How to lay paving slabs on concrete: A step-by-step guide
Laying paving over existing concrete is a smart way to refresh your patio without the cost and mess of a full dig-out. The key to getting it right is preparation.
If your concrete base is sound, well-drained and level, you can get a long-lasting finish from either natural stone or low-porosity porcelain.
In this step-by-step guide, we’ll show you how to lay paving slabs on concrete the right way. From checking the base, setting the correct fall, choosing between full-bed mortar or an exterior tile adhesive system and using primers or slurries to guarantee a bond.
With the right tools, products and a little patience, you can create a clean, durable surface that looks great and drains properly.
Can you lay paving slabs on concrete?
The short answer is yes. If the base is sound, well-drained and level, you can lay your paving slabs onto concrete; there are just some steps you’ll want to follow to get a professional, long-lasting finish.
Check the concrete base
Before you start the job, it’s important to check the health of the concrete base. You’ll want to look at:
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Soundness: Make sure there are no major cracks, heaves, or loose areas.
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Levels and falls: Can you create the right fall level away from your home? Runs should be 1:60 to 1:80. If not, you might need to add a self-leveller, use a grinder, or add a new screed layer.
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Moisture and damp: No standing water, and make a plan for the water to run off to a gully or channel.
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Thresholds and door clearances: Avoid bridging the damp proof course and allow plenty of room for slab, bed and grout thickness in your plans.
Tools and materials
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Rubber mallet, straightedge, spirit level, trowels, buckets, mixing paddle.
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Primers/slurries: polymer-modified primer for concrete, porcelain back-butter/slurry for very low-porosity slabs.
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Bedding options: Exterior full-bed mortar or C2 exterior tile adhesive for porcelain over concrete.
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Spacers, exterior grout, silicone mastic for movement joints, angle grinder.
Which laying method is best?
When laying paving slabs on concrete, there are two options to choose from:
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Full-bed mortar: This is the versatile option; it’s great for stone and concrete slabs. You’ll need to use primers with this method.
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Exterior tile adhesive: This is best for those using porcelain tiles or those with low porosity. If you have a thinner concrete bed and crisp levels, this will be your best option.
Laying paving on concrete with full-bed mortar
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Clean and degrease the concrete
Start with a spotless base. Use a pressure-washer to remove dirt, moss, grease, or any loose material. These could weaken the bond of your concrete and mortar bed if you don’t do this. Allow it to fully dry before you carry on.
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Repair and level the base
Check the surface for cracks, dips, or raised areas. Using a repair compound for cracks, mortar for hollows and grinding down any high spots. If your paving area needs water to run off in a certain direction, now is the time to adjust the levels to get proper drainage. Standing water is the enemy of a long-lasting paved surface.
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Prime the concrete base
A bonding primer will help with durability. Brush or roll on a polymer-modified primer to help the mortar stick to the concrete base. If you skip this step, when winter hits, you could hit problems with debonding.
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Mix a workable mortar
Prepare a mortar mix that’s smooth, cohesive, and firm enough to hold a shape. You need full and even support under every slab to avoid voids where water could collect or cause problems later.
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Prime the underside of slabs
Some paving materials have low porosity, porcelain included. In this case, they'll need a slurry primer on the underside of each slab to make sure it has a strong bond with the mortar.
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Set out control lines
Use string lines or chalk to ensure a straight line for when you’re laying. Lay dry slabs with spacers to get a feel for how it looks and get good spacing and alignment.
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Lay on full bed of mortar
Don’t use dabs or spot bedding, as you could end up with rocking slabs and eventual breakage. Spread a full, even mortar bed (about 30-40mm thick), then lay each slab in its place. Gently tap it down with a rubber mallet, checking with a spirit level to maintain both level and fall.
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Keep joints consistent
Use spacers to help maintain the gaps between slabs, usually 3-5 mm for outdoor paving. Consistency looks professional and makes pointing the joints easier.
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Install perimeter and movement joints
Where paving meets walls, steps, or other fixed edges, leave a gap filled with compressible foam and seal it with a flexible, exterior silicone. Movement joints are best in large paved areas to allow for expansion and contraction.
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Curing and protection
Freshly laid slabs need time to settle. Keep foot traffic off for at least 24-48 hours. If heavy rain or frost is expected, it’s worth covering the area with a breathable sheet or tarpaulin. Protection is key to giving you a strong and lasting bond.
Laying paving on concrete with exterior tile adhesive
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Prime the concrete base
Most outdoor adhesives will tell you to apply a primer to the concrete base to help it stick and reduce suction. This helps to stop the adhesive from drying too quickly and gives you a long-lasting bond. Skipping the primer is one of the most common causes of listing slabs.
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Key the backs of the slabs
Porcelain, in particular, is dense and non-porous, which means it doesn’t naturally grip adhesive. Using a slurry primer can help with this. With natural stone or textured pavers, this step may not always be needed.
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Apply adhesive
Spread your adhesive onto the primed concrete with a notched trowel. The notch size matters; typically, a 10-12 mm notch is best for outdoor paving. Trowel in straight lines so air can escape when the slab is pressed down.
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Back butter for full contact
Spread a thin layer of adhesive (sometimes called back-buttering) directly onto each slab before you lay it. This means the slab will have full contact with the adhesive bed beneath and reduces the chance of voids.
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Bed the slabs with firm pressure
Place each slab down, press, and slightly slide it into position. A rubber mallet will help you to bed it down evenly. Checking coverage is essential, as it lets you see that you have adhesive contact across the slab; otherwise, voids may lead to rock cracking or frost damage.
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Check levels, falls and joints
As you go, use a spirit level to keep the paving true to your drainage fall, so water can’t pool. Use spacers to keep consistent joint widths.
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Movement joints
Outdoor paving needs room to expand and contract with changes in temperature. Leave movement joints around the perimeter and in intervals across larger areas. They should be filled with compressible foam backer rod and sealed with a flexible sealant.
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Curing before grouting
Leave the adhesive to cure before you attempt to grout; the manufacturer’s instructions will give you a time frame. Premature grouting can disturb the slabs and affect how it takes. Protect the area from rain, frost, or direct sun while curing, as adhesives are sensitive to extremes during this stage.
Laying paving slabs on concrete
Using existing concrete as a base for your paving slabs makes sense; it’s easy to do, and it avoids the mess and costs that come with digging it out.
The most important thing to note is that you’ll have to put in a bit of preparation work beforehand to get a professional finish.
If you’re ready to start your project, Ovation Landscaping has quality paving solutions and materials to complete the job. We have a wide range of natural stone and porcelain products to suit every style and budget. Browse online today and get them delivered to your home to get your new patio underway.