You can’t afford to be complacent about the setting out of foundations – getting it wrong is all too easy. If you end up having to have extra trenches filled with concrete, this means extra cost for you right from the outset.
It doesn’t just matter to you that your first steps are correct, it matters to the local planning authority that your home is built in the position shown on the approved plans. Even when dimensions haven’t been specified, planning enforcement officers can scale off the drawings to check that your home is where it should be. The distances to boundaries and the levels are both important from a planning aspect, because of the issues of overlooking and overshading neighbouring buildings.
You and your builder are the only people who will check the setting out at the start, so take the time to get it spot on. With pegs driven into the corners and chalk lines sprayed on the ground between them, you can start digging the foundation trenches. Mark the centre line of the foundations to avoid confusion; the wall positions will be set out with pegs and string lines later when the foundations have been concreted.
Trench fill foundation
Preferred by self-builders and by most large-scale operators, trench fill avoids bricklaying below ground. The concrete is poured to within 150mm of the surface ground level, saving time and trouble. The sides of the trench play as much a part in supporting the load as the bottom, and so this foundation must only be used in stable ground, where the trench sides are firm and capable of bearing loads. Clay and chalk soils are ideal for trench fill foundations.
Usually wider, strip foundations use less concrete by being thinner. They’re typically 300mm thick. The exact thickness will need to be determined by the masonry courses of the walls up to damp-proof course. If you have a sloping site, you will need to step the foundations to keep them level. Steps should overlap at least the width of the trench when concreted and that means shuttering across them. Use plywood and retaining pegs. Strip foundations are often necessary in softer soils, such as sand, since they spread the load of the building out over a greater area.
Building control and home warranty inspections
You must notify your building control officer at certain stages and await their inspection. You can assign the task of giving notice to your contractor, but check for yourself that the works have been inspected and approved before covering that stage and continuing. Most building control authorities are happy to book inspections from a phone call, email or fax, so long as they are given a day’s notice. The same goes for your insurance warranty inspector, who will also need to inspect at some stages.
Building control inspection schedule for groundworks
- Commencing the work Give two days’ notice in writing
- Foundation excavations Give one day’s notice by phone, email or fax
- Oversite preparation Give one day’s notice by phone, email or fax
- Drains laid in trenches Give one day’s notice by phone, email or fax
- Drains testing Give one day’s notice by phone, email or fax
Foundations formed in rock and stony grounds can often be shallower than those dug in “shrinkable” soils such as clay. In the latter, at least 1m deep is normal. Nearby trees or shrubs may extend this depth to a good deal more. Your building control officer will guide you in these cases. Once the trenches have been dug and inspected, you will need to hammer pegs into the sides to mark the level for top of concrete. Timber pegs are safer than steel pins.
Make sure the ready-mix lorry can access the site. Readymix can be placed by pump if the trenches are difficult to get to or the truck’s chute can’t be extended far enough. Pumps must be booked in at an early stage. One reason for doing so is because there may not be that many of them in your area – they’re often tied up with bookings on civil engineering works. Once poured, the concrete will need to be hand-raked into level against the pegs.
Beneath a ground-bearing concrete floor slab, the ground must be properly prepared with an oversite treatment. The site should have been stripped clear of topsoil and vegetation before the foundations were dug. It’s from this reduced level the infill construction begins. Hardcore should be used in a layer, at least 150mm thick, but no greater than 600mm, in selected aggregate. It’s then compacted down in layers with a plate compactor. If this task is done badly or with the wrong material, settlement is to be expected, causing the slab to crack.
Check out your options for oversite material locally. Hardcore rubble needs to be clean, broken brick, concrete or stone that has been crushed or is naturally available in less than 100mm particle sizes. Approved aggregates are available from merchants and some offer recycled glass and concrete crushings. On sloping sites, where part of the oversite may exceed 600mm deep, it may be acceptable to use lean-mix concrete in layers with the hardcore to make up the difference, instead of switching to a suspended floor type.
Take care not to push out the external walls if a round stone fill is used. Stone just rattles around and doesn’t exactly compact. Like water, it finds its own level very quickly. Lap and tape down 1,200 gauge polythene for the damp-proof membrane and make sure it is dressed up over the walls so it can be lapped with the damp-proof course later. The insulation boards can be laid out over the damp-proof membrane and covered by an additional layer of polythene before placing the concrete. These boards should also be cut to an upstand around the external walls to encase the concrete slab fully in a warm tray.
Angular hardcore needs binding with a thin layer of sand to protect the polythene damp-proof membrane from puncturing and also to create a level bed for the insulation to sit on.
The hard work is in raking it out and tamping it into level using a straight length of timber that will span the distance between the walls on either side. Tamping the concrete helps to consolidate it in the same way that vibrating it does and, combined with a gentle sawing and tapping motion, will bring the surface to the level and finish needed.
Levels are just as critical when laying the drainage system. The invert level is the bottom of the pipe and from this point the system falls are determined. The trenches need to be carefully dug to the right depth and gradient to avoid huge amounts of bedding being necessary to support the pipes. Gravel is used to a depth of 100mm to 200mm thick for both bedding and surround material supporting and protecting the pipes at the correct fall. A gradient of 1:40 is ideal, but plastic drains will tolerate much shallower falls if it’s necessary. Look for at least half a bubble out on the spirit level and make sure they fall the right way. Eyes can be deceived when looking at drains in trenches, particularly when the ground slopes the other way.
- Lay pipes to an even gradient
- Lay pipes in straight lines and avoid bends
- Locate the soil vent pipe at the head of the run
- Provide rodding points or inspection chambers at bends and changes in gradient
- Connect water closets and soil vent pipes direct to inspection chambers
- Use at least 100mm of gravel to cover the pipes before backfilling the trench and don’t leave the trench open for longer than is necessary
Using deep drainage trenches for running in other services makes good sense. Gas, electric and water cables all have to be provided at your expense, so laying them in position now will save time and money later. Getting the supply companies to connect them when the house is complete can be a slow procedure, so get the job registered, priced and booked in well ahead to avoid delays later.
For peace of mind, a water or air pressure test should be carried out before drains are backfilled to ensure all the joints are watertight and everything works. Your building control officer may want to witness the test at this stage or when your self-build is complete. The reason for the latter is that drains can become displaced or damaged with careless backfilling and site traffic. Drainage connections are usually best left until later, but if you do connect up at this stage to the mains system, make sure you fit manhole covers and fill gulley traps with water. The air will have turned foul on site before you’ve hardly started.