Learn to Build – Starting the Build
Starting the Build
Clear as much as you can from your garden or yard.
You will need a lot of working and storage space for sand, blocks, timber etc. Now mark out where you are going to dig.
This is usually done by knocking wood or metal (preferable) pegs into the ground. Measure 3 metres from the rear wall of your house and first push the pegs into the ground.
Allow room to dig, so measure perhaps 3metres 30 centimetres, Donot hammer the pegs well down until you have checked that you are square and parallel with the rear wall of your house.
To confirm that you are coming out square from your house a builder would use the 3 – 4 – 5 rule.
Place a long straight piece of wood (at least 3 metres) on the floorwith one end fixed in place touching the rear wall of your house at the point where the extension is going to extend out and roughly at right angles (90 degrees).
Now mark with chalk 4 metres along your wall and 3 metres along the wood.
Measure the distance between the two chalk marks and slowly swing the wood (making sure the end touching the wall does not move) until the distance between the 2 chalkmarks measures 5 metres…hence the 3-4-5, this rule works with any 3-4-5 ie; metres, feet, or any multiple of these such as 6-8-10 or 9-12-15.
The smartipants amongst you will realise that this is known as the Pythagorean Theorem.
After double checking that you now have a area measuring approximately 3 metres out by the desired width (usually the width of your house) and perfectly square to the house, hammer the pegs into the ground (not into a gas pipe) and stretch a string line between the pegs.
You should now have 3 sides lined up…wall to peg 1, peg 1 to peg 2, peg 2 to wall. A perfect rectangle.
To double check that the corners are perfectly square, measure diagonally from corner to corner, the measurements should be exactly the same.
Now measure 600mm 2′ on the inside of these lines using more pegs, and you now have the area where you must dig out the footings.
That was the easy bit, now begins the donkey work. Unless you are lucky enough to have a large space of spare land to dump the excavated earth, you will need a large skip.
You will be surprised how much earth comes out of a trench. Dig down until you hit clay.
At least 1metre 3’4″ or below the level of any drains, or tree/shrub roots, say 1.2 metres
4′. Try to keep the sides of the trench as straight and perpendicular as possible especially within 300mm 12″ from the bottom.
This neatness will save you a lot of money in concrete, and impress the B.I. Get off to a good start.
The inspector will also want the bottom of the trench to be square. And as flat and compacted as possible.
These are the inspection stages:
*excavations for foundations
* foundation concrete
* oversite: inside the inner walls
* damp-proof course: DPC
* foul-water drains: from the toilet and bath/shower, sink
* surface-water drains: from roof, floor ie. Rain water
* occupation prior to completion: final works, electricals, etc
You might hear stories about not having to wait for the inspector beyond a certain period and of carrying on beyond these stages if the inspector fails to turn up.
Ignore them please. The stages are carefully worked out so that no important work is irrevocably covered up before it has been adequately inspected and approved.
If the Building Inspector thinks that you have covered up something that is wrong or that you have carried out work that is in defiance of the regulations, they have the power to order its exposure and you will bear the cost.
In the end they have the legal right of enforcement and the power to issue what is known as a ‘Stop Notice’ that will bring your entire site to a grinding halt until such time as you have either made good the incorrect work or satisfied them that the work is in order.
After he has passed your trench you can put in the concrete, well tampered down to minimum depth of 300mm 9″ for a 2 storey extension.
It’s a good idea to set more pegs at the sides at the bottom of the trench to give you the depth of the concrete when pouring.
Use a scaffold board or similar to check that the top of all the pegs are dead level. There is nothing much worse than starting to build a wall on a sloping foundation.
Steps in foundation concrete must not exceed its thickness and should overlap by twice its thickness. In other words, if the concrete is going to be 300mm thick, each individual step must not exceed 300mm and must overlap the base concrete by at least 600mm. many areas two layers of C503 mesh reinforcement are required at the top and bottom of the concrete with 75mm cover.
Whilst excavating the footings you would do well to remember that the soil within the rectangle…the space that is going to be within the walls, has to be removed also.
You must finish with an inert surface ie: clay, or gravel, rock, or sand,because , regardless of what type of floor you are going to choose, you must blind over the raw surface with a layer of concrete.
The Foundation Concrete
You have the choice of mixing the concrete yourself or buying it ready mixed.
Guess which is the easiest but, not always the most expensive by much? Do not accept the first quote, phone several ready mix companies and always confirm that they can pump it the length of your garden if that is the only access point.
Some firms will bring the raw materials on a wagon and mix only the quantity you require, and charge you accordingly.
The average extension will take 2.5 - 3 cubic metres of concrete.
You can calculate this by measuring the trench overall length by the width by the depth of the concrete in metres ie: a 7 metre wide house with an extension 3 metres out = 2 sides of 3metres and 1 side of 7 metres 13 metres X 600mm X 300 mm giving 2.34 cubic metres.
I would order 2.5 cubic metres because the floor and walls of the trench are never perfect and once you start tampering down the concrete it will shrink. Thicker is better than too thin.