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 and your budget.
A) Adjustable Riser Pedestal or Breeze Blocks on to gravel
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. Compared to breeze blocks, riser pedestals are very easy to level.
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 £3-£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.
Now place the riser pedestals or breezed blocks accordingly to the foundation plan. We recommend keeping the distance between the risers within 70-80cm. 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. Bitumen strips under the planks are good to use for levelling and keeping the frame moisture free.
B) Concrete Slab Base
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 hours!
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 sub base 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 in to 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 center 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.
C) Strip Foundations
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 floor boards 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 sub base 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 center 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.
D) Paving slabs
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 granulas. Even and level the sub base 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.
E) Ground Screws
Ground screws are a cost effective, fast and easy way to install a durable base to any type and size of garden building providing the soil is not very rocky and without a lot of stones in ground. It is also one of the best solutions for building a garden room or a shed on a sloped and uneven surface. Installing ground screws does not require any special skills or knowledge. Different ground screws are installed into 60-90cm depth, so you must be sure that there are no cables, pipes, heat pumps or similar that could be damaged.
How to install ground screws?
Making a shed base out of ground screws, in brief, is just turning ground screws into the soil and placing thick and sturdy wooden beams on to the ground screws.
U-type ground screws are the most convenient ground screws for building a base for your summer house, because foundation beams are easy to place into U-profile type of screw heads and they sit there firmly. One screw can take 500-1000kg of weight. Price per screw is around £30-50.
For building ground screw foundations, you need the appropriate number of ground screws, wooden beams, wood preservative, damp proof membrane, gravel, saw, screws, screwdriver, shovel, garden rake and 50-70 cm long wooden plank or a metal rod for turning screws into the ground. Beams should be as thick as the inner width of the U-Profile or a little less and preferably 100-120mm high. Thick and sturdy beams help you to build a firm base with fewer ground screws used and therefore less money spent.
Count the number of required ground screws and wooden beams by using your cabin’s foundation plan. If you are using 50-70mm thick and 100-120mm high beams, you can place one ground screw per each 1.5m of beam. Wooden beams must be laid perpendicular to cabin’s foundation planks and distance between the beams should be not more than 80-100mm to ensure a firm and hard floor without bending.
Cut the wooden beams in to the length and treat with wood preservative.
Mark down cabin spot and remove 7-10 cm upper soil from the entire surface.
Turn in ground screws by using a piece of wood or metal rod in the U-profile.
Cover the entire base with damp proof membrane and then infill with gravel or granite MOT 1 sub base mix. Even out with a shovel and garden rake.
Finally sit wooden beams on the ground screws and fix with screws.
F) Concrete Pier Foundation
A concrete pier foundation is one of the best base types if you are building a cabin on a steeply sloped or damp surface and it is also a widely used base type for building a summer house or a garden room high above the ground. Concrete pier foundations fit to any sized garden building or summer house. It requires far less time to construct than a concrete slab or even strip foundation base, so you save time and money.
How to build a concrete pier foundation?
For building a concrete pier foundation you need the following tools: auger drill, shovel, garden rake, cement mixer, wheelbarrow and 50-60 cm long wooden posts, level, tape measure and hand saw. Then you need concrete mix, gravel, tube forms for concrete piers with 15-20 cm diameter and a damp proof membrane.
Start with removing 7-10 cm of upper soil from the entire cabin spot.
Excavate 50-60cm deep holes for piers accordingly to your cabin plan by using an auger drill. Keep the distance between the piers under 1m to ensure a firm and stable flooring for your cabin. Holes must be a minimum 20cm deeper than the frost line and slightly larger diameter than the tube forms.
Fill the bottom of the hole with 15 cm gravel and compact with wooden post.
Cut the tube forms into length by using a handsaw. Measure the depth of the hole, then add 7-10cm length for the remaining depth and finally the desired length of how high the piers should be from the ground to get the full length of tube forms.
Place tube forms into the holes and level. Mix concrete and fill the tubes.
Cover entire cabin spot with damp proof membrane and then infill the hole spot with gravel. Hardening will take three days, before cabin installation can start.