Timber Building Foundation Issues – Part 1


Pre-manufactured timber outbuildings are wonderful. They allow property owners to purchase a high quality, attractive solid wood building without taking on the expense of having it designed and built, or taking the trouble to build it themselves. However, some thought needs to go into where and how the timber building will be placed on the site.

In order for it to last for as long as possible, a prefabricated shed, garage, or workshop will need a good foundation. There are many techniques for building a foundation for a timber outbuilding. Some are inexpensive and easy to assemble, others are best put together by a professional building contractor.

Before deciding which technique is right for a particular site and choice of building, it's important to consider the following issues:

  1. Support
  2. Levelling
  3. Tie-downs
  4. Moisture protection

The first part of this article will discuss levelling and basic support, and part two will address tie-downs and moisture protection.

1. Supporting the outbuilding

Some pre-manufactured outbuildings come complete with a wood floor deck, and some do not. Before planning a foundation, it's important to know exactly what type of support will be provided. Most likely, it will be either:

  1. walls with a flat piece of timber at the bottom perimeter and no floors; or
  2. a shallow floor deck and floor boards.

Both will need to be supported at the edges.

If there's no floor

If there's no floor, it's best to tie the sides of the structure to a rigid, slightly  elevated wall foundation and leave the floor natural, or place the structure on an pre-built wood deck or concrete slab.

Manufactured buildings without floors can use all of the foundation types listed below, but the issue of the floor surface also needs to be addressed. The fact that the wood walls can easily be elevated above the level of the slab is an advantage when it comes to moisture protection, but tie downs are very important.

If there's a floor

If the shed or workshop has an existing wood framed floor, then that floor will need to be supported on a flat, continuous surface, such as:

  • level, well drained bricks, blocks or pavers;
  • a concrete slab or pavement;
  • the boards covering a wood deck;
  • a wood deck's joists;
  • a mudsill foundation; or
  • a full foundation, below the level of frost penetration.

The local authority may require that the foundation extend below the level of frost penetration in larger outbuildings, such as garages. This will prevent frost heave. Frost heave occurs when water freezes under the foundation and pushes it upward. It can damage the foundation and make the building move around and settle unevenly.

2. Creating a level surface

Whatever type of base or foundation is used, it's important to make sure that it provides a level surface for the edge of the outbuilding and for the floor, if it has one. It may seem obvious that an outbuilding needs to be installed on a level surface, but this is often easier said than done. Existing concrete pavement is not always level.

However, the most effective levelling technique depends on the type of foundation. If there is a downward slope that's going to cause water to collect against one side of the outbuilding, then elevating the outbuilding or providing a drainage path around it will extend its life. In areas where there's a pronounced slope, the best type of base to provide for the outbuilding is an elevated wood deck on wood posts, supported by concrete tube footings or specially manufactured piers.

That's a building project in itself, but it will provide an effective and long lasting base for the outbuilding. If a paved surface for a garage or workshop is needed on a dramatically sloped site, then it's best to talk to a contractor.

In the next installment, tie downs and moisture protection will be addressed.