Timber Building Foundation Issues – Part 2


Before a ready made wood outbuilding is added to a property, a base or foundation needs to be prepared. In deciding what kind of foundation to provide, property owners should consider the following issues:

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

In the first part of this article, issues related to support and levelling were discussed. In this half, tie downs and moisture protection are addressed.

3. Tying the shed, workshop or garage down

There are two reasons to tie down a timber outbuilding: uplift and overturning. Overturning is a fairly intuitive concept. Buildings that are proportionately tall and narrow can easily be pushed over. They may topple when someone leans on them to tie a shoe. Neighbourhood children may be able to push them over. In extreme cases, they may even be overturned by wind.

If the building isn't small and light enough to be pushed over easily, uplift still needs to be considered. One of the most dramatic ways that high winds can affect buildings is by negative pressure. This means that they can be pulled upward, and combined with wind on the side of the building; this can push them off their foundations.

Any building made of wood, even a large house, is susceptible. This is why it's important to tie timber buildings to their foundations. This is usually done using bolts embedded in concrete or grout and secured through holes in the bottoms of the walls.

4. Protecting the timber building from moisture

Wood structures that are exposed to the elements can last for 30 years or more if they're properly supported and maintained. However, unprotected wood exposed to the damp ground will only last for a fraction of that.

Pre-manufactured wood buildings are usually constructed on top of a shallow wood deck. That means that the narrow ends of wood joists – the pieces of wood that lie on edge to support the floor boards - will be underneath the building. If those shallow joists are placed directly onto damp ground, they and the floor boards will begin to rot within a few years.

The most important thing to do in designing a foundation is to keep pooling water away from the base of the outbuilding. Protecting the wood from prolonged exposure to moisture will help to prevent rot and deterioration. No matter how the base is constructed, it's best to place it in an elevated area, where water doesn't collect.

It's also a good idea to have the base fit the bottom of the building precisely and to elevate it slightly above ground level, so water won't pool around the edges. It is important to remember that concrete and clay brick are wet materials. They're like sponges, absorbing and wicking water. That's why it's always a good idea to put a water resistant layer in between wood and concrete or clay.

This can be a polyurethane membrane (it looks like a garbage bag), a thin layer of closed cell polystyrene insulation (this can also help to form a seal between the foundation and at edge of the wall), or even just a coat of water resistant paint or finish. Using treated, moisture and insect resistant wood for the outbuilding can also help to delay deterioration.

The specific ways that these concerns can be addressed using different foundation systems will be explored in upcoming articles. Some methods are inexpensive, only require a couple of hours of work, and are most appropriate for small sheds. Others usually require a building contractor and can add significant expense to the project.

Even a garage is usually better off supported on a DIY foundation than placed directly on the ground. Even if the budget is tight, giving thought to drainage, tie-downs, proper support, and levelling in the context of the site will help to insure that the base of the timber building functions reasonably well. However, it's important to remember that some timber buildings may require planning approval and a building warrant, bringing more specific requirements into play. That, along with specific types of shed bases and foundations and how to build them will be discussed in future articles.