Four weeks into the build, and quite a lot achieved; the ground floor slab was cast this week – a big day! But quite a bit happened before we got to that.
After the subsoil under the footprint of the house was excavated to form a level, flat surface at the correct level, 150mm thickness of MOT Type 1 hardcore (MOT as in Ministry of Transport – the long since defunct government department that originally drew up the specification for this material back in the mists of time) was spread and consolidated with several passes of a vibrating roller. The hardcore base extends 1.2m beyond the footprint of the house. Drain connections are installed through the type 1 layer, forming ‘pop ups’ that will be connected to sinks and loos inside, and drains outside.
Then a 50mm thickness of grit forms a levelling layer on which the extruded polystyrene insulated formwork rests. The grit consists of, very importantly, 2 – 6mm particles of crushed stone with no ‘fines’ so that it consolidates when compressed but continues to allow water to pass through. It forms a flat, level surface with a tolerance of +/- 5mm, achieved by spreading and tamping in bays between two screeding bars, then removing the bars and infilling the depressions to form a level surface. The grit extends 600mm beyond the footprint of the house, and took quite a while to lay accurately, with frequent checks with a laser level. But everything else in the house rests on this layer, so it has to be right.
The insulated formwork which isolates the ground floor concrete slab from the ground arrived by lorry from Dorset … I know, delivery miles, but it’s just not available any nearer. L shaped units form the edge of the ‘trough’ with flat slabs forming the base, all of them interlocking with ‘egg crate’ profiles to form a remarkably stable and robust formwork to contain the in-situ reinforced concrete slab. The base of the insulation is 250mm thick, in two layers, and the upstands at the edge are 100mm wide; the whole lot giving a massive insulation value of 0.11 W/m²K; translating as ‘extremely toasty’ in layman’s terms. The polystyrene generated quite a bit of interest from passers by, and neighbouring builders, neither of whom has seen anything like it before.
The reinforced concrete slab which forms the ground floor of the house has a damp and radon gas proof membrane under it, steel mesh in the top and the bottom layer, and is 250mm thick; quite a lot of concrete! 20 cubic metres of ready-mix was delivered in four mixer loads from Tongland Quarry, spread by George and team, settled down between the steel mesh with a large vibrating rod, tamped and levelled.
As the surface hardened it was power floated with a machine resembling a giant floor polisher to give a smooth, level surface. This extended the working day well into the evening, waiting for the area poured in mid afternoon to become firm enough to finish, and the last patch was polished the following morning.
The theory of the insulation and the large amount of concrete is that the slab will form a ‘heat sink’ within the thermal envelope of the building, warming up during the day from solar heat gain and giving heat out during the evening and night, with the insulation preventing heat loss to the ground. Uninsulated floors can account for up to 25% of the total heat loss from a building, depending on the construction and type of soil under the house. During the summer the slab will act in the other direction; reducing overheating during hot weather … hard to imagine that just at the moment!
The exposed face of the ‘trough’ of polystyrene is faced with smart Staffordshire blue engineering bricks, although the ‘specials’ with a chamfered edge for the top course and the internal and external corners are currently on order and it’s hoped that they arrive in time to be laid before the timber frame arrives on site during the week beginning 26th March. The supplier is cutting it very fine!
The best thing about what we’ve achieved is that now I can see where the spaces will be; I can stand where the sitting room window will be and enjoy the view up the valley, and know where the sheltered sun trap will be in the angle on the south side of the building.
After all the excitement of polystyrene, steel mesh and lots of concrete it’s back to digging for George and the team; drains are next, but that’s another story for next time.