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There's a floor in your reasoning
I don't know why, but it seems to me that getting a working floor in any building is a major morale-booster. Once you got a floor -- even a rough, temporary one -- you've got a building, rather than a building site. Or maybe it's just that I've learned to associate flooring with being near the end of a job -- you can't really put down a proper floor until the rest of the building is more-or-less watertight. In any event, it's always an occasion for a sigh of relief. In some cases you may able to lay a floor over the foundation slab, with a damp-proof layer in between to keep water off the timber. You may even simply be able to paint the foundation slab and call it a floor. But, in my case, the floor had to be nearly a foot above the foundation, which necessitated some sort of joist arrangement.Skids and joists
Most modern buildings seem to use metal joist hangers for the installation of floor joists. These are set into the mortar at regular spacings, and the joists are simply cut and inserted into the hangers.However, if you have materials spare, it's quicker, easier, and cheaper to stand the joists on a pair of low brick walls. It's traditional to make a `honeycomb' wall for this in a house, because the gaps in the honeycomb can be used to pass cables and pipes through. There's no need for that in this job, but there's no point in making an elegant, neatly pointed, wall that's below the floor and never seen.
The advantage of the honeycomb wall approach over joist hangers is that you can sit the joists on top of flat skids, on top of a thick bed of mortar, and this will allow the floor joists to be adjusted to give a pefectly level floor. Mortar is extremely strong in compression, and a floor constructed this way will take as much load as the timber joists will stand. It's worth bearing in mind that the space under the floor is likely to get rather damp -- it's very close to the ground -- even if you've installed airbricks. So you need to treat all the cut ends of the joists and skids with a good dose of wood preserver. Also, it's probably best to allow some space between the timhers and the brickwork so that damp cannot be conducted from the brick into the wood.
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| A `honeycomb' wall, topped with a wooden skid, resting on a layer of damp-proof membrane (not visible in the photo), resting on a thick bed of mortar |
The floor proper
I was fortunate enough to get a parcel of solid oak, tongue-and-groove floorboards large enough for my porch second-hand for twenty quid. I could have laid these board directly on the floor joists -- that is how they were intended to be used, after all.However, to do this I would have needed either to nail through the tops of the boards with big ugly nails, or to nail through the tongues (`hidden nailing'). The first approach is ugly, the second very tiresome. So instead I constructed a temporary floor from scrap plywood, screwed to the joists, and then laid the oak flooring on top of that, making a `floating' floor.
As with the joists, ideally the flooring needs not to make contact with the brickwork. If you're fitting skirting board inside, you'll easily be able to hide a gap of 5mm or so. Otherwise some flexible, waterproof filler should do the trick.
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| The joists are fastened to the skids, not touching the brickwork at any point, to reduce water penetration. The thing on the right of the photo is an old tabletop, which served as walkway for people to get in and out of the house during the construction: it's very easy to trip over bare joists |
Approximate costs
| Brickword, timber joists and skids left over from earlier work | £0 |
| Subfloor made of scrap plywood | £0 |
| Solid oak flooring, second-hand | £20 |
| Floor varnish | £10 |
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| Total | £30 |
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