Clear guidance on low-rise house extensions is lacking, so know your foundation depths, says Geoff Wilkinson
Continuing our series on the most common problems I see as a building inspector, this month we look at foundations for low-rise extensions.
A large proportion of projects I encounter involve enlarging existing domestic buildings through the addition of a single-storey extension, often to create a kitchen or dining area. There is limited guidance within the Building Regulations dealing specifically with foundations for low-rise domestic extensions, which, unlike new buildings, tend not to involve rigorous soil investigation. As a result, many architects simply note ‘foundations to be agreed on site with building inspector’, which can in turn cause cost overrun, delay, dispute or claim against the architect.
The first step in determining the correct depth of a foundation is to look for any obvious issues during the site survey, such as adjacent buildings, basements, proximity of trees, and locations and depths of drain runs under or adjacent to the proposed extension. I often have to advise contractors or owners that they will need to excavate deeper because it didn’t occur to anyone to lift the manhole cover and check the depth of the drain under the proposed extension.
Once such basic checks have been carried out, the minimum depth of foundations should then be determined. Despite folklore to the contrary, the minimum depth of a foundation is not one metre, and should be set as the greatest of the following:
• A depth to a suitable bearing stratum for the loads imposed by the foundation;
• In clay soil, a depth between 0.75m and 0.9m, depending on the shrinkability of the clay;
• In sands, chalk and other frost-susceptible soils, a depth below the zone of frost action, which is normally taken as a minimum of 450mm;
• Where new foundations adjoin an existing drain or basement, make sure they won’t impose any load on the existing services or structure.
The tool most commonly used to determine the suitable foundation depth is probably NHBC Standards, chapter 4.2. These standards were developed in the 1970s in response to hot, dry summers, when the issue of clay shrinkage became widely recognised. This has meant that many extension foundations are far deeper than those of the original buildings. As a result, traditional strip foundation can undermine adjoining buildings, and should be excavated in sections not exceeding one third of the length of the wall (similar to underpinning).
Where the foundation design exceeds 1.5 metres in depth, traditional trench fill starts to become less economical and, under CDM,
requires greater consideration to ensure that it can be safely constructed. When the depth reaches 2.5 metres, NHBC guidance requires that the foundation is engineer-designed. Possible alternatives include rafts or pile and beam foundations, both of which require less excavation and are frequently more appropriate on a constricted site.
I often see foundation depths that differ markedly from those of the existing building. For an extension to move in harmony with a main building that is built on shallow foundations, the foundations of the extension would need a similar depth. This can lead to damage at the junction between old and new structures. Indeed, the insurance industry spends approximately £350 million a year on repairs to properties that have been damaged after foundation movement. Many of these cases involve differential movement between the building extension and the original structure. To help accommodate this potential movement, I recommend that the foundations are stepped away from the existing building in 300-500mm steps.
It’s also important to incorporate a movement joint where new brickwork abuts an existing wall. As the joint could allow ingress of moisture, debris and possible insect infestation, a filler should be incorporated that is not only weatherproof and flexible, but will also tolerate both tension and compression throughout the life of the structure.
Traditionally, two types of material are used for jointing in brickwork – filler boards and pre-compressed, impregnated foam tape, together with a suitable sealant to seal the exterior face of the joint against the weather. The first group, ‘flexible’ filler boards, are easy to apply but do not work under tension and because they have insufficient compressibility, they are not particularly suitable. This is not the case with either sealants or pre-compressed foam tapes, both of which are appropriate if correctly specified and installed. In any case, it will be necessary to apply sealant to the exterior of a joint to ensure a durable, watertight finish. The gap in the inner face of the wall may be hidden by dry lining.
Further information can be found in BRE Good Building Guide 53 and NHBC Standards.
Geoff Wilkinson is managing director of approved inspectors Wilkinson Construction Consultants.