How to Build – Building the Footings
Starting the Blockwork
When the B.I. has passed your beautifully smooth and perfectly level concrete, the build can begin.
You have to decide at this time what type of wall insulation you are intending to use once you are above the ‘damp proof course’ DPC, just above ground level. There are 2 main types of wall insulation.
The full fill cavity sort made from recycled glass, or the part fill cavity sort that fits close to the inner wall, made from polyisocyanurate PIR.
This is a super insulation similar to polystyrene but made from polyurethane and is stronger, also excellent for underfloor insulation.
Points to consider; if you use the glass fibre bats, these must be 100mm 4″ thick and therefore your cavity must be 100mm wide.
With the polyisocyanurate (Kingspan) you can build a 75mm 3″ cavity…50mm Kingspan bats and a 25mm 1″ airspace between the bats and the outside wall.
There are pro’s and cons for both. Neither of them transfer water so completely filling the cavity is not a problem.
The glassfibre full cavity has the advantage that in the case of fire, it can withstand high temperatures and by completely filling the cavity it stops the transfer of smoke from room to room, or to neighbouring houses.
If you use the Kingspan PIR you must use a full cavity ‘sock’ that fills the cavity, between any adjoining cavities, for this reason. I mention this at this stage because you must decide right from the beginning what width cavity you are going to have.
The Underground Brick/Block Work
These must be of a suitable compression hardness) of at least 7 Newtons.
Sounds complicated but, all builders merchants will already know what you want. such as engineering bricks or concrete blocks.
Measure and mark out your corners with chalk.
Mark the centre of the concrete, then, if you are going to use a 100mm cavity, measure 50mm each side of the centre line.
Ensure that your corners are exactly square, this is your last chance.
Any mistake made at this time will continue all way up the building. Sand & cement mixed together is called mortar, or sand/cement composition, often referred to as ‘compo’ by brickies, and hereafter in this article.
Lay a good bed of compo at both sides of the corner and place 2 blocks at right angles to each other on the compo with a smearing of compo on the end of the block that abutts up to the other.
Tap them down, making sure that they are square and level. Take your time with these first 2 courses, it’s essential to get the first course right, the rest of the courses as you build up will follow on.
If you have never laid bricks or blocks before I can thoroughly recommend the purchase of a ‘brickie’ from Amazon.
This was invented by an Irish bricklayer called Marshal and comes with the best instructional DVD that I have ever seen.
You can actually watch everything being done as I am explaining here.
Could be the best £30 I ever spent. Lay the first blocks at all corners, and once again check diagonal corner to corner measurements.
Get it wrong at this point and it will be wrong all the way up. Once your corners are set up, stretch a builders line from one corner to another.
From the house to the first corner and begin to lay the rest of the blocks between the corners.
Lay the blocks with a 10mm compo filled space between each block or brick.
The building line should be on the outside edge of the blocks, and each block should lightly touch the line as a guide.
Once you get in your swing you won’t worry too much about every block or brick being perfect.
After laying each course, you can look along the line and tap any blocks/bricks that are slightly out of line. The compo bed that each block is laid on, should also ideally be 10mm thick.
The ‘brickie’ tool that I mentioned earlier automatically ensures that all joints and beds are 10mm thick, and keeps you pretty straight.
Complete the first course of the outer wall and then lay the first course of the inner wall leaving the correct cavity space.
This is the worst part of the build because you are working in a confined space with an aching back.
The good thing is that you are getting plenty of practice where nobody other than the building inspector will see it.
Every second course 450mm you must insert a stainless steel tie across between the outer and inner wall at approximately 750mm centres.
It will be a miracle if you do the complete build without forgetting to insert some of these ties, and have to go back to them.
Once out of the ground and at DPC level you will rejoice at not having your legs wedged down a narrow gap.
Before continuing to build further you must fill in the cavity space with concrete up to ground level and to within 1 block below the DPC level approx 225mm 9″ down.
Get this passed before continuing the build. Hopefully by this point you will have decided upon what type of ground floor you are going to have.
The smart money is now in favour of underfloor heating.
If this is your preference, (and I would strongly recommend it) you must choose between a suspended wooden floor or a solid concrete one.
My personal preference is for concrete because wood tends not to be a good conductor of heat.
If you choose concrete, you must lay a layer of hardcore to a minimum thickness of 150mm or more to reach the desired height, bearing in mind that you must allow for the sand blinding, and a minimum 100mm thick concrete.
Blind it over with sand and wacker it down well with a wacker plate (hired for a day).
When this is well compacted down, cover it with 1200g thick Visqueen PVC from the builders merchant.
This is usually 1 metre wide when bought but unfolds to 4 metres.
Allow the extra width/length to come up the walls, say 500mm 20″ This forms a vapour barrier between the walls and the floor.
As usual, the B.I. must check and pass this work before you can pour the concrete.
The B.I. may sometimes ask for reinforcing mesh in the concrete.
With a wood floor the insulation 125mm thick EPS or PIR goes under the concrete, but with a screeded underfloor heated finish the insulation goes on top of the concrete.
The concrete should be a minimum of 100mm 4″ thick.
In the screeded case, the insulation is placed on top of the concrete to stop the heat from the underfloor heating from going down and heating the ground.
The last B.I. we encountered had to have this concept explained to him before we could continue.
As I said earlier, some B.I.s are unsurprisingly struggling to keep up with all the new regulations.
Obviously, you don’t lay the over concrete insulation until the extension has been completed, and you are ready to run the underfloor hot water pipes or electric mats.