The very first step was to build the concrete pad and pier foundations, which I describe in this post.
On the right is a photo of one of the six piers just after it had been filled with concrete.
It was important that this concrete was still soft as I would be driving some steel into the wet concrete to ‘pin’ the wooden frame to the foundations.
These are the large beams which sit on top of the concrete piers and support the floor joists. Like all the structural timber of the tiny house, the beams were made from Douglas Fir. Their sections were 70 x 200mm and the lengths were 4.8m (sides) and 3.6m (ends). The 4.8m lengths were supported in the middle by a concrete pier.
As for calculations, I think the floor beam size was simply suggested by Rob, the project’s ace carpenter, who knew from experience it would be sufficient.
Before putting the beams in place, I attached the joist hangers. It was easier to nail them on at this stage. I must say that the hangers were very easy to use. Just position them at the right depth, nail them on, then bend over the excess ‘tabs’ at the top. Remember to use the special twist nails that are made for them. Neither the hangers nor the nails are particularly expensive. I got mine from Screwfix in the UK and brought them with me for the build. These are the sort of items that you don’t want to be running around trying to find locally at the last minute.
After nailing on the hangers, the floor beams were put on top of the concrete piers and joined using heavy duty, galvanized corner brackets.
A piece of steel rebar was driven into the wet concrete through a second angle bracket in order to ‘pin’ the floor beams to the concrete piers.
I was assured by Rob that this was good enough. The weight of the house means there is no need to bolt the floor beam down. We just need to prevent to beam from moving laterally relative to the concrete piers.
I didn’t use any damp proof membrane between the beam and the piers as I reckoned that the weatherboarding would prevent rainwater from getting to this area.
The photo below shows the installed floor beams ready to accept the joists.
Floor Joists and insulation
The floor joist section was 45 x 200mm and they were spaced at 60cm. You can find online calculators such as this one to help choose joist sizes and spacings according of the span and load of your project.
Installing the joists was simple, I just slotted them into the hangers and secured them with some screws.
Next I nailed some battens along the sides of the joists. I then loose fitted some wooden boards between the joists. These simply rested upon the battens and were necessary to retain the hempcrete insulation.
This looks like a lot of work but it was actually done in an afternoon. Using the chop saw with the end stop meant that the boards could be cut to the exact length very quickly.
One mistake I made was to fit the boards together quite tightly.
I should have left an expansion gap of a few mm between the boards. After all, the boards were only acting as a kind of permanent shuttering.
What I later found was that mositure from the hempcrete caused the boards to expand and some of them buckled in a sort of concertina fashion. This wasn’t a major problem as the hempcrete remained in place, but it was a bit annoying all the same.
Anyway, the photo below shows the floor ready to receive the hempcrete.
The hempcrete was simply wheelbarrowed up a ramp, tipped into the spaces between the joists and raked out. It was compacted slightly so as to leave an air gap between the top of the hempcrete and the chestnut flooring that would be laid on the joists.
I used a ‘lightweight’ hempcrete mix of 10 litres of lime, 50 litres of hemp and roughly 10 litres of water. In terms of buckets, this represents a ratio of 1:5:1 respectively.
This mix, which is relatively weak in lime, could be used as the hempcrete didn’t need to have any mechanical strength. All the floor loads were taken by the joists and beams. The lime did add a little bit of thermal mass though.
The photo below shows the hempcrete infill almost complete.
I used tongue and grooved OSB sheets to cover the floor and provide a working platform for the rest of the build. From memory, the OSB was 22mm thick. I butted the sheets quite tightly together and screwed them in place. The photo below shows the floor at this stage.
Fitting the OSB was simple enough but I did have a problem later in that it expanded and caused a slight bump in the floor surface. This was rectified by cutting out the raised area and fitting a patch of new OSB in its place.
With hindsight I should also have glued the boards to the joists and ran some glue into the tongue and grooved joints. I would also have fitted them together less tightly. This might have stopped the buckling issue. Ideally though, I would have been able to wait until the hempcrete insulation had fully dried before fitting the OSB. Time constraints meant that I couldn’t.
It was a full year later before the OSB floor ‘platform’ was covered by the final chestnut floor. By this time the tiny house had been built and the interior walls had been plastered and limewashed.
I chose the least expensive grade of solid chestnut from a local sawmill. This had a few more knots than the dearer stuff but still looked nice and went together just as easily. The boards were just over 20mm thick, came in bundles of various lengths and were tongue and grooved.
Upon Rob’s advice the boards were screwed through the tongue into the joists in order to get a firm fixing that would minimise squeaks.
After a few tests, I found some woodscrews that were the right diameter and length to do the job. This system meant drilling a pilot hole and countersinking each screw, which was obviously slower than simply nailing the boards down. However, with 2 cordless drills, work progressed quickly enough.
Overall, laying the floor was quite straightforward. I left an expansion gap at the edges and used a scrap piece of board and a hammer to tap each board home before fixing. The boards that were cut at the end of one row were used to start the next row. Being accurately machined and super smooth, the boards didn’t need any sanding.
In order to protect the floor I chose an eco friendly oil that was sold by the board manufacturer. This gave a clear finish and went on very nicely but I later found that it didn’t seem to offer a great deal of protection. This was disappointing as I had previously used similar oils with good results. Furthermore, the oil was quite expensive.
A few weeks later, I re-coated the floor but this time I used linseed oil. This is a natural product too but is much cheaper and is easily available in DIY shops and even some supermarkets in France. I highly recommend it. It’s important to add some drying agent to the oil, otherwise it could be sticky for a long time. I coated the floor with linseed oil in the morning and then went out for the day. Although the weather was very warm, the oil was still very slightly tacky when I got back at night. I just about got away with it! Ideally, I would have left it for 24 hours (if I had somewhere else to stay that night).
Although I had given the floor a good wash and let it dry before oiling, it was still somewhat stained from dirt being brought in from outside. That, plus the linseed oil, had the effect of darkening the floor slightly.
I was still happy with the final result, pictured below, and it’s completely in keeping with the rustic theme of the build.