The first patio I helped lay had three problems by year two: some slabs had sunken in the corners where they hadn't been fully supported, a few had rocked from day one (not fully bedded at the edges), and the grouting had started to crack and fall out in sections. All of these came down to sub-base preparation, bedding technique and jointing. Not materials or slab quality.
A patio done properly will not move or crack in normal conditions for 20-30 years. The preparation takes longer than the laying.
What you need
- Patio slabs (calculate quantities with the Patio Slab Calculator)
- MOT Type 1 for sub-base (calculate with the MOT Type 1 Calculator)
- Sharp sand and cement for bedding mortar
- Kiln-dried sand or pointing mortar for grouting
- Vibrating plate compactor (hire)
- Rubber mallet
- Long spirit level and straight edge
- String line and pegs for setting out
- Bolster chisel or angle grinder for cutting
- Pointing trowel and brush
Step 1: Plan and set out
Calculate your area and decide on the slab pattern. A simple grid (each slab the same, joints aligned) is easiest. A random pattern or mixed sizes looks more natural but requires more cutting.
Set falls. A patio must drain away from the house. A fall of 1:80 (12mm per metre) is the standard for hard paving. Steeper falls look and feel tilted; shallower may allow water to pond.
Set out with string lines. Mark the perimeter with pegs and string. Check the corners are square (3-4-5 triangle method: a 3m and 4m measurement from the corner should give exactly 5m diagonal if square).
Step 2: Excavate
Calculate the excavation depth:
- Sub-base depth: 100mm (pedestrian patio, firm ground) or 150mm (heavier use)
- Bedding mortar: 40-50mm
- Slab thickness: varies (typically 40-50mm for natural stone, 18-22mm for porcelain, 50mm for concrete)
Total excavation = sub-base + bedding + slab thickness + 10mm allowance
Mark this depth on your pegs and excavate. Remove all topsoil and organic material - these compact and settle under load.
Step 3: Install and compact the sub-base
Spread Type 1 in layers of no more than 150mm loose depth. Compact each layer with a vibrating plate compactor before adding the next.
After compaction, the sub-base surface should be firm, consistent in level, and to the correct falls. Use a long straight edge to check flatness.
Do not skip the sub-base. Patio slabs on soft ground will sink and move regardless of how good the bedding mortar is.
Step 4: Prepare the bedding mortar
Mix ratio: 1 part cement to 5-6 parts sharp sand (semi-dry consistency)
The mix should hold its shape when squeezed in your hand but crumble apart when dropped. It is much drier than normal building mortar. This semi-dry consistency is important: too wet and the slab sinks; too dry and it doesn't bond.
Lay the mortar at approximately 40-50mm depth in a consistent layer. For large slabs (600mm+), use a full bed to ensure even support. For smaller slabs, some installers use five contact points (a blob in the centre and one near each corner) - the butter-and-blob method - but a full bed gives better support and prevents cracking.
Do not add plasticiser or admixtures to slab bedding mortar. These change the consistency in ways that make a solid bed harder to achieve.
Step 5: Lay the slabs
Set the first slab at a corner. Place it onto the mortar bed, then tap it down with a rubber mallet to set it to the correct level. Check with a spirit level in both directions and check the fall.
Press down firmly and evenly. The entire base of the slab should be in contact with the mortar. If you tap one corner and the other rises, the bed is uneven - lift the slab, add mortar to the low points and try again.
Use consistent spacers for joints (plastic tile spacers or pieces of slab offcut). Standard joint width for patio slabs is 10-15mm.
Check level between adjacent slabs and across the area regularly. It is much easier to correct a slab before the mortar stiffens than after.
Allow 24 hours before walking on laid slabs.
Step 6: Point and grout the joints
Once the bedding mortar has hardened (24-48 hours), fill the joints.
Options:
- Kiln-dried sand: Swept into joints with a soft brush. Cheap, easy, but provides no bonding and can wash out. Suitable for informal patios.
- Dry mortar pointing: 1:5 or 1:4 sand:cement brushed in dry, then watered in with a fine spray. Traditional method - can look patchy if not done carefully.
- Pointing mortar or flexible grout: Pre-mixed jointing compounds (Rompox, Sika, Groutex etc.) are more expensive but much more durable and resistant to weed growth. Recommended for natural stone and porcelain.
Apply pointing on a dry day. Rain on fresh pointing washes it out and stains the slab surface.
Common mistakes
| Mistake | Consequence |
|---|---|
| No sub-base | Slabs sink and rock |
| Sub-base not compacted | Uneven settlement |
| Mortar bed too wet | Slabs sink, difficult to level |
| Not full bedded (hollow spots) | Slab cracks under load |
| No falls | Puddles on patio |
| Falls towards house | Water at DPC, potential damp |
| Grouting in damp conditions | Staining, poor bond |
My tips on laying patio slabs
Hire the plate compactor, don't skip it. The sub-base preparation is 60% of the job. A vibrating plate compactor can be hired for £50-80/day and is the single most important tool in the process.
Lay slabs dry first. Before mixing any mortar, dry-lay a row or section to check sizes, cuts and falls. It is much easier to adjust a dry arrangement than to relay a mortared one.
Wet the backs of natural stone slabs. Dry stone, especially sandstone, pulls moisture from the mortar too fast. Spray the back of the slab with water just before laying. Don't soak it - damp is enough.
Keep mortar off the slab faces. A grout smear on a textured natural stone slab is very difficult to remove after setting. Wipe spills immediately with a damp cloth.
Use the Patio Slab Calculator and Sub Base Calculator to estimate materials before you start.