This article is an excerpt from Residential Construction Details – US Edition. The book is packed full of both 2d and 3d details all of which are available to download as both dwg cad files and .skp sketchup files. Find out more about Residential Construction Details.
Principles of foundation design
The main role of foundations is to structurally support the building by transferring the loads of the building through the walls into the surrounding soil. In terms of a stick frame structure, the foundations must also protect the timber from moisture ingress by lifting the members above the ground.
The type of soil on the site will have a strong implication to the foundation design.
Different regions will have different soil types, the table below briefly demonstrates the types of soil and its suitability as a foundation material.
From the table you can see how important it is to establish the soil information on the proposed site. This often means a soil study or report is carried out.
The second important site factor to consider when designing foundations is cold and permafrost climates. These climates see a level of ground permanently frozen, and the concern is that the soil under the foundation could thaw and lose strength. Specialists should be consulted if designing foundations in an area of cold or permafrost climate (generally the far north – Canada, Alaska and so on).
The remaining northern half of the United States and mountainous regions are considered a cold or under heated climate where the frost depth is generally 12in. or greater. The design for this type of climate is a little more straight forward with the following measures taken:
- Providing foundations below the frost depth
- Providing a basement
- Insulating the exterior to reduce the chance of cold ground temperature reaching the structure.
Types of foundation
There are four common foundation types in residential construction that all work in quite different ways but each requires a support around the outside edge of the building. The four types are:
- Slab on grade
- Pier and grade beam
- Crawl space
- Basement
For the design of foundations, building codes should be consulted along with local codes to determine appropriate frost depths and design requirements. Foundation choice is dependent on many factors, such as soil type, site, climate and the process of choosing your foundation system goes beyond the scope of this article.
Footings
Footings lie under the basement, crawl space or foundation walls and transfer the structural load from the walls to the supporting ground. Typically these footings are cast in place concrete. In order to prevent damage or heaving that can be caused by freezing water in the soil, the footings must be cast below the frost depth.
Footings should be aligned so that the supported wall is as close as possible to the center line of the footing.
Slab on Grade
Slab on grade consists of a shallow footing perimeter, with a concrete slab as the ground floor. This type of foundation is common in warmer climates, where the frost line is close to the surface and therefore the footing is usually shallow – with the ground floor a concrete slab. A slab on grade system often sees the footing, foundation and subfloor cast in place at the same time.
A stem wall with slab on grade supports the wall above and can also provide a ledge to support an exterior masonry veneer. The wall is exposed to soil on both sides, so waterproofing or damp proofing is generally not required.
Insulation to a stem wall is often situated on the exterior of the masonry. If insulation is placed on the interior, care must be taken to insulate the joint between the slab edge and the foundation wall to avoid thermal bridging.
The build up for a slab on grade system can vary, but the key is to provide a solid support for the slab and to control the ground moisture.
Soil can require compaction to ensure a solid base. It can also be chemically treated to prevent issues with termites in certain regions. The gravel layer is a porous layer that is used to level the ground and assist with draining water away from the slab. The moisture barrier is the moisture defence for the slab.
Slabs require expansion joints which will allow the slab to expand and contract with the changes in temperature without causing any cracking or damage to the slab itself.
Pier and Grade Beam Systems
The pier and grade beam is a good foundation system for expansive soils or hillside locations.
The grade beam is a continuous reinforced concrete member that spans across non load bearing areas, and is supported by pilings or piers. This type of foundation is commonly seen in areas with expansive soil.
The piers are drilled down to a solid bearing (rock), and the grade beam spans between the piers. Piers can also be constructed using reinforced masonry (brick or concrete block) which are supported by reinforced concrete pad footings.
Foundation walls
There are two main types of foundation walls, concrete block or concrete cast in place. Concrete block is the most common system as it does not require formwork, it is flexible to more complex designs and layouts. It does work best when the foundation is designed based on standard dimensions of concrete blocks i.e. in 8in. increments.
All foundation walls should be tied to the footings with a vertical rebar placed at the corners, next to all major openings and generally at regular internals. A continuous horizontal bar is also required at the top of the foundation wall.
The design of the width of the foundation wall is dependent on the number of stories the wall will be supporting, and the depth of the backfill. The minimum height of a foundation wall will allow for sufficient clearance of beams and joists from the crawl space.
Crawl Spaces
The crawl space system has concrete footings which support the concrete or concrete block foundation walls. These foundation walls support the timber suspended floor, which creates a crawl space that is accessible for utilities and services such as plumbing and ductwork. This type of foundation system can be seen in all climates but more often in warmer regions.
Moisture build up in the crawl space can cause significant damage by mildew, fungus and other organisms that require moisture. It is key that moisture is avoided.
Soil under crawl spaces will naturally carry some moisture which will end up in the crawl space in the form of vapour. The most efficient way to prevent this is to install a vapour retarder directly on the ground within the crawl space. The vapour retarder – a dark plastic polyethylene – will prevent plant growth, and moisture. The vapour retarder is then protected by a layer of concrete – usually about 21/2 in., to ensure it is not damaged. If the space is to be used for storage, a thicker concrete slab may be used.
There are two methods used to prevent moisture build up in crawl spaces.
Vented Crawl Space
Cross ventilation can remove unwanted moisture and is achieved by installing vents to all areas of the crawl space. Research has shown however, that the use of a moisture retardant ground cover eliminates the need for vents in many locations. If the crawlspace is vented, the floor, exposed pipes and ducts are usually insulated. Unvented crawlspaces must have a floor covering to minimise moisture.
The code has detailed requirements of the vent areas required according to the under floor area. The vents should allow for cross ventilation to all areas of the crawl space, and therefore vents should be located near corners and on opposite sides of the space. Access doors must also be provided.
These traditional vented crawl spaces can often be damp, and inhabited with pests.
Unvented (or conditioned) Crawl Space
In humid climates, ventilation to the crawl space can actually bring in moisture during summer months so it is common that the crawl space will be insulated in those regions and unvented. These spaces will be conditioned with internal air. However, it is not solely warm climates that benefit from the use of insulated crawl spaces, as research suggests insulated crawl spaces is a more efficient method of design.
An unvented space should be treated as a mini basement. The floor should be left uninsulated, with a vapor retarder, and the walls should be insulated and air sealed – with air conditioning using indoor air. The crawl space is insulated on the perimeter, not between floors.
The cold foundation wall must be protected from the interior moisture laden air, so the rigid insulation is positioned on the inside face of the wall.
The rim joist must be insulated with air impermeable insulation or insulated on the exterior. The joints must be sealed to provide an air barrier.
This article is an excerpt from Residential Construction Details – US Edition. The book is packed full of both 2d and 3d details all of which are available to download as both dwg cad files and .skp sketchup files. The book features 24 2d foundation details and 24 3d foundation details along with further details of flooring options and basements. Find out more about Residential Construction Details.
Hi!
Is this type of explanation included in the UK version of the Book you released prior?
Thanks!
Hi!
The UK version does not currently have this level of detail. However, I’ll let you into a secret. I am currently updating the UK version to a second edition – which will have WAY more information in it. This is due to be complete within the next month or so. If you would like more info, drop me an email emma@firstinarchitecture.co.uk
Excellent book
Thank you!
nice