Introduction: A Hut on Wheels
I built a wooden hut on top of an old caravan (trailer) chassis. The inspiration came from the design of traditional English shepherds huts and also a mobile home on wheels which I saw on Instructibles built by a guy called Paleotool. I've wanted to build one of these for years and used to sit on the train to work, sketching designs. Eventually I left that job, to pursue my own interests, including this project.
I bought Douglas Fir cladding & framing timbers from a local sawmill which specialises in home-grown timber. It was not "cheap" but was good value, I think. The trailer-load of timber pictured was fairly expensive at around £700-£800 but it was locally sourced and native and "fair trade", from a sustainable source. I think this is important . I'm sure that bulk purchase or home milling or using standard construction grade framing timber could have saved money, but would have been a lot more work (time) and perhaps not from a sustainable source.
In total, including the stove & pipework, it cost me £4,500 GBP for the materials. I could have reduced costs by using salvaged door, windows & stove. Plywood is expensive but strong for its weight & bulk.
It took me about 500 hours to build. There was a lot of time spent problem solving, so a second build would be quicker.
The hut measures 2m wide x 3.6m long. It is wide enough for a full size bed to go across the width.
Step 1: Floor Is Built on Top of the Old Chassis
I sourced an alloy chassis to avoid corrosion. Cost was £75 GBP. The weight rating of these chassis is usually stamped on a plate somewhere. Note: the tow hitch may have a different plate with a different (usually higher for safety) rating.
The floor is made from two layers of 9mm structural, waterproof ply with a sandwich of 50mm rigid insulation between 75mm x 50mm batons. I used home-grown (Scottish) Douglas Fir for the battens for its lightness & strength. All joints were glued with polyurethane, expanding glue and screwed. I wanted it to be rigid, light & strong enough to handle the jolting of road travel.
Step 3: The Framed Walls Are Built
I bolted the now complete floor to the chassis using the original mounting points plus two extra ones (probably unnecessary) giving a total of 7. I drilled upwards from underneath to avoid error. I tightened the dome headed bolts until the 9mm ply compressed about 3-4mm. I dragged the chassis "indoors" to a covered car-port (Costco £240 GBP) for weather protection. I used the completed floor as a workbench and template for assembling the walls. Again using 50mm x75mm Douglas Fir studs. It's a very pleasant wood to work with, takes screws well and is relatively lightweight for its strength and also pretty weather durable if you use the heart wood. Watch out: it is listed as an aggressive irritant if you inhale the dust. Great smell when cut.
Step 4: Walls Framed Out and Skinned
Walls were skinned with 9mm OSB (oriented strand board) called "Smartply 3" It's weather resistant, FSC approved and relatively "green" (being made from waste material) compared to plywood. Cheaper too. Again, all joints screwed & glued. OSB fixed with 50mm ring-shank galvanised nails from the trusty nail gun (a godsend)
Step 5: Making the Arched End Walls
I cut the arched, gable wall tops from 9mm OSB. They were notched, ready to take the roof purlins. The radius of the roof was 1.6m. Tip: I later wished I had made a spare template of these arches, it would have saved a lot of time. The arches were framed with 50mm x50mm Douglas Fir battens. 50mm rigid insulation was placed in between.
Marking the radius onto the ply was a bit tricky. I used the now-finished floor as a table, squared the ply up by marking a centre line on both it and the floor, and used a tape measure with its zero end screwed through onto the table. A pencil held at the 1.6m (1600mm) mark of the tape was used to draw the curve. I had tried using string but it was impossible to pull it taught without stretching it, and thus making errors in the radius. You need a material that is rigid. A piece of long, thin timber with two holes at 1600mm would have done a good job too.
Step 6: Raising the Walls
With all the walls now built as flat-pack, I raised each one, temporarily held it in place on the vertical with long battens tack-nailed onto wall & floor (working alone can be a pain, get a helper if you can). Note that you will need to think about how your corners are built; one wall will "overlap" the other at the corners. Draw a sketch first to work out your corner details. I made detailed drawings of each wall and its stud-work before I started. Note: I let the "skin" of the ply wall overhang the floor by 50mm. This would allow any water or condensation to overshoot the floor and fall on the ground. It also gives another surface for screwing the wall to the sides of the floor, for extra strength.
