Laying Tarmac Over Concrete: The Professional Guide

You can lay tarmac over an existing concrete driveway, as long as the concrete is structurally sound, drains properly, and there’s no threshold problem at the entrance. Done right, it’s a faster, cheaper way to get a fresh tarmac finish than breaking the concrete out. Done wrong, the joints and cracks in the concrete telegraph straight through into your new surface within a year or two. This guide walks through how we assess and do it across Kent, and the situations where we’ll tell you to break out instead.

Key points

• Tarmac over concrete works when the slab is stable, level and intact, and fails when it isn’t.
• The single biggest risk is reflective cracking: existing cracks and joints in the concrete mirror up through the tarmac.
• A tack coat bonding agent between the two surfaces is non-negotiable.
• Adding 40–70mm of tarmac raises the surface, so thresholds, doors and drainage all need checking first.
• An overlay over sound concrete is far cheaper than a full break-out and rebuild.

Assessing your existing concrete base

Everything starts with the slab. Before we quote an overlay, we check four things.

Structural soundness. The concrete has to be stable, with no rocking sections, no sinking corners and no soft spots. Tap or load-test suspect areas. If a slab moves under weight, tarmac laid over it will move and crack with it.

Cracking. Hairline surface cracks are usually fine. Wide, active cracks (especially ones that run right through the slab or have lifted at the edges) are a warning sign, because those are the cracks most likely to reflect through the new surface.

Levels and falls. The concrete should already shed water away from the house. If it ponds, that water has nowhere to go once it’s sealed under tarmac, and it’ll attack the bond and the base from below.

Surface condition. It needs to be clean, dry and free of oil, paint, curing compounds and laitance (the dusty top film on old concrete). Tarmac won’t bond to a contaminated or dusty face, so prep here directly affects how long the job lasts.

If the slab is sound on all four, an overlay is a strong option. If it fails on structure or drainage, we’d steer you to a break-out, because no amount of surface work fixes a base that’s moving or holding water.

Managing threshold heights and step transitions

This is the detail that catches people out. A tarmac overlay adds roughly 40–70mm of height across the whole area, and that has to go somewhere.

The places it bites:

• Door and garage thresholds. Raise the drive 50mm and a door that used to clear the surface may now scrape it, or worse, you’ve reduced the step that kept water out of the garage.
• Air bricks and the damp-proof course. The finished surface must stay clear of the DPC and any air bricks, or you risk bridging the damp barrier and pushing moisture into the wall.
• Drainage gullies and channels. Existing ACO channels and gully gratings need to finish flush with the new level, which sometimes means lifting and resetting them.
• Steps and paths. Where the drive meets a path or step, the new height changes the rise, and an uneven transition becomes a trip hazard.

We design these transitions in before we lay a thing. Often the answer is a feathered edge or a tapered margin near the threshold; sometimes it’s a small drainage channel at the garage line. The point is that height is a planning decision, not something to discover with a roller running.

Preventing reflective cracking

Reflective cracking is the main reason tarmac-over-concrete jobs go wrong. Concrete is laid in slabs with joints between them, and it cracks as it ages. When you surface over the top, thermal movement and traffic flex the slab at those joints and cracks, and that movement works its way up through the tarmac until the same crack appears on the new surface directly above the old one. It’s been well studied in pavement engineering: reflective cracking is driven by thermal and traffic-induced movement at the joints below.

You can’t eliminate the risk on a jointed concrete base, but you can manage it down a long way:

• Treat the cracks first. Wide and active cracks are cleaned out and sealed before overlaying, rather than buried as-is.
• Lay enough thickness. A thicker overlay resists reflective cracking better than a thin skim; research consistently shows crack resistance rising with overlay depth.
• Use a stress-absorbing interlayer where it’s warranted. On larger or more heavily trafficked areas, a glass-fibre geogrid or stress-absorbing membrane interlayer (SAMI) laid within the tarmac spreads the movement and slows cracks reaching the surface. Geogrid systems are typically specified with a minimum overlay thickness (often around 70mm), so this is a design decision made up front.

For a domestic drive over sound, lightly jointed concrete, good crack prep plus adequate thickness usually does the job. For a commercial yard or a slab with significant jointing, an interlayer is worth the spend.

Bonding agents: the role of tack coat

A tack coat is a sprayed bitumen emulsion applied to the concrete before the tarmac goes down, and it’s what fuses the two surfaces into one. Concrete and tarmac are different materials that don’t naturally stick, so without a tack coat you’ve simply stacked one on the other. Under traffic and temperature swings, an unbonded overlay slides, blisters and lifts.

The tack coat does three jobs: it glues the tarmac to the slab, it seals the concrete against water getting between the layers, and it helps the finished build behave as a single structure rather than two that fight each other. It’s cheap, it’s quick, and leaving it out is one of the clearest signs of a job being rushed.

Cost comparison: overlay vs full break-out

The cost case for overlaying sound concrete is strong. You keep the slab and the ground works, and pay only for prep and a new surface.

On top of the per-m² gap, a break-out adds skip or grab-lorry charges for the broken concrete, which is heavy and expensive to take away. The overlay also takes one to two days against several for a full rebuild.

The honest caveat: an overlay over the wrong concrete is the most expensive option of all, because you pay for the surface twice. If the slab is moving, cracked through, or holding water, the break-out is the cheaper choice once you count the repeat visit. That’s the call we make on site.

Frequently asked questions

Can you lay tarmac over a concrete driveway? Usually yes, provided the concrete is structurally sound, drains away from the house, and there’s no threshold issue at the entrance. We apply a tack coat bonding agent between the surfaces and treat any significant cracks first. Where the slab is failing, a full break-out is the better option.

How thick does tarmac need to be over concrete? Thicker is better for resisting reflective cracking, so we’d typically lay 40mm or more, often in two layers on heavier-use areas. A thin skim bonds but cracks sooner. The exact build depends on the slab and the traffic it carries.

Will the cracks in my concrete come through the tarmac? They can, and that’s reflective cracking. The risk is managed by sealing the cracks first, laying adequate thickness, and using a stress-absorbing interlayer on larger or more cracked slabs. On a sound, lightly jointed slab the risk is low.

Do I need to remove the concrete first? Not if it’s sound, level and draining. Keeping the slab is the whole point of an overlay and where the saving comes from. We only recommend removal where the base is compromised.

Does tarmac over concrete need planning permission? Resurfacing an existing hard surface usually doesn’t. We confirm drainage and permitted-development rules during the site visit.

Thinking about tarmac over your concrete drive?

We’ve been laying tarmac across Ashford, Tenterden, Canterbury, Folkestone, Hythe and the wider Kent area for over 40 years, and we’ll give you an honest answer on whether your slab is a good candidate. See our tarmac resurfacing and overlay service, or book a free site visit for an itemised quote.