Garden slab installation needs a firm sub-base, a full mortar bed, and a gentle fall away from walls for drainage.
Ready to lay a patio that stays flat, drains well, and looks sharp for years? This guide gives clear steps, practical checks, and time-saving tips. You’ll plan the layout, dig to the right depth, compact a solid base, set a safe slope, bed each flag on fresh mortar, and finish clean joints.
Plan, Measure, And Mark Out
Pick the spot, then sketch a quick plan to scale. Note doors, vents, and any inspection covers. Keep the finished surface below the damp-proof course by a safe margin. Set a slight slope away from the house so rain moves off the surface instead of back toward brickwork.
Run string lines at the final height. Peg the perimeter, pull the line tight, and check with a long level. A gentle fall around 1:80 suits most patios; on heavy soils, 1:60 sheds water faster. Trade guidance on falls and gradients sits here: Pavingexpert on gradients.
Tools And Materials
Stage gear before you dig. Keep heavy bags close to the work zone to cut carrying time.
| Item | Use | Notes |
|---|---|---|
| String Lines & Pegs | Set levels and edges | Match the planned fall |
| Spade & Shovel | Excavate the area | Square edges give tidy borders |
| Wacker Plate | Compact sub-base | Several overlapping passes |
| Long Level & Straightedge | Check plane and slope | Two-metre length catches humps |
| Rubber Mallet | Bed slabs | Firm taps, no chips |
| Angle Grinder + Diamond Blade | Cut flags | Eye, ear, dust protection |
| MOT Type 1 (DTp1) | Sub-base | Well-graded, compacted |
| Sharp Sand & Cement | Mortar bed | Common ratio 4:1 |
| Slurry Primer | Boost bond | Useful for porcelain |
| Jointing Compound | Fill gaps | Pick for slab type |
Installing Garden Slabs Step By Step
1) Excavate To The Right Depth
Dig down to allow for the compacted sub-base, a full mortar bed, and the slab. Many patios sit on about 100–150 mm of compacted Type 1 plus a 30–50 mm mortar bed. Keep the bottom flat. Trim the sides square. If the soil is soft, go a little deeper and extend the base past the finished edge for better support.
2) Lay And Compact The Sub-Base
Spread Type 1 in layers about 50 mm loose. Compact with a plate in overlapping runs. Aim for a tight, bound surface that follows your slope. Recheck string lines often so you don’t build a hump. A poor base is the main reason slabs rock later.
3) Set Levels And The Fall
Reset string lines to finished height. Mark the high side near the house and the low side where water should go. A 1:80 fall drops 12.5 mm per metre; at 4 m that’s about 50 mm. If puddles linger on your site, 1:60 gives extra runoff. The trade page linked above explains these ratios in plain terms.
4) Mix A Stable Mortar Bed
Use sharp sand and cement near 4:1 for a firm, workable mix. It should hold shape when squeezed, not slump. Many brands set out the same ratio for patios, like the guide from Bradstone on mortar mixing. Keep batches small so the bed stays fresh under each course.
5) Prime Smooth Backs
Porcelain and very smooth stone benefit from a slurry primer on the underside. Brush a thin coat and lay the flag while the primer is tacky. This boosts bond strength where frost and foot traffic stress the joints.
6) Lay The First Course
Start from a straight edge or a corner by the house, working outward with the fall. Trowel a full, even bed 30–50 mm deep. Skip “dot and dab.” Full support spreads load, limits cracks, and keeps water away from the back of the slab. Bed the flag, tap down with a mallet, and check both plane and slope.
7) Keep Joints Consistent
Use spacers or timber gauges to hold gaps steady. Many flags sit well with 5–10 mm joints; porcelain often uses narrower gaps set by the maker. Sight along the bond every few rows so lines stay straight across the full area.
8) Cut To Fit
Measure twice and mark with a wax pencil. Support the piece and cut with a diamond blade. Take shallow passes. Keep the cut edge to the border where you can. Dry-fit awkward shapes before mixing more mortar.
