Summer 2018 was a record-breaking season with the warmest and sunniest weather in over 40 years. Such exceptional weather was ideal for building a 200 sq m roof as we were working under the sky for the entire summer. The overhang (500 mm) adds a lot of extra area to the roof – the overhang being necessary to protect the walls from driving rain. We had already covered a lot of the roof with cladding and now needed to build the roof deck to support the insulation and the living roof.
Part 1: Roof deck
The roof deck involved fixing square timber to round timber, which added a level of complexity and difficulty to the task. The first layer of the deck consisted of 150 mm x 50 mm (6″x 2″) timber laid over, and in opposition to, the roundwood rafters at 400 mm centres, where possible (for greater span we used some 220 mm x 50 mm).
Building the deck on the L-shaped part of the roof (north and east side) was reasonably straightforward as the surfaces were quite even and only slightly sloping. However for the reciprocal roofs, it was a very difficult and time consuming task, as the roof surface is made up of many different planes which needed be connected and covered. The complexity of this task considerably lengthened the time budget for the roof. We once again engaged Osian to help us with the carpentry on the roof and to speed up the process. The roof deck took 16 weeks to complete.
Each reciprocal roof required over 100 separate and different pieces of OSB to cover it – like solving a big wooden jigsaw puzzle. Each piece had to be individually measured, cut, offered up, altered, glued and nailed.
Timber size and strength dictates the span it can be used for (see span tables). An alternative approach for our roof could have been to use larger timber e.g. 225 mm x 50 mm (9″ x 2″) to enable greater spans to be achieved, and would have used slightly less timber. However, the complexity of covering the reciprocal parts of the roof would remain. Another alternative would have been to make up grillages on the ground to cover this part of the roof and use a crane to lift them into place (a more expensive option).
Part 2: Fascia
The purpose of the fascia is to cover and protect the roof’s supporting timbers and insulation. Due to the size of our roof rafters and the deck we built, the height of the fascia in places is almost 1,000 mm. It also follows the contours of the roof, so that it is wavy in appearance.
For most of the roof, the fascia consists of 3 lengths of 225 mm x 20 mm fascia board which is biscuit-jointed, glued, and screwed together. It was then nailed on to the roof. The fascia was built in various sections, which were then fitted together onto the roof. This work took place at the beginning of September. The weather was still good for roofing and we were hoping to complete our roof covering before the weather changed.
Part 3: Vapour Barrier
The purpose of the vapour barrier is to prevent the build up of moisture in the roof’s structure when cold and warm air meet. We contacted Laydex who supply systems for green roofs. The Vapour Barrier we used was Alutrix 600, which is a high performance barrier. Applying this was straightforward, a primer was painted on and then the Alutrix was applied (there are excellent videos to help do this).
The completed roof…
Part 4: Roof Lights
During the building of the deck, Osian made four wooden dodecagons (12-sided shaped) which were affixed to the roof openings, once the rafters were cut back and levelled. We recycled some of the Larch we had kept from our framing bed for this.
We purchased two roof lights online and they were relatively easy to install, although this depended on the weather. As it was now November, we had to wait for a calm, dry day to complete the installation.
Unfortunately, time and the weather were against completing the roof in 2018. It still needs to be insulated and covered with the waterproof barrier. We were a bit disappointed because ideally the roof should have been water-proof by the end of the year. It may well be that our time scales were over-ambitious as there is a high level of unforeseen complexity to the work, which, as amateurs, we found difficult to factor in.
Towards the end of 2018, we covered the entire roof with 1000-gauge polythene to protect the vapour barrier. We will be aiming to complete the roof in 2019, with perhaps a more realisable timescale.