PART 4: How the walls and rafters were constructed
I constructed the walls as a series of timber-framed panels. The concrete base provided a suitable level surface to work on.
The framing is nominal 2×3 regularised studwork. Many sheds and garden buildings from DIY centres are made with much smaller gauge timbers. 2×3 is much stronger and create a really solid, robust building. 2×4 studs are used structurally in modern timber-framed house construction, even several floors high, so 2×3 will be perfectly adequate for garden building purposes!
I chose 2×3 studs for a number of reasons:
- Strength – I need to be able to carry the weight of a green roof.
- Depth – plenty of room for insulation, electrical conduit etc.
- Cheap – as this is a standard construction material it is relatively cheap.
- Handling – ‘regularised’ timber is nice – semi-planed and with the corners rounded off. It hardly ever gives splinters and is easy to handle.
As you can see in the photo above, I laid out the studwork and screwed it together flat on the ground. I then screwed a sheet of 12mm OSB board to it, which gives the studs great rigidity. The edge of the OSB acts as a square, keeping the panels true. The OSB overshoots the top of the frame as it will be trimmed to fit the rafters later.
You can nail the studwork frame together but personally I prefer screws – slower and more expensive, but easier to undo and adjust.
Next, I stapled a modern breather membrane to the outside of the OSB. This will keep water that gets through the cladding out of the building whilst allowing any moisture inside the building to diffuse out. The membrane was allowed to hang down a few inches below the bottom rail to cover the joint between it and the concrete base.
This level of construction is virtually identical to that of a modern timber-framed house. Practising it on a garden building is good way to get experience if you ever consider building your own home.
Erecting the walls
The wall panels were then lifted up from one edge until they were vertical. Temporary diagonal braces held the panel upright until an adjacent panel was raised and screwed to it. By erecting two corner panels they quickly become self-supporting. Note the OSB board used as a square to make sure the walls are a right angle.
Although this method of construction is relatively quick and most of the work can be done by one man, it is advisable to have a second pair of hands available for a few minutes to assist when a panel is being raised. It is also advisable to avoid windy days!
This image shows the walls at the almost-complete stage. The tile batten which will carry the cladding is clearly visible over the breather membrane.
The door and window frames were built in while the panels were laying flat on the ground simplifying the construction.
Although the structure is clearly not weather-tight at this stage, the materials are effectively weather-proof for even several months – little harm would come to them from even heavy rain: OSB is quite moisture resistant in the short-term. What you would have to worry about, though, would be strong winds which could wreck the structure at this stage as the walls are like sails. For that reason if you did need to leave it plenty of props and cross ties would be in order.
Raising the rafters
For a flat roof, and especially a green roof, there needs to be a fall of about 1:50 minimum. The usual method is to buy wide joists, say 2×7 inch and cut them along their length so they taper from 7 inches at one end to 4 inches at the other. I find long rip cuts awkward and tedious. Also the timbers are expensive. With the vision of all this ahead of me I tried to take a short-cut: I made my rafters out of two 2×4 timbers screwed together, one set at an angle to the other, so that they effectively created a single rafter with tapering dimensions, without a rip cut in sight! This certainly saved time, and if I had not built a green roof on top of my building I might have got away with it. Trouble is, such composite beams are never going to be as strong as a single piece of timber: with the weight of the green roof the rafters bowed significantly!
Before the rafters went on a 2×3 top rail was screwed all the way round along the top of the studwork framing to tie everything together and provide a solid bearing for the rafters. The horizontal, ceiling rafters (joists?) rested on the toprail within the OSB upstands of the panels. The diagonal, roof rafters were then fitted into a notch cut into the upstand of the OSB board, using a jigsaw, so that they projected out at each end to create an overhang. The ends were cut to an angle for appearence as they would be visible when the building was finished. At the higher end the diagonal roof joists rested on 2×3 noggins fitted between their companion horizontal ceiling joists. This provided a consistant fall.
Noggins were placed between the rafters to create a level top edge, and the remains of the OSB upstand cut off flush. The breather membrane was then carefully cut and folded over the top of the building and stapled in place, and tucked in around the joists to make a tight fit. You can just make out all of these details in the image above.