Tuesday, 5 July 2016

Building the extension slab

The rain held off for 24 hours so we cracked on with shuttering and filling the trench with a strong concrete mix, and reinforcing bar because of the clay underneath. Our trench was around 18-inches deeper than the bottom of the hardcore, and a further 36-inches below finished floor level.

We had to lay some old smashed concrete on the slab area just to walk on it without collecting clay-mud on our boots.


Because the house itself has no foundations, and the bottom of the stonework lays at the exact level we are building UP to with the concrete, we were in great danger of causing a collapse of the house. So an important task we did in carefully managed stages (not many pics to illustrate it, sorry) was to build a large concrete buttress below the corner of the house.

We did this by creating a form-work out of concrete blocks, and carefully digging in to the clay below the corner, We hammered in several rebar sections and then filled the area with a strong concrete mix 2ft deep and 4ft long. 



Whilst that set, and before we did any more work in scooping out the clay near the house, we set about moving back in to the slab area tonne after tonne of hardcore. It took 2 full days with 4 people to hand-shift bricks, concrete, tiles, slabs and stones, smashed down to half-brick and smaller sizes to fill the slab area and make a stable base on top of the clay. Worst job we've ever had to do.


In fact, we ran out of 'clean' hardcore so we had to start sifting through the earth which we'd dug out of that hole to get large stones and bricks to throw back in. We ended up with around 10 tonnes of earth left over, which a kindly local farmer took away to fill some holes!


The amount of materials used in the extension has boggled my mind. My rough calculations are that, to get to finished floor level, we've hand-shifted in barrows 23 tonnes of 'stuff'. I'm not kidding when I say that I now have a knackered back and a new appreciation for labourers. 


By day 4 of having the Builder on site, we'd got the first blocks up towards finished floor level.

Here's a day's effort of filling that level with hardcore. The top of those blue bricks is finished floor level to give some appreciation of the task in hand.


Our trusty barrows - three at a time in operation during the day.


On top of the hardcore went down a layer of scalpings, which were whacker-plated to flat. You can see that below. On top of THAT went a layer of sand to smooth off any sharp corners, and then a layer of Kingspan insulation.


And eventually, after what was only 8 days but which seemed like an eternity of barrowing, the Builder got the concrete laid and level, and it started to look like a floor. Hoo-flippin-rah.

Foundation trench for the extension

Oh good, it's forecast to rain for the next week, just in time for the heavy clay foundation trench to fill up like a swimming pool. How we laughed.

When we finished laughing we strung up an enormous tarpaulin to try to keep as much of the rain off the trench as possible. You see, once the rock-hard clay/stone mix of ground got even the slightest bit wet, it turned from ground which you'd have serious trouble getting a spade in to, to ground which you'd have serious trouble getting your shoes out of.


That was a horrific couple of days of soul-destroying shoveling to get the trench to final depth (4ft-plus below finished floor level) and the entire area of the slab down to a flat and stable 3ft below finished floor.

Clay, you see, shifts and moves and grows and shimmies as it absorbs water, so the regs say you have to go deep, and provide a solid, heavy, reinforced base which won't cause your extension to crack. To hell with the fact that your house's foundations are (literally) above the finished floor level you're aiming for, and that it's just plonked down on the clay.., that's old building with stone and lime versus modern thinking and materials.

And the rain, as if to illustrate the fact that the clay was absorbent, kept on coming. Our tarp served to redirect most (not all) of it in to a bathtub we rigged up, where we pumped some away, and watched as more filled our trench repeatedly. It was frustrating, shall we say.


But in the end we got there, and you can see the bottom of the trench, where we threw in some spare Glapor to give us a walking surface, versus the finished floor level (see the gravel line on the house wall).

It took 3 days of manual digging and fettling to get the levels right.