Before you start your paving project, concrete slabs, gravel, paving blocks and asphalt all need a sub-base to support them. This consists of hardcore (crushed or broken bricks, block and stone) that's compacted with an earth rammer or vibrating plate compactor, and then topped with a layer of sharp sand or ballast (called 'blinding') to fill any gaps.
If you're laying slabs on a drive that will need to bear the weight of several cars, it's a good idea to compact the sub-base with a single-drum vibrating roller, and putting down a layer of concrete as well.
Ensure you read our handy guide before you start your project.
How to prepare your patio sub-base
You'll need to remove a depth of top soil equal to the depth of your sub-base, plus the thickness of your paving and mortar (at least 25mm). If you have firm well-drained soil, your sub-base should be 100mm deep - but if you have soft ground, it should be deeper. You'll need to clear an area that's slightly larger than your paved area will be. And if you're building a patio next to your house, make sure the level of the finished surface is at least 150mm below the damp-proof course.
Top tip - How deep is your drive?
The depth of the sub-base for a driveway depends on what you plan to pave it with. If you're laying concrete slabs, you'll need to allow 150mm of compacted hardcore plus 125mm of concrete. Or if you're laying paving blocks, allow 150mm of compacted hardcore and 50mm of sand
Mark out the area for your sub-base with builder's lines attached to wooden pegs. Use a builder's square to ensure the corners are square before you start digging. And as a final check, measure the diagonals to make certain they're the same.
Cut the turf in strips with a spade and roll it up. It's well worth saving some of it in case you need some to put in between your lawn and the paved surface.
Dig the area to the right depth and use marker pegs to mark the finished hardcore level - taking the needed fall into account.
Tip in enough hardcore to fill your sub-base to just above the top of the pegs.
Compact the hardcore with an earth rammer or petrol-powered vibrating plate compactor.
Cover the surface with a thin blinding coat of sharp sand or all-in ballast, then rake it level.
How to set the fall (slight slope) in a sub-base
All paved areas need a slight slope (or 'fall') so that any surface water can drain away. Generally, you only need to have a fall in one direction - the surface in the other direction can be level. In normal conditions, the ideal fall for a patio is 1 in 80 (that's a 12.5mm drop in level per metre), about 1 in 80 (across the width) for a path and about 1 in 40 for a drive. You can achieve this by using a spirit level, a one metre-long straightedge and a shim, and a small strip of wood cut to the depth of your required drop in level per metre.
First, decide which way you want your paving to slope. If you're building a patio against your house, it must slope away from the walls.
Start by cutting a number of softwood pegs. Measure from the top of each peg and mark them with the depth of the sub-base. Then hammer in a row of these pegs across the high side of the site.
Cut a shim to the correct size and nail it to the underside of one end of a 1 metre-long straightedge. Then hammer in the next row of pegs in line with the first, exactly one metre away. Put the straightedge between two pegs with the shim on the lower peg, and lay a spirit level on top. A level reading means the lower peg is deeper by the exact depth of the shim - the right amount to give the fall you need. Carry on with this process at metre intervals across the whole area that you're paving.
How to set the fall (slight slope) in your paving
Although the fall is already established in the sub-base, you'll still need to run builder's lines that mark the top edges of the paving to make sure you keep the right slope as you lay your slabs.
Mark out the edges of your patio with builder's lines attached to pegs. Use a builder's square to make sure the corners are true right-angles.
Hammer in pegs at the four corners of the patio to mark the finished slab height, allowing a depth of at least 25mm for the mortar. To check the fall, rest a straightedge and spirit level between opposite pegs at the high and low ends of the site. The depth of the shim on the lower peg needs to be the drop per metre multiplied by the length of the paving. Check the pegs are level in the other direction. Then fix lines between the pegs, which you can nail into the top with large-headed nails.
How to lay the first paving slab
Paving slabs are bedded in mortar that's mixed with four parts sharp sand to one part cement. The first slab is the important one, as it's the guide for all the others - and you should position this in a corner. You may need to fit slabs around drainage gullies or manhole covers. Think about how you can arrange them so you have as little cutting as possible.
Make sure you look after your back. If you're laying slabs that are 600mm x 600mm or larger, you must get someone to help you lift and lay them.
Put five blobs of mortar on the ground for the slab - one at each corner and one in the middle. (If you're laying a drive, you need a continuous layer of mortar between the sub-base and the slab.) Wet the back of the slab with a brush, as this will improve adhesion and make it easier for you to slide it into position.
Carefully lift the first slab, and lay it on top of the blobs of mortar.
Use a piece of timber and a club hammer or rubber mallet to tap the slab into place. Be very careful not to crack it.
Fill any gaps under the slab with mortar, and cut it neat with the edge as you go.
How to lay the remaining paving slabs
Whichever method you use to cut concrete slabs, it's hard to get perfectly straight edges. So always place the cut edge against a mortar joint, where it'll be less noticeable.
Top tip - Cutting slabs
It's best to cut concrete slabs on a bed of sand. First, mark your cutting line in pencil on all its surfaces. Then - wearing safety goggles - cut a groove along the line using a sharp bolster chisel and club hammer, cutting gradually deeper until the slab breaks. For a quicker, cleaner cut, use an angle grinder with a stone-cutting disc. As well as safety goggles, you must wear heavy-duty gloves and a gauze dust mask while you work.
Keep children and pets well away from an uncovered manhole. If necessary, build a temporary guard around it while the new concrete dries.
Starting from your key slab, lay the first row of slabs in the direction of your slope. Fit 5mm spacers in all the joints to make sure they're the same size. Use a spirit level and shim to check and recheck the surface is flat, and that the fall is correct. If you put a slab down and it rocks on the mortar, take it up and re-lay it - you probably won't be able to correct it by trying to push mortar underneath it when it's in place.
When you've finished your first row, lay slabs along the two closest outer edges. If you're using a grid pattern, the first and last slabs on alternate rows will need to be half slabs.
Fill in the central area and work back row by row. Nail a line to pegs at either end of each row before you begin laying it. Check throughout that the fall is correct and the surface is flat in both directions.
How to fill the joints in your paving
Leave the mortar to set for 48 hours before you walk on the slabs - and only then fill in the joints. If the slabs are wet or rain is forecast, it's best to wait for a drier day.
Alternatively, you could choose to fill the joints in with dry jointing compound. To do this, brush the compound in and compact it down with a jointing tool. It dries hard in a few hours - but make sure you protect it from the rain until then.
Mix a dry mortar of three parts sharp sand to one part cement. Remove the spacers and brush mortar into the joints. When you've filled all the joints, push the mix down firmly with a trowel or piece of wood, then brush in more mortar. Repeat this process three or four times, otherwise holes will appear in the mortared joints. Finally, brush away all the excess mortar carefully. It's worth taking your time over this, as it's easy to end up with mortar on the slabs.
Using a watering can with a rose attachment or spraying device, lightly wet the whole surface. This will make the mortar set.