What Sort of Foundations are Needed for my Garden Shed?

The foundation of a shed is the first and most important part of the installation process. Having a suitable base is absolutely necessary in order to build a functional, long-lasting structure. Irrespective of how strong the building construction is, if the foundation is poor your shed will not stand the test of time.
In order to achieve a simple, inexpensive foundation appropriate for your shed, think about its size, use and location.

Size

Small Sheds

The smallest sheds don’t need a foundation at all. They can often be situated on an existing patio or hard-standing.

Sheds up to 8’ x 6’

Smaller prefabricated sheds (up to about 8’ x 6’) can be sited on crushed stone with either treated wood foundations, such as timber sleepers/railroad ties, or concrete foundation blocks or paving slabs.

An innovative plastic shed base is also now available as an alternative to traditional methods. Its use can save time, effort and money.

Sheds bigger than 8’ x 6’

For sheds bigger than 8’ x 6’ more substantial shed foundation plans are required.

A concrete shed base may be the solution. A concrete slab foundation has an advantage over timber floors if water is to be used on the shed floor or if wet equipment is to be brought in often.

Alternatively, a shed pier foundation is an easily constructed option guaranteed to save you time and money.
When deciding on your shed foundation type, remember that the larger the shed and the more often it is used, the more substantial the base will need to be.

Location

Does the Site Slope?

If the ground slopes it can make construction slightly trickier; as the foundation grade becomes steeper, the problem becomes greater. Slight slopes can be accommodated by paving slab and timber sleeper foundations through cutting into the slope slightly on one side and building up the crushed stone on the other. For severely sloping ground, there are two main solutions for building a shed foundation:

Post and beam timber deck

This route accommodates the steepest slopes. A post and beam type timber deck is built to form a level platform for the shed to rest on. The up slope side is shored up by a timber beam on crushed stone. The down slope side is supported by timber posts on concrete pads.

Retaining wall

For shallower slopes, a retaining wall can be used. This requires cutting into the slope to create a level platform with a retaining wall to hold back the ground above the shed.

Floating Shed Foundations

Small sheds will shift no more than an inch or two in the course of a year, a barely noticeable movement of no consequence.
However, for larger sheds, foundation movement can be a problem for three reasons:

The shed is connected to another structure with a different foundation type that is moving in a different way
Services are connected in to the building.
If one end moves and the other does not, the floor will start to slope and the building may crack or distort.

These issues can be avoided by making a foundation that is sufficiently deep enough to combat frost heave and the volume changes triggered by trees.
Frost heave occurs when the ground freezes during the winter months. This causes the ground to expand and push up whatever is on top of it. To avoid this, the base of the foundation should be set below the frost zone (600mm in the UK).
The volume of some soils, mainly clays, can change due to moisture content. Moisture content is affected by particularly wet or dry seasons. It can also be changed by trees growing in it and sucking the moisture out, which causes the soil to shrink. Conversely, if a tree is removed or cut down the moisture content increases and the soil expands. To circumvent the adverse effects of volume change, avoid locating large sheds close to rapidly growing trees or areas where a tree has recently been cut down.

All sheds need a solid, square foundation. However, the good news is that constructing a suitable base does not require advanced skills, special tools or a large amount of money; just the patience to plan properly and do the job right.