A level, firm and stable foundation is one of the most important stages, if not the most important stage and starting point for building a summer house, shed or any other garden building.
A good base ensures stability of the cabin, smooth assembly, longevity and a good look. A poor and improper base results in the reverse outcome and puts a great pressure on the whole cabin which will lead to a shorter life span and numerous other problems like gaping between the wall planks, poor overall appearance as well as problems with the functioning of doors and windows.
The following base types are all good foundations for garden buildings. Each foundation type has its own pros and cons, so we will take a good look at each one of them to help you in choosing the best base type depending on the garden surface, cabin size and your budget.
IMPORTANT! Please email us for detailed dimensional plans to use for base preparations and to help decide if the cabin is suitable for your intended use. Plans can be requested from [email protected]
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Whether you are building a small storage shed, 4 x 5m garden room or very large 5 x 10m office building, you can’t go wrong with a concrete base. It is the most durable and solid foundation but like with all the other good things in life, a concrete foundation is one of the most expensive bases to build and involves more working!
How to prepare a Concrete Base?
If you are building near a fence, trees, wall or similar, leave at least half meter provision around the cabin for roof overhangs, convenient installation and maintenance. Mark the area and excavate approximately 20cm depth accordingly to your cabin plans. Add 5 cm to all sides for wooden boxing that you will install later.
Now infill with 15cm of granular subbase or gravel. Level with garden rake and compact it by using a roller or tamper.
Next step is to build a wooden frame or shuttering within which to pour the concrete. Support the frame from the outside by knocking into the ground some posts along the outside edge of the timber. Install the wooden boxing so that the top of the concrete base will be 4-5cm above the ground level. This will result in water running off rather than pooling around the base of the cabin. Inner area of the wooden boxing must be in strict compliance with your cabin plan and well levelled. Measure up all four sides and compare diagonals, they must be an even length. Level the boxing by using a spirit level.
Now cover the whole base surface with damp proof membrane. This will protect the cabin from moisture.
For cabins, greater than 10m2 we recommend using a steel reinforcement mesh (steel bar). This will significantly add strength and crack resistance. Steel bars should be lifted up to the centre height of the slab.
Next stage of building a concrete base is pouring the concrete in to the shuttering. You can either mix your own concrete or get a ready mixed supply in. You will need a wheelbarrow, shovel, long piece of wood with straight edge and one extra pair of hands. For mixing your own you will also need a cement mixer.
Start pouring from one side and work towards to other side. Bring concrete by using a wheelbarrow, pour down and evenly and level by using shovel and long piece of wood. Hardening will take 2-3 hours on warm day and 4 hours when it is cooler. Pull out supporting posts and knock off wooden boxing around the base. Final hardening will take 3-4 days before the assembly can start.
Strip foundations are an easier, lighter and less expensive alternative to a concrete slab. Fits to almost any size garden building if the ground is not sloped. It is easy to remove strips if you buy a new garden room in the future and the old one needs to be knocked down and a new base built. Concrete strips are firm, strong and stable.
How to prepare a Strip Foundation?
Start with marking down strips by using your cabin’s foundation plan. Strips must be perpendicular to internal foundation planks of the cabin with up to 90-100cm distance between the strips to ensure that foundation planks and finally floorboards would be firm and sturdy enough without bending.
Now excavate 15-20 cm deep and 15cm wide strips in compliance with your cabin’s foundation plan. Infill 10-15cm granular subbase or gravel. Build wooden boxing approximately 5 cm above the ground, which will result to top of strips being same distance from the ground ensuring good ventilation beneath the cabin thus keeping cabin moisture free. Support wooden boxing from the outside by tapping posts into the ground and nailing them to boxing. This will strengthen the whole wooden shuttering to handle concrete pressure while hardening.
For extra strength, we recommend using steel reinforcement mesh inside the strips. Before you pour concrete, lift the mesh 4-5 cm from the bottom, so that it would be height-wise in the centre of the strip.
Pour concrete into the wooden boxes. Since strip foundations require several times less concrete compared to a full concrete slab, and therefore mixing your own concrete should be an easily achievable task, or you can get a ready mixed supply in. You need a cement mixer, a wheelbarrow for taking concrete to building spot, a shovel and a 20-25cm long piece of wood with straight edge.
Start pouring concrete from one end of the box working towards the other side. Pour down concrete and level by using shovel and long piece of wood. Hardening will take a few hours on a warm day and 4-5 hours when it is cooler.
When strips have been laid, clean the ground between the strips. Remove approximately 5 cm of ground, cover the surface with damp proof membrane, geotextile or similar. Finally fill in gravel, sand or subbase granular. This will keep grass away of growing into the cabin that would lead to decay and rotting.
Pull out supporting posts and knock off wooden boxing around the base. Final hardening will take 2-3 days before you can start cabin’s installation.
A paving slab base is a good base for small and medium sized garden buildings. It is one of the easiest bases to build, relatively cheap and does not involve concrete work. On the other hand, it looks nice, it is easy to install a cabin on paving slabs and it is easy to remove or extend in the future should you want to build a larger cabin.
How to lay a paving slab base…
Excavate a 15-18cm deep hole accordingly to your foundation plan and you may leave 10-15 cm extra beyond the boundaries of the cabin to allow for an extra area of dry ground around the building. Cover the surface with a damp proof membrane or similar.
Lay down the subbase of 15cm of gravel or MOT 1 granules. Even and level the subbase by using a garden rake and compact it by using a roller or tamper.
Now lay down 3cm of sand, even with shovel and rake but don’t compact.
Start placing down paving slabs. Leave 3mm distance between the slabs, push and tap them to place and level.
Finally infill the gaps between the slabs with sand. Sand in the gaps must be well compacted, therefore push the sand tightly into the gaps.
If you are looking for a firm and durable base for a shed or garden room then this option comes with a very reasonable price. The adjustable riser pedestal is superb if you already have hard standing surface that needs leveling.
How to prepare a Pedestal Foundation?
First count the number of risers you need for your cabin. For this purpose, use the cabin’s foundation frame plan. Price per riser is around £5. Alternatively, you can use breeze blocks on gravel.
Dig out 20cm of ground and leave 10 cm extra beyond the boundaries of the cabin to allow for an extra area of dry ground around all the outer surface.
Build a barrier by the inner sides for the excavated area by using wooden planks. This will help you when laying gravel and when levelled keeps the gravel base in place.
Cover the bottom with membrane to avoid grass growing through the base beneath the cabin. Fill the hole with MOT 1 subbase granular or gravel and level with a garden rake. You can then compact the ground to ensure there is no movement overtime.
Now place the riser pedestals or breezed blocks accordingly to the foundation plan. We recommend keeping the distance between the risers within 50-60cm. Support all foundation planks, including all outer and inner planks.
First screw the full foundation frame structure together accordingly to your cabin plan, lift it on the risers, level by adjusting the hight of risers and screw the planks to the risers.
When using breeze blocks, first place the blocks, then screw together the whole foundation frame, then level and finally screw outer planks to blocks.