Shed Base, Concrete Base and Concrete Garage Base
Garage and Shed concrete base information for prefabricated garages.
Garage foundations are required to transmit the load of the building safely to the ground. Therefore, all sectional buildings should have adequate foundations (normally concrete), which will vary from one project to another depending on the circumstances of each case.
These concrete foundations can be cast as deep-fill (filling most of the trench) or shallow-fill (where the minimum thickness to transfer the load to the soil is provided).
There are other types of concrete foundations that may be used if the ground conditions do not make trench fill practicable. It is advisable to contact a structural engineer or speak to building control for further advice.
Factors to be taken into account of when designing a foundation:
Type of soil
The type of soil that the foundation will sit on is important for two reasons:
Firstly, it should be able to bear the weight (load) of the foundation and the extension - different soils have different load bearing capabilities.
Secondly, the way it reacts to variations in moisture content (such as in prolonged rainy or dry seasons) can lead to the soil expanding or contracting. This is a particular issue with some clay soils. These changes mainly occur up to a certain depth (typically about 0.75m) therefore foundationsÂ should be made deeper so they are not affected by ground movement (although see "Trees" below).
It is important to ensure that the excavation for the new foundation does not undermine adjacent structures. In general it is good practice to excavate at least to the same depth as the bottom of the foundation to the adjacent building. If the excavation runs alongside an existing footing then care will be needed - for example, by excavating and concreting the foundation in shorter sections to avoid undermining a whole length of an adjacent structure (see also guidance on the Party Wall Act).
Trees will draw moisture from the ground around them and beyond through their root system. As moisture is drawn from the ground it will have a tendency to shrink. How much the ground will shrink will depend on the following factors:
Type of soil - Clay soils shrink more than other types of soil. Therefore excessive movement of the ground could cause damage to the foundation and the structure it supports.
Size and type of tree - How large a tree or shrub will grow (its mature height), and the tree type will determine how much moisture it generally draws from the ground.
The presence of trees in clay soil areas can mean foundations need to be significantly deeper than might be first expected, although if the trees are far enough away, there may be no impact. Note: If existing trees are removed or significantly reduced in size, all or some of the moisture in the root system will be released over time into the soil and, if the soil is clay for example, could cause swelling of the soil and damage to nearby foundations and structure(s) supported.
Drains and sewers
As the weight (load) from the foundation of a building is transferred to the soil it spreads downwards outside the footprint of the foundation at a typical angle of 45 degrees. If a drain or sewer is within the area covered by that 45 degrees area there is a risk that it could be affected by the load from the foundation and possibly crack. Therefore,the foundation excavation should normally be at least to the same depth as the bottom (invert) of the deepest part of the drain, sewer or its trench.
Size and construction of new building
The foundation will need to support more weight (load) from a two storey building compared to a single storey. This has a significant factor in determining design, particularly in respect of its depth and width. This is directly related to the bearing capacity of the soil supporting it. The width of the foundation is also governed by the wall thickness.
Generally the topsoil is taken away and good undisturbed ground is found i.e. ground that has not been built on. In some cases there are areas which have previously been backfilled, such as above where drains have been laid or to level a site, which consist generally of soft, mixed soil with foreign objects. The foundation can not be poured until undisturbed ground has been found.
Some properties have been constructed on landfill sites which may require a more extensive form of foundation like piling as the depth of undisturbed ground could be many metres deep. An alternative may be a "raft" foundation. A structural engineer will be able to advise you further.
For health and safety reasons, care should be taken when working in trenches due to the risk of collapse causing potentially serious injury.
This is an introductory guide and is not a definitive source of legal information and has been extracted from the government planning portal which was updated October 2008.
NOTE: This guidance relates to the planning regime for England. Policy in Wales and Scotland may differ. If in doubt contact your Local Planning Authority
Concrete Shed Basework.
A solid concrete base is required to erect the building on.
We will not carry out erection on paving slabs or tarmac.
A base minimum 3” (75mm) longer and 3” (75mm) wider than the external size of a shed is recommended.
The concrete must be a minimum of 4” thick in the center and 12" around the perimiter.
Concrete Garage Basework
All of our sectional buildings require a concrete base as a foundation.
Your local groundworks contractor can normally offer a base laying service, or alternatively you can consult a local builder.
The basework will have to conform to the following guidelines.
The concrete base will ideally be slightly higher than the surrounding area so water does not drain from the ground to the base.
The concrete itself should be a minimum of 4" (100mm) thick, and 6" (150mm) longer and 6" (150mm) wider than the external size of the garage.
In order to check that your base is square, measure out a 3-4-5 triangle before securing your shuttering prior to laying concrete, as per the following diagram:
To double check your measurements, measure along the diagonals and ensure both measurements are equal, as per the diagram below.
If you are close to an existing building or boundary wall, ensure that you allow for the roof overhang (Knight Pent roof 4" (100mm) either side, Apex roof 6" (150mm) either side).
This does not include gutter width. After the garage has been erected, we strongly recommend that a sand and cement fillet is pointed around the inside (not outside) of the building to prevent ingress of water between the panels and the base, we can provide this if required.
A 3 parts building sand : 1 part cement mix is recommended, with a PVA added to the mix.
Seal the garage on the inside only, not the outside of the panels.
Diagram showing how the concrete base should be built, a polythene membrane
should be placed between the blinded hardcore and the cement to prevent the damp rising