Specifying a Car Park Build-Up: Sub-Base to Wearing Course
A car park build-up is the designed sequence of layers from the ground up: the subgrade (the prepared existing ground), a compacted granular sub-base that spreads the load, and the bound surfacing layers, typically a binder (base) course and a wearing (surface) course of asphalt. The build-up is specified to suit the traffic and loads the car park will carry (cars, vans, delivery vehicles or HGVs) and the strength and drainage of the ground beneath, with weaker ground or heavier loads needing a thicker, stronger build-up. Get the specification right, adequate sub-base, the correct bound layers, proper compaction and drainage, and the car park carries its loads for years; under-specify it and it deforms and fails early.
A car park is only as good as the layers you cannot see. The surface takes the credit, but it is the build-up beneath, the subgrade, sub-base and bound layers, that carries the loads and decides how long the car park lasts. For architects, engineers and developers specifying a car park, getting that build-up right is the whole job. Here is how it is designed, layer by layer.
The Layers, From the Ground Up
- 1Subgrade: the existing ground, prepared and compacted. Its strength is the foundation everything else is designed around.
- 2Sub-base: a compacted granular layer that spreads the traffic load over the subgrade and provides drainage and a working platform.
- 3Binder (base) course: the lower bound asphalt layer that provides the structural strength of the surfacing.
- 4Wearing (surface) course: the top asphalt layer that takes the traffic and weather, and provides the finished surface and skid resistance.
Matching the Build-Up to the Loads
The build-up is not one-size-fits-all, it is designed for what the car park will carry. A staff or customer car park seeing cars and the occasional van needs less than a service yard taking loaded HGVs and skip lorries, which need a much stronger, thicker build-up to resist rutting and deformation. Under-specifying for the traffic is a classic cause of early failure: the surface ruts and cracks because the layers beneath were never designed for the loads crossing them.
The Ground and the Sub-Base
The other half of the equation is the ground. Weak, wet or variable subgrade needs a stronger, thicker sub-base (and sometimes stabilisation or a capping layer) to provide a sound platform, whereas good ground needs less. Assessing the subgrade and designing the sub-base to suit is fundamental: a beautiful surface on an inadequate foundation will still fail. Drainage ties in here, keeping water out of the build-up so it retains its strength (see car park drainage).
Getting the Specification Right
A sound specification balances the loads and the ground: adequate, compacted sub-base, the correct bound layers, proper compaction throughout, and drainage designed in, with each layer laid and compacted correctly, because compaction is as important as thickness. Mactar builds car parks and hardstandings to specification across Leinster, with our own crews and Kilsaran materials, and can advise on the build-up for the traffic and ground. See what car park surfacing costs, where the build-up drives much of the price, and car park resurfacing for renewing an existing one.
Frequently Asked Questions
What are the layers of a car park build-up?
From the ground up: the prepared subgrade (existing ground), a compacted granular sub-base that spreads the load, a binder (base) course providing structural strength, and a wearing (surface) course of asphalt taking the traffic and weather. Each is designed and compacted to suit the loads and ground.
How thick should a car park build-up be?
It depends on the traffic and the ground. A car park seeing cars and light vans needs less than a service yard taking loaded HGVs, which needs a much thicker, stronger build-up. Weak or wet ground also needs a thicker sub-base. The build-up is designed for the specific loads and subgrade.
Why do car parks rut and fail early?
Usually because the build-up was under-specified for the traffic, or the ground and drainage weren't dealt with. If the layers beneath aren't designed for the loads crossing them, or water gets into an inadequate build-up, the surface ruts, cracks and fails. Correct specification and compaction prevent it.
Need a Car Park Build-Up Specified?
Mactar is a tarmac, asphalt and civils contractor based in Duleek, Co. Meath, working across Leinster on commercial surfacing, car parks, roads and reinstatement, with our own crews and Kilsaran materials behind us. Build-ups are specified with reference to National Standards Authority of Ireland guidance on pavement materials and construction. Tell us about the site and we will assess it and quote the right repair or resurfacing solution.