Why there are no default thicknesses
We could fill the boxes with figures that sound sensible. We don't, because the right thickness depends on the building's weight, the ground, what will stand on it and what the maker requires for the guarantee. Many garden-building suppliers publish base instructions; use theirs. If you are building your own bar, a local groundworker can tell you what your soil needs. Our base guide explains the options and when each makes sense.
Reading the results
- Area is the base size, including any margin you add beyond the walls.
- Edge length is the perimeter: the length of timber formwork for a slab, or edging for a paved base.
- Concrete and sub-base are volumes in cubic metres. Suppliers sell ready-mixed concrete and aggregate by volume or by weight, so ask them to convert if needed.
- Bags of mix appears when you add the yield printed on the bag. Round up and keep a spare: running out halfway through a pour leaves a weak joint.
- Slab count works on a grid, so a base that doesn't divide neatly shows how many slabs will be cut.
The waste buttons add a margin for spillage, uneven ground and breakages. Choose it yourself based on the ground and your experience.
Before you dig
- Check the planning starting point. A base raises the building, and height is measured from the ground.
- Look for buried services. Pipes and cables often cross gardens in unexpected places.
- Plan drainage so water runs away from the building, not under it. See damp and ventilation.
- On sloping ground, read building on a slope first. The volume of a level base on a slope is not a simple rectangle.
- If the bar will go on decking instead, read garden bars on decking.
Comparing base types
Concrete, paving, ground screws and timber frames each suit different gardens. Our comparison of ground screws vs concrete sets out the trade-offs, and the build vs buy guide shows where the base sits in the wider project.
Next step
Put the results into the base line of the budget planner once you have a quote for materials or labour.
Quick answers
How thick should a garden bar concrete base be?
Follow the building maker's base instructions, or ask a groundworker who has seen your soil. The right thickness depends on the building, the load and the ground, so we do not publish a single figure.
Should the base be bigger than the building?
It depends on the maker. Some want the base to match the footprint so rain does not sit on a ledge by the walls; others allow a margin. Use the margin box only if your instructions call for one.
How do I turn cubic metres into bags?
Look on the bag for the volume one bag makes, usually given in litres, and enter it in the yield box. One cubic metre is 1,000 litres.