out accurate patio levels and drainage fall using perimeter timber battens, packers, and taut mason's string lines. Use the string line grid to verify sub-base depth and lay primed natural slate patio slabs onto a mortar bed at the exact height and gradient.
The full step-by-step Spetz Specs for this project (every part, size, diagram and step, free): Patio String Line Leveling and Slab Laying Guide Spetz Specs.
- Difficulty: intermediate
Safety
- Wear eye, ear, and hand protection when drilling into masonry walls with an SDS rotary hammer drill and when handling heavy stone slabs and cement mortar.
Must know before you build
- Set 10mm per meter drainage fall on string lines to prevent water pooling.
- Place timber packers behind battens for string line clearance; forgetting them ruins the setup.
- Use BS5534 graded roofing battens; they are straighter than standard treated timber.
- Brush paving slurry primer onto slab undersides for strong bond to mortar bed.
- Allow 60mm total depth below string line for 40mm mortar bed and 20mm slab.
- Consider a vacuum suction lifter for heavy slabs; it allows clean placement between lines.
Tools
- String line (high-visibility mason's line)
- Corner blocks (plastic mason's line blocks)
- Spirit level (box beam spirit level, Stabila)
- Rotary hammer drill (corded SDS rotary hammer drill, Bosch, Boschhammer)
- Claw hammer (hand hammer)
- Tape measure (pocket tape measure, Stanley, 8 m / 26 ft)
- Hand saw (wood hand saw, Irwin, Plus 880)
- Wheelbarrow (builder's wheelbarrow)
- Plate compactor (petrol vibrating plate compactor, Bomag) (optional)
- Primer brush (wide masonry brush)
- Vacuum suction lifter (battery-powered vacuum slab lifter, Grabo) (optional)
- Rubber mallet (white rubber paving mallet)
- Tile spacers (plastic cross spacers, 5 mm) (optional)
- Cement mixer (electric drum mixer) (optional)
Materials
- Roofing battens (treated timber batten)
- Timber packers (timber batten offcuts)
- Masonry hammer fixings (nylon wall plugs and masonry screws)
- Cement mortar (sand and cement mortar bed, 40 mm bed depth)
- Natural slate patio slabs (natural stone paving slabs, 20 mm thickness)
- Paving slurry primer (tile adhesive primer)
Step by step
Step 1: Set Initial String Line and Drainage Fall
Run an end-to-end string line along the wall and set a 10 mm per meter drainage fall by positioning the spirit level bubble just past the indicator line.
- Run end-to-end string line along the wall. Stretch a taut string line from the left-hand side of the patio perimeter to the right-hand side along the masonry wall to establish your baseline.
- Set 10 mm per meter drainage fall on the spirit level. Place a long spirit level along the line and adjust the slope until the bubble sits just past the vial indicator line, creating a 10 mm drop for every 1 meter of run.
- Tip: Set the spirit level so the bubble sits just past the black indicator line to achieve a 10 mm per meter fall for water runoff.
- Common mistake: Setting the bubble dead center in the vial, which leaves zero fall and causes rainwater to pool on the patio.
Step 2: Attach Leveling Battens to Perimeter Walls
Drill through the timber battens into the perimeter masonry walls, insert timber packers behind the battens to leave a gap for the string line, and secure the battens at the desired fall angle with hammer fixings.
- Drill clearance holes through the timber batten. Lay a length of roofing batten on the ground and drill clearance holes near both ends using the rotary hammer drill so wall fixings can pass through.
- Anchor the first end of the batten with a packer. Drill into the masonry wall at the starting mark, place a short timber packer behind the batten to space it off the wall, and hammer a masonry fixing through the batten into the wall.
- Level the batten to the fall gradient and fix the opposite end. Rest the spirit level on top of the batten to verify the bubble sits just past the line, drill through the second hole into the masonry, slip a timber packer behind the batten, and hammer the fixing home.
- Tip: Always slip a timber packer behind the batten next to each fixing before hammering the screw tight so the string line can slide underneath and behind the batten.
- Common mistake: Tightening the masonry fixing before inserting the timber packer behind the batten.
- Continue installing battens around the remaining perimeter walls. Repeat the drilling, packing, and leveling process along the remaining brick and block walls so the bottom edge of the battens forms a continuous level guide around the patio.
- Tip: Place packers right next to the masonry screws so the clamping pressure holds them securely in place.
- Common mistake: Placing a packer too far from the screw fixing, allowing it to fall out from behind the batten.
- Tip: Fix packers directly to the wall first and then screw the batten into the packers if you want an easier way to keep them from slipping.
Step 3: Verify Sub-Base Depth Against String Line
Stretch a string line between the perimeter battens and verify there is 60 mm of clearance underneath to accommodate a 40 mm mortar bed and a 20 mm patio slab.
- Stretch string line across the sub-base and check 60 mm clearance. Pull a string line taut from the back batten to the front border and gauge the gap underneath across the compacted sub-base—using a 60 mm reference like the toe of a work boot—to ensure room for a 40 mm cement bed plus a 20 mm tile.
- Tip: Use the 60 mm height of your work boot toe (or a 60 mm gauge block) under the taut string line to quickly check that the compacted sub-base is deep enough for a 40 mm mortar bed and 20 mm slab.
Step 4: Prime and Lay Patio Slabs to String Lines
Brush slurry primer onto the back of the slate slab, lift it with a vacuum suction lifter onto the mortar bed alongside the tensioned string lines, and tap it down with a mallet.
- Set parallel string lines across the patio area. Hook two parallel string lines from the front border corner blocks back to the perimeter wall battens to guide the height and alignment of the next row of slate slabs.
- Brush slurry primer onto the back of the slab and attach suction lifter. Coat the underside of the slate slab evenly with paving slurry primer using a wide brush, flip the slab face-up on the wheelbarrow, and engage the battery-powered vacuum suction lifter in the center.
- Tip: Use a battery-powered vacuum suction lifter to pick up primed slabs cleanly without getting slurry primer and mortar on your hands or disturbing neighboring tiles.
- Lower the slab onto the mortar bed and space joints by eye. Carry the primed slab by the suction lifter, lower it squarely onto the troweled mortar bed beneath the string lines, release the vacuum, and tap it down with a rubber mallet while spacing the joints by eye.
- Tip: Skip fixed 5 mm plastic spacers when laying natural slate slabs; because natural stone varies slightly in size, adjust the joint widths by eye to keep the overall pattern aligned.
- Common mistake: Forcing uniform 5 mm plastic spacers between dimensional natural stone slabs, which throws off the alignment of the pattern.
Step 5: Lay and Level Slab Using Intersecting String Lines
Manoeuvre a primed slab underneath the intersecting string lines using a vacuum lifter and tap it down with a rubber mallet until the surface just touches the lines.
- Slide the slab underneath the intersecting string lines onto the mortar. Grip the slab with the vacuum suction lifter, hold the intersecting string lines clear with your free hand, angle the slab underneath the lines onto the furrowed mortar bed, and release the lifter.
- Tap the slab down with a rubber mallet and flick-test the string lines. Tap the corners and center of the slab down with a rubber mallet until the stone face is flush with the string lines, then flick the lines with your fingers to confirm the slab is just touching without pushing the strings up.
- Tip: Flick the string line with your fingers after tapping the slab down; the line should just kiss the surface of the slab and move freely without being pushed upward.
- Common mistake: Leaving a proud slab pushing upward on the string line, which throws off the level reference for every subsequent slab along that line.
- Tip: Two parallel string lines are plenty for most rows; remove extra crossing lines once your plane is established so they don't get in the way.