To lay a patio that survives the British winter, you must dig down at least 150mm, compact a solid 100mm sub-base of MOT Type 1, and lay your flags on a full 35-40mm bed of 5:1 sand and cement mortar, never use the ‘dot and dab’ method.
I have seen countless DIY patios sink, wobble, and crack after their first wet winter because the groundwork was rushed. Laying flags is not just about making them look level on day one, it is about ensuring they stay that way for decades. Whether you are using Indian sandstone or concrete slabs, success lies entirely beneath the surface. In this guide, I will walk you through the exact process I use on UK sites, from digging out to the right depth to creating the perfect mortar mix.
- Essential tools and materials for laying slabs
- The cost to DIY vs hiring a pro
- Why the 'dot and dab' method will ruin your paving in winter
- Step 1: Digging out and preparing the ground
- Step 2: Mixing the perfect mortar (the 'dough' test)
- Step 3: Laying, levelling, and setting the falls
- Step 4: Pointing the gaps to weatherproof your patio
Essential tools and materials for laying slabs
Before you start, you need the right gear to get the job done safely and efficiently.
Mat’s tool bag:
- Shovel and wheelbarrow
- Rubber mallet
- Spirit level (at least 900mm)
- Pointing trowel
- String line and pegs
- Wacker plate (hire this locally)
- Cement mixer (hire this locally)
- PPE: Steel toe-capped boots, heavy-duty gloves, and knee pads
The cost to DIY vs hiring a pro
For a standard 10m² UK patio, hiring a professional tradesperson will cost around £1,000 to £1,500 in labour alone. If you do the work yourself, your main costs are materials and tool hire. Hiring a wacker plate and a cement mixer for a weekend from a local shop usually costs around £80 to £100. By putting in the sweat equity, you can save over £1,000 on the project. If you are doing a full garden makeover, you might also be looking at the cost to lay turf to finish off the edges.
Why the ‘dot and dab’ method will ruin your paving in winter
I recently ripped up a sinking patio in Yorkshire and the problem was obvious immediately. The previous builder used the ‘five-spot’ or ‘dot and dab’ method, placing a dollop of mortar in each corner and one in the middle. In the UK climate, rainwater pools in the empty voids beneath the slab. When January hits, that water freezes and expands. This ice physically snaps the paving slab in half from underneath. You must always use a full mortar bed to prevent this costly damage.
Step 1: Digging out and preparing the ground
You need to dig down a total of 150mm to create a stable foundation. This allows for 100mm of sub-base, roughly 35mm of mortar, and a 15mm slab. If you are paving a front garden, check the Gov.uk Planning Portal for rules on permeable surfaces to prevent localised flooding.
The Patio Dig-Out Calculator
The Patio Dig-Out Calculator
Enter your patio dimensions below to find out exactly how much soil you need to excavate and how much MOT Type 1 sub-base to order.
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Soil to Excavate (150mm depth): 0 m³
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Why the sub-base (MOT type 1) is non-negotiable
Do not lay your slabs directly onto soil or sand. You need a 100mm layer of MOT Type 1 aggregate to provide a solid, load-bearing foundation. Rake it out evenly and run a hired wacker plate over it until it is completely compacted and flat. If you need a refresher on materials, read our guide on dry mix concrete.
Step 2: Mixing the perfect mortar (the 'dough' test)
Forget resting slabs on little piles of sand because you need a full mortar bed to support the weight. I recommend a mix of 5 parts sharp sand to 1 part cement. Always use sharp sand, not building sand. Building sand is too soft and will not support the weight of the patio. For more details on mixes, see our complete guide to mortar mixing ratios.
Slowly add water to your mix until the consistency is firm and doughy.
My quick test: Grab a handful wearing heavy-duty gloves and squeeze it into a ball. If it holds its shape without oozing water or crumbling apart, it is ready to use. I highly recommend hiring a cement mixer for this, but if you are doing a small area, use a proper spot board or wheelbarrow.
Step 3: Laying, levelling, and setting the falls
Spread a full 35-40mm layer of mortar evenly to seat your first flag. Lower the slab carefully into place and tap the corners with your rubber mallet until it sits firmly in the bed. Place your spirit level across the slab, but remember you do not want it perfectly flat.
You need a 'fall' of 1:60 to ensure rainwater drains away from your house bricks. This means for every 60cm of width, the patio should drop by 1cm. Use your string line to keep the entire row straight and maintain this gentle slope.
Step 4: Pointing the gaps to weatherproof your patio
Leave the slabs to cure for at least 24 hours before stepping on them. Once set, you must fill the gaps to prevent weeds from taking root and stop water from getting under the slabs. You can use a traditional wet mortar mix pushed in with a pointing trowel, or a modern brush-in joining compound for speed. Pack it in tightly and brush off any excess before it dries completely.