Step 7: Fitting the Central Arch & Purlins
I made a central arch out of three sections of 22mm plywood glued together and cut to the radius of the ceiling. Note, the arch was 100mm wide, so the radius of the lower curve is 100mm less than the upper curve. Notches were cut for the purlins, but only to half depth, the purlins also being notched. Joints were glued with a long, thin screw inserted to tighten the joint while the glue dried. Note: By the time I finished the hut, I had cut the curve shape eleven times in four different materials. Like I said, I really wish I'd made a template.
Step 8: Top of Walls and Ceiling
I ripped a 2x4 for the top of the long walls, with an angle which corresponded to the roof curve. This was easy on a table saw but could also be done with a circular saw and a jig to keep everything true. The ceiling was formed from four sheets of 8x4, 3.5mm ash-veneered ply, bent over the roof ribs and screwed in place using the ribs to hide the joins. This worked well and is lightweight & pre-finished. I put a coat of danish oil on the ply's internal surface. It is quite pricey but makes for a quick job.
Step 9: Wiring and Internal Walls
I ran wiring through the studs, keeping to 300mm above the floor for all horizontal runs. That way I know where the wires are, after the walls are finished. three wall lamps and three double sockets were provided for. I used a "Garage Consumer Unit" to provide a protected power supply. I allowed a generous margin for safety in the spec of the cable I used. Note that if electrical cable is run in an insulated space it's current carrying capacity is downrated slightly. The length of the run is also relevant.
The internal walls were insulated with the 50mm Kingspan.
Step 10: Internal Wall Finishings
The walls were finished with 3.5mm maple veneered ply with one coat of danish oil. I debated a lot about what material to use internally the alternative being T&G cladding. I reckoned thin sheets would be faster & simpler but I'm not so sure now. I had to cut some complex shapes in thin sheets with little margin for error. Not much fun. T&G would have allowed more flexibility as I went along. Having said that, I do hate varnishing and the though of sanding & varnishing T&G was off putting. Wiping the ply with danish oil was quick & easy.
I also used reclaimed church pews for cladding for the gable walls. The ply was looking a bit flat & boring and needed some relief. The old pine looks nice and gives some much needed character to offset the man-made materials.
I used this strips of wood, cut on the table saw, to finish the joints between the sheets. I left the small screw heads exposed, I think this looks OK.
Step 11: External Walls
The external walls were covered with breather membrane and then battened for the cladding. Vertical 15mm battens were added first, to allow an air circulation gap behind the cladding. Next, horizontal 25mm battens were nailed on. These would be used to fix the cladding onto.
Step 12: Roof
The roof was made by screwing 75x50mm CLS battens onto the ceiling purlins, through the thin ply ceiling. This provided a gap for the roof insulation. First though, I fixed a layer of shiny Protech 200 vapour barrier to the roof for condensation protection. The weather was very rainy throughout and I had to use a tarp to keep things dryish.
Step 13: Roof Insulation
The roof was insulated with 75mm re-cycled glass-wool "batts". (fiberglass). I seriously thought about using sheep's wool but was put off by tales of damp rot, mice & moths. It may well have been OK, but I was very concerned to make a hut which would last for years with almost no maintenance. I was reluctant to put an organic, perishable material into an inaccessible space above my head.
The fiberglass was covered with breather membrane to deflect any condensation drips from the tin roof. This was stapled into position. I left an un-insulated frame for the chimney flue to poke through later on. The flue requires a 50mm gap to any combustible material.
Step 14: Wrinkly Tin Roof
The roof was finished with corrugated steel from a local supplier. They also make pig arks for farmers and are used to bending the stuff. I forgot to add the depth of the roof joists (75mm) to the radius of the tin roof and just made it the same as the ceiling (1.6m). This meant I had to make the roof joists in three different thicknesses from centre to edge, to fit the resulting crescent. Oops. Note, you can't stack same radius curves on top of one another AND keep a consistent gap between them. (I know I'm not explaining this well)
The corrugated roof has a "non condensation" anti drip coating. Essentially, a slightly wooly finish on the underside which traps any condensation and allows it to evaporate slowly into the air, rather than drip. I also opted for a ploymer coating on the tin which adds a few years to its lifespan. I probably would skip this finish next time, since it's probably unnecessary and I'd like to have the option of collecting rain water off the roof for washing & perhaps drinking (once boiled) The tin for the roof & fixings cost around £270 GBP.
