Notes on Wall Construction

As I have mentioned before, the detailed design for this build, as set out by the structural engineer, specifies a construction method for the walls that all the builders we have spoken to say makes no sense. Our current builder is proposing to change it to :

1. Make it easier for him to build
2. Increase the thermal efficiency so as to satisfy the local council planning department.

When I spoke to the builder about this a couple of days ago, I scribbled down some notes which are reproduced below. Unfortunately, my terrible writing, coupled with Blogger-Bots insistent on shrinking all my pictures to the point of invisibility, makes it less than useful :

The top sketch shows the wall as designed. On the outside is 125mm of stone, with 75mm concrete block backing. The cavity is 85mm of which 35mm is insulation, and 50mm clear void. The inner wall is 100mm concrete blockwork. According to the builder, 125mm thickness of natural stone is difficult to work. It depends whether he is going to use old or new stone (he has not sourced this yet), but either way, the standard is 150mm, so cutting it down to 125mm will involve both hard work and lots of wastage.

His proposal is shown in the lower sketch. 150mm stone, 100mm thermal backing block (with greater insulation properties), 75mm cavity of which 25mm is insulation. The inner wall blockwork at 100mm remains the same.

As I said in one (or maybe more) of my recent entries, the inner retaining wall forms the outside edge of the cavity so you can see that these two construction methods make a difference to the positioning of any wall that needs to adjoin the existing building. For the first, the front face of the stone(i.e. the outside of the building) is 200mm in front of the cavity (125mm stone plus 75mm backing block). For the second, this distance is 250mm (150mm stone plus 100mm backing block). This is a whopping 5cm difference.

In practice, it doesn't actually matter that much as both walls of the existing house to which the extension is to be attached are to be rebuilt. It should be possible to vary their position by 5cm without too much trouble. In fact, there really isn't much latitude in the front wall as it has to sit on a 450mm foundation beam. The total depth of the wall construction has increased to 425mm, giving only 12.5mm on each side !

I was surprised to find that the groundworks contractors were interested in this front wall (I call it front, because it is at the front of the site, but in fact it is at the back of the house !). This part of the build is all at ground level, so there is no subterranean component, or retaining wall needed. However, they have to lay at least the first course of blocks in order to contain the cast floor slab - otherwise it would leak out all over the garden ! This has led to more hard questions. To build this first row of blocks they want to know where the patio doors are going, how wide they are going to be, and even details like how thick the plaster will be. These are all questions for which we have no answers (yet).

One final piece of good news from the week. The invoice for the piling has come in and is as per the estimate. We were expecting the written equivalent of sucking of teeth, with reference to standing time, and greater than expected pile depths, so it is welcome that there has not been any increase in this bill.