Laying Self Binding Gravel Over Concrete or Tarmac
Laying self binding gravel over an existing concrete or tarmac surface is possible in the right conditions and can save significant time and effort compared to a full excavation and new-build installation. The existing hard surface acts as a ready-made sub-base, eliminating the need to excavate and lay fresh aggregate beneath. Whether this approach is appropriate for your site depends on the condition of the existing surface, the drainage situation, and the depth available between the existing surface and any adjacent features such as door thresholds or drainage channels.
When Is Laying Over an Existing Surface a Good Option?
Laying self binding gravel over concrete or tarmac is a practical approach when specific conditions are met.
The existing surface is structurally sound. A sound concrete or tarmac surface, free from significant cracking, settlement, or crumbling, provides a stable and even base for the self binding gravel layer above it. A surface that has large cracks, sunken areas, or sections that flex underfoot is not a suitable base and needs repair or removal before any new surface material is added.
There is adequate depth to work with. The self binding gravel layer needs a minimum compacted depth of 50mm to perform adequately for pedestrian use, and 75mm for vehicle use. Before committing to this approach, measure the available depth between the existing surface and the threshold of any adjacent buildings, the top of any drainage channels, and any step edges or level changes. If there is insufficient depth, the self binding gravel layer will be too thin to hold together properly.
Drainage is acceptable. Laying an additional surface over an existing hard surface changes the drainage characteristics of the site. The considerations involved are covered in the section on drainage below.
Preparing the Existing Surface
Preparation of the existing concrete or tarmac surface is the most important step in achieving a good result when laying over an existing base.
Clean the surface thoroughly. Remove all moss, algae, loose debris, and any vegetation growing through cracks. A pressure washer is the most effective tool for this. The self binding gravel needs to sit against a clean surface, and organic material beneath the new layer can create soft spots and promote further weed growth.
Repair significant cracks and voids. Fill any significant cracks in the concrete or tarmac with a suitable filler compound and allow it to cure fully before proceeding. The goal is to create an even base surface without large voids that the self binding gravel can migrate into.
Check the drainage falls. The existing surface was presumably installed with drainage falls to direct surface water to a specific point. Identify where those falls run to and check that the new self binding gravel surface, laid on top, will maintain adequate falls in the same or an appropriate direction. A very flat existing surface may require care in spreading the self binding gravel to maintain positive drainage falls.
Consider edge containment. The edges of the existing surface define the edges of your working area, but self binding gravel laid up to a free edge without a physical restraint will gradually break down and crumble. Installing edging around the perimeter of the area, even where the existing surface appears to provide a defined edge, extends the life of the finished surface significantly.
Does a Geotextile Membrane Go Between the Hard Surface and the Gravel?
A geotextile membrane is not strictly required when laying self binding gravel over an existing concrete or tarmac surface because the hard base prevents soil and root ingress from below. However, a membrane can still be useful in some situations.
On a concrete surface with minor cracking, a membrane helps prevent the self binding gravel fines from gradually washing down into the cracks over time, which would cause the surface layer to settle unevenly.
On a tarmac surface with a rough or open texture, a membrane can prevent the finer particles of the self binding gravel from bedding into the tarmac surface and reduce the risk of contamination between the two materials.
In most standard situations, however, a membrane between the hard base and the self binding gravel is optional rather than essential.
Drainage When Laying Over Concrete or Tarmac
Drainage is the most important consideration when covering an existing impermeable surface with self binding gravel, and it requires careful thought before you proceed.
Self binding gravel retains some permeability after compaction. Surface water that falls on the self binding gravel layer will pass through the aggregate and reach the concrete or tarmac below. From there it has nowhere to go except along the drainage falls of the existing surface to the nearest outlet. This means the drainage outlet capacity of the existing system needs to be adequate for the new surface area.
If the existing drainage falls are adequate and the outlet is clear, this is generally not a problem. The self binding gravel layer actually improves the situation compared to bare tarmac in one respect: the aggregate slows the rate at which rainfall reaches the base surface, reducing peak flow to the drainage outlet.
If the existing surface has no positive drainage falls and water sits on it, covering it with self binding gravel will not improve the drainage situation. Water that cannot drain from the base of the self binding gravel layer will saturate the material from below, softening the surface and potentially causing the aggregate to become unstable. In this case, either the drainage must be improved before proceeding or a different installation approach is needed.
If you are covering a driveway that previously drained to the public highway, check whether planning or drainage requirements apply in your area. Covering an existing impermeable surface with a semi-permeable layer typically does not reduce overall drainage performance, but local regulations on surface water management vary.
For the detailed answer to whether self binding gravel itself is permeable and what that means for drainage planning, the is self binding gravel permeable page covers this specifically.
How to Lay Self Binding Gravel Over an Existing Hard Surface
The installation process when laying over an existing hard surface is simpler than a new-build installation because the sub-base work has been eliminated.
Step 1: Clean and prepare the existing surface as described above. Allow the surface to dry fully before adding the self binding gravel.
Step 2: Install edge restraints around the perimeter if not already in place. Ensure edging is set at the correct height to give the self binding gravel layer the right finished depth.
Step 3: Spread the self binding gravel at approximately 75mm depth for pedestrian areas or 100mm depth for vehicle areas. These pre-compaction depths will reduce to approximately 50mm and 75mm respectively after compaction. Spread with a rake from the far end of the area, working backward to avoid standing on freshly spread material.
Step 4: Lightly dampen the surface with a watering can or fine hose spray before compaction. Do not saturate the material.
Step 5: Compact with a plate compactor, working in overlapping parallel passes. On harder base surfaces such as concrete, the compaction feedback will feel different from compacting over a sub-base, and the self binding gravel layer will reach a firm finish more quickly. Check the surface level after compaction and top up any low areas.
For the full installation process and equipment guidance that applies to new-build installations, the how to lay self binding gravel guide provides a complete step-by-step reference.
Can You Lay Self Binding Gravel Over Tarmac That Is Breaking Up?
Deteriorating tarmac that is crumbling, potholed, or has large areas of surface loss is not a suitable base for self binding gravel. The uneven surface creates voids and inconsistent depth in the self binding gravel layer above it, and the loose tarmac material will continue to degrade beneath the new surface, causing progressive settlement and soft spots.
In this situation, the options are to repair the tarmac surface to a sound and even condition before proceeding, to remove the deteriorated tarmac entirely and proceed with a full new-build installation, or to level the tarmac surface with a sand and cement mortar screed before laying the self binding gravel on top.
The choice depends on the extent of the deterioration, the available budget, and the planned life of the new surface.
Frequently Asked Questions
Will self binding gravel stick to concrete? Self binding gravel does not bond chemically to concrete. It sits on top of the concrete surface as a compacted aggregate layer and relies on its own internal binding action and the surrounding edge restraints to hold its position. Movement of the self binding gravel layer relative to the concrete below can occur over time, particularly at edges, which is why edge restraints are important even when laying over an existing hard surface.
How thick should self binding gravel be over concrete? A minimum compacted depth of 50mm for pedestrian areas and 75mm for vehicle areas applies when laying over concrete, the same as for new-build installations. Going thinner than 50mm produces a surface layer that is too shallow to hold together reliably under traffic.
Can self binding gravel be laid over block paving or slabs? Yes, in principle. The same conditions apply as for concrete or tarmac: the existing surface must be sound, level, and have adequate drainage, and there must be sufficient depth available between the existing surface and adjacent thresholds. Laying over block paving tends to produce a slightly less even base than laying over concrete, because the joints between blocks can create minor height variation that the self binding gravel layer bridges.