What are the construction methods for my off-grid house?

General Approach

As described under House: Why? I want to do a self-build - doing as much of the construction myself as possible. This means choosing construction methods which a reasonably healthy (I hope) but not particularly strong, fit or skilled person can manage on their own. This leads to a quite different solution to the one which would be adopted by a professional builder where the main aim is to get skilled workers on and off site as quickly as possible.

A good example is the construction method used for the roof. Typically, modern houses are built using roof trusses which are constructed off-site and craned into place. They save considerably on the length of time and amount of labour involved in construction. However, they're not the best method to use for a single person building in an out-of-the-way place:

• They would occupy more space and therefore require more road-miles to transport than the (somewhat larger) amount of timber needed for a site-built roof.
• A crane with operator and probably at least two other people would be required for the (relatively short) installation period.
• Trusses are made to an exact size and are relatively intolerant of any variation in the structure below.

In general, a site-built roof should not, with a bit of care, take too long to construct. Looking at the roof of my current late 1960s house, which is obviously site built, nothing looks very difficult at all although the purlins and principal joists (if that's the right name for them) are pretty heavy and, for solo installation, would need much careful work with a Tirfor or similar, lots of ropes, chains and ratchet straps and the odd block to do safely.

Still, it might be quicker than organizing a B&B for the crew then dealing with the aftermath when the crane driver returns there around midnight very drunk, throws up on the way in to the house then falls asleep in the dog's basket. I don't know if this happens on all builds.

In History I describe the background to my thoughts on the subject of build methods. Over time, though, I've settled on a fairly conventional overall structure with only a few more wacky embellishments.

Overall Structure

As described on the main page the overall layout of the house will be single story in the form a simple east/west rectangle. The basic construction will be post and beam infilled with cellulose filled stud walls.

Rather than build extensive foundations the plan is to use a number of separate concrete pads on a fairly regular grid to support the posts with an undercroft crawl space below the house which can be used for storage and service access.

The reasons for choosing this structure are:

• Simplicity of construction: I can get contractors in to do the foundation pads which are easy to specify. The rest can be built on them without much further external interaction.
• Flexibility of site: by keeping the building clear of the ground it should be possible to use sites which might require extensive earth works with other methods.
• Access: to be able to make changes below the structure if there are problems or modifications are found to be useful.
• Insulation: ease of installing and maintaining insulation below the building which is often difficult to access in most structures.

I'll need to take advice on the detailed structure but what I have in mind is that the main load-bearing posts should be mostly in the inner skin with a lighter outer skin attached in a way to minimise cold bridging. One possible makeup for the walls between the posts would be:

• Plasterboard
• 19 mm service void
• OSB providing main anti-racking strength
• 89 mm vertical studs
• 89 mm horizontal studs
• 89 mm vertical studs, all voids between these studs filled with cellulose
• 60 mm of woodfibre board (with waterproof breather surface)
• 50 mm battens with air gap between
• Rainscreen cladding

An alternative might be to use I beams to provide 300 mm thickness for the cellulose. If the cold bridging can be kept under control then there's less need for the outer woodfibre board which would be replaced by Panelvent or similar.

Floor would be similar construction but with suitable flooring materials, of course. Roof would also be similar (i.e., warm loft) but with significantly thicker insulation, 450 mm or so. Again, the main load-bearing joists and purlins would be in the inner skin.

Rainscreen

The rainscreen cladding on the walls and roof would be different depending on the direction faced. For those which get sunshine in the winter (east, south and west walls and south roof) the cladding in each case would be some form of solar collector.

The north facing roof would have a metal cladding for rainwater harvesting.

Rest of construction