THE JOURNAL FOR THE SOCIETY OF GARDEN DESIGNERS

Sub-bases for paving: a basic guide

Portuguese limestone setts being laid on a non-permeable mortar bed over a 150mm compacted DOT Type 1 sub-base. Note visible falls to central drain. Photo: The Outdoor Room


Landscaper David Dodd explains the ins and outs of garden paving foundations


The sub-base is the main load-bearing layer of paving construction. A well-designed sub-base should evenly spread the load of traffic, whether pedestrian or vehicular, down to the sub-grade, also known as the formation level.

Best practice usually stipulates a geotextile non-woven membrane (e.g. Terram 1000), which acts as a filtration/ separation layer between the subgrade and sub-base. If the sub-base isn’t correct, a beautifully laid terrace may well become useless in a relatively short space of time. Failed paving construction can very often be put down to the sub-base being the wrong material, an insufficient depth or being poorly installed.

With the emergence of many new paving products on the market, and legal compliance for Sustainable Drainage Systems, recently a whole new world of best practices and materials have opened up for sub-bases and bedding, so it seems timely to revisit the basics of this important element of garden construction.


Reclaimed York stone detail – flexible construction



Flexible sub-bases

In the good old days, a paving sub-base used to be ‘hardcore’ comprising old bricks and broken up lumps of concrete, all of which were of varying sizes and shapes. Although often strong enough as a material, it was the random sizes that caused voids, which had to be blinded in with gravel afterwards. The irregular sizes often led to weak spots that would eventually cause sinkage or cracking of the paving above.

Don’t be afraid of using hardcore, as it’s an excellent way of recycling old material. You can buy or hire a crusher for any hard materials being broken out, but these are slow and very noisy. Therefore we tend to clear it away in skips or grab loaders and then buy it back as a 50mm diameter to dust ‘crushed recycled aggregate.’

The most common granular aggregate sub-base is DoT (Department of Transport) Type 1 and Type 3 Limestone. They are both crushed stone: Type 1 has a maximum permitted top size of 63mm, graded down to dust; and Type 3 is an open-graded 40mm unbound mixture with a reduced amount of fines. Both are porous, but Type 3 is becoming more and more popular if the finished surface is to be free draining or have a permeable jointing material.

There are also Type 2 and scalping sub-bases, which are usually 40mm down to dust, but with a higher percentage of fines, therefore having less load-bearing capability.


Brick on edge paving detail – flexible construction



What depth?

The thickness or depth of sub-base depends on the weight of traffic the paving is likely to incur. It also depends of the sub-grade/soil type. For sticky clay, it is advisable to go deeper, but for freer-draining, more stable sub-grades such as chalk or sandy soil, a general rule for a Type 1 sub-base could be as follows:

Light Duty (pedestrian patio or garden path) – 100mm to 150mm

Medium Duty (driveway for cars or vans not exceeding 7.5 tons) – 150mm min

Heavy Traffic (225mm for vehicles not exceeding 20 tons) – consider poured concrete

Highways – consult a civil engineer or the Highways Department

It’s essential that any flexible sub-base is laid and compacted properly, and compaction should be done in at least two layers using the correct equipment. A vibrating plate compactor may be suitable for patios and paths, but a much heavier-duty vibrating roller will be required for a driveway.


Rigid sub-bases

With the growing desire for hard surfaces to be more porous, many designers, architects and contractors are looking at alternatives to solid mass concrete, but is a flexible, granular sub-base as good as a rigid one? In my opinion, it certainly has its place, particularly when a heavy load is required or where unstable ground may be a problem. In this instance, it’s also worth considering reinforced concrete.

Paving an area of more than 6m x 6m will require expansion joints, and to comply with SuDS you will also need to ensure there is a suitable drainage system to manage surface water run off.

An example of a rigid sub-base specification for a driveway with medium to heavy duty load could be: Terram T1000 geotextile membrane; then 150mm compacted DOT Type 1 (compacted with a vibrating roller in 2no layers of 75mm); and 120mm RC25/30 concrete with 1 layer A142 fabric reinforcement.

A concrete sub-base is a lot more expensive than a granular one, but if it’s well laid and the stone is well calibrated, or if you’re using the increasingly omnipresent porcelain, the laying of the finished surface is far quicker, as you’re effectively gluing the paving to the base using a 5mm bed of exterior adhesive.