The foundations are the supporting layer of the construction and are generally divided into two categories: Shallow foundations and Deep foundations.
Shallow foundations are those such as Pad, Strip or Raft foundations.
Pad foundations are generally used to support a single column or post and are square or rectangular holes that are then filled with concrete. A staddle stone would normally be used to sit the post on. Our staddle stones come in two types – tapered and flat. Flat stones would be used against existing buildings or where garage doors are fitted. Tapered stones would be used elsewhere. These staddle stones are made of reconstituted stone. We do not recommend sandstone as we have found that sandstone soaks up moisture and when it freezes the stones are liable to split or chunks fall off. If a client would like a different type of staddle stone then they can arrange their own and we will be happy to fit our building onto them.
Strip foundations are the most common and are used to support the walls that the oak frame sits on. The width of the footing is dependent on the width of the wall going on top of it. There should be a minimum of 50mm of footing showing each side of the wall that sits on it. The depth of the footing will depend on the load bearing ability of the soil you are going into. The building control officer dealing with your job will agree the depth you have to go and may advise a different type of foundations if they feel it more appropriate.
Raft foundations are used to spread the load from the structure over the whole area of the structure. They are often needed on soft or loose soils with low bearing capacity. This type of foundation must be designed by a structural engineer and is specific to your site.
Deep foundations are used to transfer the load of the structure down through weak layers of soil to stronger layers below. This is usually at depths of over 3m. An example of this is a Piled foundation.
Piled foundations are relatively long slender members that transmit foundation loads through low load bearing capacity soil to deeper soil or rock strata that has a high bearing capacity. These foundations are usually carried out by specialist contractors.
If the ground floor of your building is to be insulated then you have the choice of putting the oak soleplate at finished floor level (FFL) or having a dwarf wall for the frame to sit on.
Soleplate at FFL
This is our preferred method especially for larger span buildings as the frame can easily be fastened to the oversite for structural purposes. The plinth wall is built up higher than the concrete level by the thickness of the insulation plus the thickness of the screed (usually about 150mm).
This is a low cavity wall which is usually used in a Garden Room for the frame and joinery to sit on. There is a soleplate that sits on the outside brick skin which is used to distribute the weight of the building along the whole wall and not just the points where the posts sit.
This soleplate is made of Douglas Fir rather than Oak so that there is no shrinkage which will affect the internal window board. The internal blockwork is also thicker than normal and the width of the blocks may need to be varied depending on the height of the wall. We at English Heritage Buildings also consider the suitability of any foundations and plinth walls which our frames sit on where as some of our competitors do not. They will give you the structural calculations on the frame but never mention that the foundations are not correct to take it. The first time you know that there is a problem is when the building control officer brings up a query from the engineer who is checking the calculations.
Oversite / Floor
The oversite is the internal floor of your building and again this is split into Un-Insulated and Insulated.
This type of floor will be used for garages and outbuildings where the ground floor is not going to be insulated. It is made up of a compacted sub-base, sand blinding, damp proof membrane and then concrete on top which would normally have reinforcing mesh in it. In a building with an open front we suggest that you put a fall on the concrete of about 25mm so that any rain / water that lands on the oversite runs out again rather than stays inside.
If the ground floor of your building is to be insulated then you will need the floor to be insulated as well. It is the same makeup as the Un-Insulated Floor but on top of the concrete you put a layer of insulation then a sand and cement screed. On top of this goes the floor covering. If required underfloor heating can also be incorporated within the screed layer. The thickness of the insulation will depend on the U-Value you require for the floor.
Putting the insulation and screed on top of the concrete also makes it much easier when you have a situation where one part of the building is being insulated and another part is not. If you have this scenario then there should be a difference in floor levels of at least 100mm to comply with building regulations. When we have this situation the concrete level is the same throughout the building and we use a 150mm high wall throughout so that the soleplate is at the same level throughout and the insulation and screed can just be put into the insulated sections.
There are other types of foundations that can be used like pad & beam and beam & block. They each have their use in special conditions and advice should always be sought from your Building Control Officer or your Groundworker as to what will be the best and most cost effective foundations for your project.