There are several different types of foundations for timber frame construction and which is used will depend on the type of structure desired.
When adding timber decoration or supporting structures to an existing building, the current foundations will usually suffice. Although hardwood timber such as Oak is a heavy material, a foundation designed to support concrete or brick should be strong enough.
However, when building a new structure, the chosen location must be assessed to ensure suitable foundations are used. During planning, a building control officer should be consulted to ensure building regulations are followed.
This can be one appointed by the local authority planning department or an independent Approved Inspector. Part A (Structure) of the current building regulations deals with foundations and is regularly updated so each new job should be assessed individually.
The type of foundation used is dependent on three main factors – ground type, size of the structure and materials used. Different types of subsoil have differing weight-bearing tolerances and this must be taken into account to ensure a safe, stable structure.
The size of the structure itself will dictate the stresses it places on the ground beneath and unequal distribution of weight can lead to collapse.
Lastly, if the structure is solely constructed from hardwood it will not always need a similar foundation to a brick and hardwood combination. There are two groups of foundations, shallow and deep and which is used will depend on the above factors.
These consist of three types – Pad, Strip and Raft. Shallow foundations are typically used where the loads imposed by the structure are relatively low compared to the bearing capacity of the soil beneath.
These are used to support single columns or posts, for example when building a decorative garden arch or gazebo. Beneath the site of each post, a square or rectangular hole is excavated and concrete is poured in to create a saddle stone when set. The post then sits on top of this to support its weight.
Concrete is the preferred material for its strength, because when set it will not absorb water that can freeze and split the saddle. Saddle stones are usually tapered as the broader base distributes weight more effectively. They are sometimes straight-edged if they are to be placed next to an existing structure, for example as the support for a garage door. This type of foundation is particularly straightforward and suited to many decorative outdoors projects.
Strip foundations are by far the most common type of foundations used for timber frame buildings. These consist of a continuous, level strip of concrete as a base for a linear construction such as a wall or row of oak joists. A three-course layer of bricks should then be laid on top of the strip below floor level.
Strip foundations are best for subsoils of good weight-bearing capacity and do not generally require reinforced concrete. This makes them easier and quicker to construct than other foundations. They are best suited to relatively light loads such as domestic structures or external buildings like large summer houses. The strip itself should extend to a minimum of 75mm on either side of the wall it supports and its depth will depend on the weight of the structure. Again, a building control officer should be consulted on this point.
This type of foundation consists of a ‘raft’ of reinforced concrete slabs of uniform thickness on a compacted hardcore base. They extend across the entire footprint of the structure to evenly distribute the weight of the building. Raft foundations are used on looser soils with a low bearing capacity where especially deep strip foundations would otherwise be required.
This can make them a cheaper, more efficient choice as extensive excavation is avoided. The concrete slabs are reinforced with steel for added strength and to prevent cracking under stress. They can also be cast with thickened areas where additional support is needed. Foundations of this type should be designed by a structural engineer to suit the specifics of the job. This type of foundation could be necessary for the construction of larger projects such as garages or larger timber-framed extensions.
For areas of soil with a particularly low weight-bearing capacity, deeper foundations will be needed to transfer the loads to the stronger layers below.
Piled foundations incorporate a long slender member, usually made from steel, but occasionally timber. This pile then transfers the foundation load to a lower stratum of harder soil or rock with the strength to support it.
Piled foundations are usually used for large structures with very high loads and are therefore rarely used in the construction of timber framed buildings. Work of this nature must be carried out by specialist construction firms. However, for those seeking to undertake a domestic improvement project, deep foundation work should not be necessary.
As can be seen from the types described above, foundations for Oak framed buildings are relatively straightforward in their construction. As a result, they should present no real issues for the customer with a modicum of skill and ability in construction.
Even if professionals are employed, most are relatively inexpensive and quick to construct due to their simple design. This means there is no reason to delay improving the look and function of your property with a bespoke timber framed structure.