Building foundations safely distribute the weight of your new home and prevent the subsoil from spreading, avoiding an unequal settlement of the structure. This is a critical stage of a housebuilding project and could prove very costly if things go wrong.
It’s crucial at this stage to take the time to double check all the measurements are correct and, if they are not marked on the plans already, write them on. (Make sure everyone on site is working from the same set of plans, too.)
If you aren’t sure which foundation system to choose, then make sure you take a look at our guide to foundation systems and soil types.
Whatever type of foundation design is assumed at the start of your project, be prepared for that to change due to the ground conditions discovered during the excavation.
Then, whether you’re project managing, taking on some of the work yourself, or want to have an overview of what your builder is about to do, here is a step-by-step guide for laying out your building foundations.
You will typically need:
- mesh reinforcement in the foundation concrete
- compressible material for lining the trenches
- a polythene slip membrane
- cut metal pins to use as markers ready on site
- several rakes and pairs of wellies (you’ll be surprised how many people turn up for work in trainers)
- to be aware of any lead-in times. If the engineers have designed a foundation that requires reinforcement cages, these may have to be manufactured off site
- to think ahead to getting the services into the house, so get some lengths of pipe or ducting to take through the concrete.
1. Start Building Foundations by Marking Out a Level Datum Point
First you will need a level datum point. If you are building an extension this is usually the finished floor level or damp-proof course (DPC) of the existing building. For new builds, this will be marked on the plan as a fixed point such as a manhole cover or a marker in the road or pavement.
The datum point is important: all your height measurements are relative to this point, so any errors could cause problems later on, particularly if there is a height restriction placed on the building.
What is a Datum Point?
A datum point is a point of reference from which further measurements can be made. The point can be based on the finished floor level, an existing building or a benchmark.
Summer Sale! Get 5 issues for £5
For even more advice, information and inspiration delivered straight to your door, subscribe to Homebuilding & Renovating magazine.
2. Calculate Material to be Removed
Next, calculate how much material needs to be removed from the site to end up back at finished floor level once all the layers of flooring materials (insulation, concrete, screed, etc) have gone down. Time taken with a site level at this stage will make things easier later on when the digger may not be able to reach any areas that have not been dug deep enough.
3. Use String Lines to Mark Out Your Building Foundations
Start marking out for the walls. Set up timber hurdles at the end of each wall, out of the way of where the digger will need to be. These carry the string lines that represent the wall faces of your building. Where space is limited, use corner pins instead and set up the hurdles after the concrete is in.
4. Mark Out the Trenches for Your Building Foundations
Begin lining up your footings by referring to your datum point. When it comes to extensions, never assume an existing building is square; pick the side you need to be square to and set up a line, then square off that line.
The easiest way to do this is using Pythagorus’s theorem (the square of the hypotenuse is equal to the sum of the square of the other two sides). If you measure the length of your first wall and then secure a tape measure at each end, by working out the length of the hypotenuse you can use both tapes to give you a third point exactly 90° to your first line.
Alternatively, most modern laser levels have a square function which does this for you.
5. Mark All Corners
Once you have two lines set up at 90°, all other measurements can be taken from this point, with all corners and junctions marked. A lot of digger drivers are quite happy for you to mark ‘centre of dig’ lines, but you may prefer to mark both sides of the trenches.
Check the width your walls and allow 150mm both sides, then use a string line as a guide to mark the trenches with marker paint.
6. Start Digging The Foundation Trenches
Now you’re ready to start digging. Have your site level set to the depth you need to dig and check it often. Ultimately it is down to the building control inspector to decide how far down you need to go; they will inspect at this stage and advise.
7. Introduce Depth Pegs and Pour
Once Building Control has visited, you are ready to pour the footings. You should place depth pegs in the trenches to show you how deep to fill, but you may find that marking the sides of the trenches with marker paint also works well, with a final onceover with the site level and a concrete rake to get them spot on — the more accurate you can be with your levels the easier (and cheaper) the next stage will be.
Also make sure that ducts are installed for any pipes or cables that need to cross the trench below the concrete.
If your site is difficult to access, it may be worth considering a concrete pump. This can work out more cost-effective than a few bodies with barrows.
Keeping the Site Tidy When Building Foundations
Before you fire up the digger you’ll need to decide what’s going to happen to the spoil. If it’s staying on site you may need to hire in a dumper so that it can be stored in a suitable place where it won’t interfere with the build.
Otherwise, you’ll have to organise lorries to take it to a tip. The turnaround time, or distance to the tip, will dictate how many lorries are needed to maintain continuity on the dig.
You will also need to decide whether you’ll be able to load the concrete directly into the trenches or whether a pump will be needed. If you do decide on a pump – always a good idea if more than three loads are anticipated – then it will need to be booked and the concrete suppliers advised so that their mix and turnaround time can be adjusted to facilitate a continuous pour.
Getting Lorries to Site
If the concrete is being directed from a lorry into the trench then you need to make sure that there is hardstanding and stable ground for it to park.
Most mixes can be ‘pulled’ around the trenches using rakes, but if the sides of the trenches are unstable this may cause collapses and unwanted contamination of the concrete. (Shuttering the trenches prior to pouring can aid in this instance.)
Adding a Basement When Building Foundations
If your ground conditions appear difficult, you might do well to consider building a basement. If you are expecting to spend, say, £30,000 on getting out of the ground, then you are maybe halfway towards the cost of a basement and you may find that you are able to add considerably more value to the house than the additional cost of a basement build.