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Quite often households inherit gravel areas or simply decide they no longer want a gravelled garden. Gravel is sometimes seen as a cheaper alternative to paving creating seating areas quickly.
The problem with this cost cutting idea is the same budget objectives leads to incorrect installation. Low budget gravel areas are typically not based up correctly. Most of the time gravel is simply laid on membrane straight over the top soil.
After a few years soft, saturated soil particles can seep up between membranes in wet weather. An eventual build up of leaves and dirt lead to the establishment of weeds. Most of the time these gravel areas become messy and uninhabitable.
Consequently replacing gravel with paving is one of the most favoured options. As with most landscaping or building work there are no short cuts to sound construction.
Paving requires a solid base
In order to install any successful, hard wearing, surface like paving you need to avoid ground movement. The way to do this is by removing the top, dark coloured layer of topsoil.
This dark colour is due to the presence of 'humus' of decayed organic matter. When moist this topsoil layer can expand and displace under pressure.
This is why you need to remove the topsoil layer when excavating for hard standing surfaces.
The problem with about 80 percent of gravel areas is they have been laid straight over virgin soil. This means most gravel surfaces do not have a sufficient base for laying paving.
You certainly cannot lay paving straight over loose gravel. Although most gravel materials are hard wearing collectively they displace under pressure. This makes them too unstable to provide a compact base work for paving.
Can you use gravel as a patio sub base?
A sub base for a patio should be made up of materials which compact well with no movement.
Traditional sub bases for paving such as mot type 1 derive their compactable strength from angular particles. These particles when compacted lock together tightly creating a dense surface with no movement or displacement.
Type 1 sub base also has varying particle sizes from 40mm down to a millimetre. This means when compacted there is no air or free moving particles left within the surface structure.
This makes it the perfect, level base for paving.
Gravels on the other hand typically consist of evenly graded particles. This means they shuffle and disperse when any force is applied.
Even gravels with angular particles fail to compact tightly as their even grade results in air pockets. This unstable structure means all gravels are not suitable for patio sub bases.
The typical reason for wanting to use gravel for a patio base is recycling existing materials. It is much more expensive to skip used gravel and buy in fresh sub base.
However it is simply not worth cutting corners as you will always regret it in the long run. There is no point paying for brand new paving if it ends up moving or cracking.
Can you lay block paving on gravel?
Block paving is a popular choice for driveways and is laid on sand instead of solid mortar. This surfacing method is known as flexible paving and allows for slight compression movements.
This is perfect for accepting heavy loads such as motor vehicles. The surface of block paving locks together tightly, this allows for slight undulations when uneven loads are applied.
These movements are unnoticeable to the human eye due to the flexible nature of the surface. If a driveways pavers are bedded straight onto solid mortar there is a risk of cracking.
The compacted screed of sharp sand provides a perfect shock absorber for heavy loads. If you laid block paving straight onto gravel there would be too much movement.
The gravel would not provide a stable, compacted surface or be possible to screed level enough. Hence you cannot lay block paving on gravel.
As block paving still needs a solid, compacted base gravel would not be a suitable sub base.
Can you pave over gravel grids?
Gravel grids are effective at stabilising suitable depths of gravel but are certainly not suitable paving bases. Gravel grids have built in flexibility which can cause movement under the pavers.
To ensure any new hard wearing surface does not fail you need to remove such surfaces. By correctly excavating out soft ground, gravels, membranes and grids you can guarantee a secure base.
This will ensure new paving to be fully durable and last the test of time.
Can you use gravel under concrete?
You have more chance of reusing old gravel if you intend to spread it under concrete. Old gravels can be spread under concrete bases but only if they are clean and not too deep.
If any concrete surface is laid upon to much loose aggregate there is a risk of cracking. The threat of this can be reduced if you incorporate reinforcing mesh.
Concrete laid on top of unstable gravels on gradients can gradually shift downhill. If you must spread loose gravel under concrete mix them with cement and ballast to base under stronger concrete.
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Paul Nicolaides has over 30 years of recreational gardening and 20 years of professional landscaping experience. He has worked for landscape contractors including design and build practices across London and the South East. In 2006 he qualified with a BA Hons degree and post graduate diploma in Landscape Architecture. In 2009 he founded Ecospaces an ecological landscaping practice which aims to improve social cohesion and reduce climate change through landscaping. In 2016 he founded Buckinghamshire Landscape Gardeners which designs and builds gardens across Buckinghamshire and the South East. This blog aims to provide easy problem solving information to its audience and encourage others to take up the joy of landscaping and gardening.