9) Finish Joints
Let the bed firm up. Brush dust from the joints. Use a resin jointing compound or a sand-cement slurry suited to your slab. Follow the cure time. Wash marks before they set, and keep traffic off the surface until the joints harden.
Drainage, Rules, And Good Practice
Hard surfaces need a home for rainwater. A small patio can shed water across the lawn. Bigger builds may need a channel drain at the low edge feeding a soakaway or a planted bed. For front hardstanding, there are rules on permeable surfacing and runoff. See the Planning Portal guide for front gardens, and the government leaflet on permeable surfacing.
Set Safe Heights
Keep the finished level well below the damp-proof course. Many installers aim for about two brick courses of clearance under doors and weep holes. Bridging to the DPC invites damp patches indoors.
Pick The Right Base And Bed
Pack a sub-base of well-graded Type 1. Many trade guides suggest around 100–150 mm compacted depth for foot traffic, then a 30–50 mm full bed. Brands like Marshalls publish notes that back these figures in their concrete paving guide.
Common Mistakes To Avoid
No Slope
Water sits, algae forms, and joints stain. Build the drop into the base and keep checking as you lay each course. Large areas may benefit from a central high line that sheds to two edges.
Patchy Bedding
“Dots and dabs” create hollow spots that crack or sound drum-like. A full bed spreads loads and keeps water off the back of the flag.
Skipping Compaction
Loose stone settles later and makes ripples. Compact in thin layers and overlap plate passes. If the plate sinks, the layer is too thick or too wet.
Poor Joint Choice
Match the product to your slab and gap size. Some compounds drain; some cure hard. Stick with one batch colour across a single area.
Cutting, Edging, And Patterns
Border Details
A soldier course or a contrasting block sets the patio off and hides cut edges. Bed border units first, then infill. Where grass meets paving, add a concrete haunch or an edge restraint to stop creep.
Layout Patterns
Stretcher bond reads clean in tight spaces. Random layouts break up large areas. Dry-lay a metre or two to judge bond lines and joint rhythm before you mix mortar.
Neat Cuts
Mark straight, support well, and take light passes. On porcelain, fit a blade rated for that material. For curves, cut small facets, then dress the edge for a smooth line.
Aftercare And Long-Term Results
Cleaning
Wash with a soft brush and a mild patio cleaner. Lift stains early. Avoid strong acid on limestone. On resin joints, keep pressure washers gentle so you don’t gouge the fill.
Sealing
Natural stone may benefit from a breathable sealer. Test on an offcut. Apply thin coats and let each coat dry fully. Many porcelains already have low porosity and need no sealer.
Frost And Movement
Full beds, tight compaction, and a sound sub-base help against freeze-thaw. In icy spells, avoid de-icing salt on soft sandstone. Re-sand or top up joints where traffic washes them low.
Mixes And Depths Cheat Sheet
| Layer | Typical Depth | Spec Or Mix |
|---|---|---|
| Type 1 Sub-Base | 100–150 mm compacted | Well-graded MOT (DTp1) |
| Mortar Bed | 30–50 mm | Sharp sand : cement 4:1 |
| Surface Fall | 1:80 to 1:60 | Drop 12.5–16.7 mm per metre |
Quick Checklist Before You Start
- Sketch the area and pick a layout that suits your slab sizes.
- Set string lines to the final height and a gentle fall away from walls.
- Dig deep enough for base, bed, and flag thickness while keeping room below the DPC.
- Compact Type 1 in thin layers to a tight base that follows the slope.
- Mix a firm 4:1 mortar and bed each piece fully; skip hollow spots.
- Check plane and fall after each flag; adjust with the mallet as you go.
- Finish joints cleanly and protect the area until the bed and grout cure.
What This Method Draws On
This approach aligns with trade notes on base depth, mortar ratios, and surface falls. See the Marshalls concrete paving guide for base and bed advice, the Paving Direct patio page for typical depths and a 5:1 note, the Pavingexpert gradients page for falls, and the Planning Portal guidance for rules on permeable surfacing.