Groundwork is the invisible half of every construction project and the half that determines how the visible half performs. Foundations, drainage, ground preparation and site levelling all happen in the first few weeks, get covered over quickly, and are usually forgotten. But every problem that surfaces in a building years later – subsidence, damp, cracked walls, waterlogged garden, blocked drains – almost always traces back to something in the ground that was done in a hurry or done wrong. Time and money spent on groundwork is time and money invested rather than sunk.
The starting point is understanding the ground itself. Bournemouth sits on a mixture of ground conditions across the area – some sandy, some clay, some with poor drainage, some with high water tables near the coast. A soil investigation before design finalisation is one of the highest-value things a project can do, and it is remarkable how often it is skipped. A trial hole dug at each corner of the intended footprint tells the Structural work and knock-throughs engineer what depth foundations are required, what type of concrete mix to specify, and whether any special measures like piled foundations are needed. Guessing at this and finding out during the dig is an expensive way to discover that the ground is not what you assumed.
Foundation depth depends on the ground and on what is being built over them. The minimum for a domestic extension on stable ground is often around a metre, but any clay soil – particularly clay that shrinks and swells with moisture – needs deeper foundations to sit below the zone where the ground moves seasonally. Trees near the site complicate the picture further because their roots draw moisture from the soil and can cause differential movement. A single-storey extension close to a mature oak may need foundations two metres deep or more; the engineer’s calculation, based on the actual site, is what determines this.
Concrete for foundations is a straightforward material but the placement matters. Trench-fill foundations, where the whole trench is filled with concrete rather than just a footing at the base, are the modern standard for most domestic work. They are simpler, they eliminate the risk of a poorly-built block foundation wall between the concrete and the ground level, and they perform better on marginal ground. The concrete should be placed continuously rather than in staged pours, and it should not be left exposed to weather for days before being covered.
Drainage is the groundwork item most likely to be underdone and most likely to cause future problems. Foul drainage from an extension needs to connect into the existing soil stack or the mains sewer, with the right falls and the right pipe sizes, and it needs inspection chambers at any junction or change of direction. Surface water drainage, from the roof of the extension, needs somewhere to go – a soakaway, a connection to a surface water sewer, or a permeable feature – and dumping it onto the existing garden is not acceptable under modern regulations. Both types of drainage need to be tested, and Building Control will want to see the tests.
The damp-proof course and membrane are inserted at the boundary between the ground and the building, and they are the primary defence against rising damp. A horizontal DPC in the walls at ground level stops moisture from wicking up from the ground through the masonry. A horizontal DPM under the floor slab stops moisture from rising up through the floor. Both need to be lapped and jointed properly at any junction, and any penetration through them – a service pipe, for example – needs to be sealed. Missing or badly-installed damp-proofing is one of the classic reasons that older extensions develop damp problems in walls a few years after completion.
Site levelling and drainage of the surrounding garden matter more than most homeowners consider. Water needs to drain away from the building, not towards it. The final ground level around a new extension should be at least 150 millimetres below the damp-proof course, and it should slope gently away from the walls. Concrete or paving butted right up against a wall without a proper drainage detail creates a splash zone that keeps the base of the wall wet in every downpour, and that is how damp finds its way into a house through walls that were built dry.
A groundwork contractor who cannot explain what depth of foundation the engineer has specified, what type of concrete is going in, and how the drainage is being routed, is not the person to trust with the invisible half of your project. The visible half – the extension itself – will only be as good as what is under it. Get the groundwork right and everything above stays true. Get it wrong and no amount of visible finish makes the underlying problem go away.
