Setting out foundations for a house
As it would get quite a bit of use, I decided to buy a rotating laser from ebay and then re-sell it once the project was done. I bought a second hand Makita SKR60 for about £70. I will let you know how I get on with it as the project progresses.
Som how to mark out footings? What is the footing markign procedure?
During demolition I found that the wall I had demolished had not been built straight (see demolition pages) so decided to check my existing building was square before I set out the extension.
First, I used the laser vertically, the beam hitting the ground (pic 2) to check that the two parts of the building, that were built at different times, were planar (ie) built in a straight line. It was only about 5mm out.
To check the existing building for square (see pic 3), I ran a string line along the end of the building A - B, extending it to point C. B - C being 4 meters. I measured 3 meters along the back wall of the house and made a mark at D. If my existing building was square, D - C, according to Mr Pythagoras, should be 5 meters. However, D - C measured around 4.89 meters, meaning angle A,B,D was greater than 90 degrees.
I now had a decision to make.
If I used the rear wall of the building as my datum and built square to that, the new wall running B - C (pic 4) would appear to kink out slightly from point B. A - B -C would not be planar (pic 4). I wondered how odd this may look when seeing the end of the building.
However, if I drew a line continuing the angle from A - B and used that as my datum (pic 5) I would not be building square to the back wall. Though this may look fine from the end wall, I soon realised it would have lots of knock on effects. The new roof would not meet squarely with the existing and would be a nightmare. Also, I was sure this would look a whole lot weirder than the end wall not being straight.
In the end it was obvious that the first option was the one to take. I decided that I would also step the building in half a brick at point B to create a visual
break and help disguise the kink. I couldn't step in more than half a brick without my footings encroaching onto my well! As Planners usually request a break in the line of old building and extension I was sure this minor change would not cause a problem with planning, so went ahead.
I fixed nails into the battens at measured points for inside and outside of the 300mm cavity wall and inside and outside of the 600 mm footings. (as shown in red and green in pic 6)
Using my CAD drawing for accurate dimensions, I screwed battens to the back wall at points A,B,C and D. Provided I then drew lines out square from the existing back wall, I could accurately mark the positions of all the perpendicular walls (red) and foundations (green).
Drawing out square with the building is probably the trickiest bit and the area open to greatest error. Because they are nice simple figures to work with, I used the 3,4,5 method of creating a right angle. With pins fixed at A and C, I could run out two tape measures. C - B should measure 5M, so where the 4M mark on the tape measure running A - B met the 5M mark on the tape measure running C - B, I inserted a pin into the ground at B. I then drew a string out from A, through B and extended this line as far as necessary. I did this for each wall.
I hammered pegs into the ground and fixed battens across them with nails to accept the squared string line. I fixed the battens well outside the footings area to allow room for the digger to get in. I could now draw strings at either wall or foundation width.
Simple linear measurements allowed me to find the position of the walls that ran parallel to the back wall. I fixed battens and marked widths for wall and footings.
I found the 3,4,5 method very convenient, however, if one needs to extend the line well beyond the 4M mark, inaccuracies can occur. I used my CAD drawing and plotted diagonal measurements across various corners. With pins and strings in position, I then double checked everything and made a few minor tweaks.
Concrete is expensive and my digger man is a bit digger happy so I decided to dig a trial pit first and have it ok'd by the Building Control Inspector. I could then set up a level and make sure my digger man didn't go too deep - concrete is expensive!
Despite the disadvantages I have had to deal with over site access, the one great advantage I do have is great ground on which to build. I hadn't gone very deep before I hit sand then sand with lots of cobble. The ground was getting hard to dig so I stopped and phoned Building Control. My BCO turned up later that day to stare into the hole. Following some prodding with a spade he said the depth was fine provided there was not variation within the site.
I attached all my string lines to the footings width of 600mm and used line marking spray paint onto the ground to guide the digger. I then took the strings away. As Pete, the digger guy, worked his way round, I kept checking depth with my staff and laser (just as well, as he would have dug way more out!) The digger could not quite dig it all without running over and collapsing the trench, so when we ran out of room, the digger reversed off site. Any further digging would have to be done by hand, so I hoped it was deep enough.
As my wife went away to Paris for a few days and took the camera, I don't actually have a pic of the footings all dug out but at the end of the day it's just a series of trenches where the yellow lines were, so no great loss. I cleaned up the trenches by hand and did a little digging to even them up. I called the BCO back and he jumped in the trench and ferreted about with spade and tape measure. He ok' d the trench, so now I could think about how much concrete I needed.
6 cubic meters of concrete is about 150 wheelbarrows. Access to my plot is difficult, and I planned to do it mid-week when helpers were in short supply. Despite the £260 price tag, I decided to hire a guy with a concrete pump. Still no pics - unless you wanted some of my wife enjoying Paris! However, I will say that it may well have been the wisest £260 I have spent so far. I had arranged the delivery with the readymix company. Once the pumping guy was on site, they wanted a call and they would send the concrete. You really don't want it the other way round. The cement mixer pours the concrete into a large hopper, the pump then pumps it through a 4" pipe, the end of which is left hovering over the trench. They can add more or less water to make the concrete easier to rake level. We kept moving the pipe around the trench and raking the concrete. 6 meters turned out to be spot on, so we sent the mixer on its way and raked the concrete close to level. Being a little runnier than normal, it had filled my shuttered off areas, but I would dig that out once the concrete had firmed up a little. A couple of hours later and I went all round the trench with my staff to ensure the laser hit my 450mm mark. A little raking followed by tamping with a long stick and a helper, and the concrete seemed level. I dug out the shuttered areas and left it alone.
I needed the concrete to come to 450mm below finished floor level. This would mean that two blocks and two mortar beds would bring me neatly up to DPM level. Using my laser level and staff I set a peg in the ground at FFL and marked the staff. I then marked a point 450mm above that on the staff.
I waited 'til twilight so the laser showed up much better in rotation mode. I then went all around my trench with my staff and when the 450mm mark lined up with the laser, I stuck a 6" nail into the side of the trench to mark the required height of the concrete. I set nails about every two feet as a concrete guide.
I shuttered off a couple of areas where I knew drainage pipes would need to pass through the footings. The areas I shuttered off were 225mm deep (675mm below DPM). This would mean a 215mm block and 10mm mortar joint would bring those areas back to the level of the concrete. A gap would be left in the block for the pipe and a lintel would go over the top.
Measuring down from the nails to the trench bottom, I could now estimate how much concrete I would need with simple volumetric calcs - height x width x depth. I estimated I needed 6 cubic meters.