It was tricky to get the tin sheets to line up without "steps" at the edges. Perhaps because one edge of each sheet is going over its neighbour and the other is going under, resulting in a very slight cone shape to each sheet.
Step 15: Wall Details
The top of the walls, under the tin roof, was fitted with an "eaves comb", designed to keep out birds and small mammals. (This does seem a bit unkind, but I didn't want guests in the walls or roof space)
I fitted an insect mesh to the top and bottom of the air-gap battens to keep out wasps etc. I'm not sure how effective these measures would be
Step 16: External Cladding
The walls were clad "batten & board" style with 15mm think Douglas Fir cladding. Each board was nailed in its centre, so each nail on;y passes through one cladding board. This allows expansion & contraction of boards between wet & dry weather.
Boards were fitted with any "cups" facing inwards (ie convex side in). You can work out which way a board will cup from its end grain pattern. I used stainless steel nails throughout. This proved wise because the single galvanised nail I used (a 3" nail on a tricky corner) started to produce staining within weeks. The stainless nails are worth the slight premium in price. All fired form a nail gun which, as well as being quick, also means less damage to the boards.
I cut all the boards too long and then ran along the bottom with a circular saw before the counter battens were fitted. I angled the saw to produce a drip edge. The cladding overlaps the floor by about 25mm
Step 17: Windows & Door
I intended using standard window sizes for cheapness but could not find any that I liked. I really wanted astragals in the windows and not many suppliers had them as standard. I ended up getting custom made windows, pre-finished, with toughened safety glass. They are double glazed and were pretty expensive. They are "casement" windows (open like a door). I made an error when estimating the window size. I forgot that an opening window has two frames, not one, so the actual glass area was less than I envisaged. Oops.
Next time, I will source cheaper windows before I start and build the walls to suit. I may even make the frames myself.
The "stable" door was made by Neil Philip woodcraft in Creif. It is very well made and is fitted with draught excluding strips> I am very pleased with it.
Step 18: Internal Work
I made a sliding bed frame which converts from single to double by pulling out the interlocking section. Using old caravan cushions it was easy to create a flexible seating/bed arrangement. (that's what they are designed for).
The stove was installed according to domestic building regulations. I could not find any regulations for huts. Cutting the elliptical holes in the curved ceiling and tin roof was tricky. It is important to establish an accurate centre line for your flue. ( I used a plumb line for this) Everything else follows from this point. Once I established the centre line, I drilled a small hole in the ceiling and used this as a datum for the cutting of the ceiling & roof.
I was reluctant to cut a hole in my lovely tin roof and rely on a rubber gasket to keep out the rain. Perhaps next time I'll take the flue out through the wall and then upwards under the roof overhang. That way, it doesn't need to be a watertight penetration. However I'd need to work out how to cut the tin neatly first. My method of drilling lots of holes leaves a ragged edge. (currently hidden by the chimney flashing)
The stove is the "Wendy" 2.5kw woodburner from Windy Smithy blacksmiths in Devon (UK). It is an idea size for such a small space and reasonably priced.
Step 19: Finishing.
The hut will be used as a quiet retreat in a garden to do art & crafts & relax. I didn't plan this hut as a "touring" affair. It would not be aerodynamic and it's not designed for the open road. The wheels are simply to facilitate moving the hut onto its final site and to allow it to be built in one place and situated in another. If I build another I will maybe use a heavy duty, twin axle trailer.
Having said that, I did build it so that it can withstand the movement of being towed. The plywood skinned frame is very rigid, much more so than I had hoped. When I wound up the corner jacks for the first time after building it, I expected the building to sag just a little at each end but it didn't move at all. The chimney unhooks to reduce height and I'd put a seal over it for transporting, otherwise the "venturi" effect would maybe suck ash out of the flue.