June 13, 2014
Despite some heavy rain at times, over the last 2 weeks works have been progressing well, and we are now well under way with the roof construction, which is very exciting! The two weeks began with the placement of strips of the Intello airtightness membrane over all of the first floor wall heads. This will connect with the rest of the same membrane that will be fitted once the building is wind and water-tight, and before the ceiling is fitted on the first floor.
It is good to see the ceiling height now on the first floor: we have deliberately kept the ceiling height uniform over both levels in order to increase the feeling of space throughout.
We can now also see the positions of the 2 rooflights: one is over the shower in the master bedroom (below), and the other will also function as the access hatch to the roof and will be located in the hallway.
A section of ceiling over the double height space is formed with a triple Kerto-S beam which sits at the line of the cripple stud at the edge of the corner window. From there to the external wall on the west the roof joists run the opposite way to effectively trim this section of roof in a similar manner to a ladder truss.
There are a few changes in direction of the joists due to the location of the load-bearing walls below and the maximum spans for the JJI joists. Despite this, and the large number of joists required in the roof due to the 300mm centres, they are certainly easy to lift up to roof height and quick to install.
At the perimeter of the double height space there is some structural steelwork to form a more effective connection between the large cripple studs and the roof rim beam. This steelwork sits within the inner third of total wall insulation, and is completely covered by more than one layer of continuous insulation, which ensures there will be no cold spots or issues with potential condensation. We tried to minimise the amount of steelwork in the building as much as possible, but unfortunately the combination of double height space, flat roof and the large corner window to the south and west meant a small amount was necessary.
The next stage was to fit a plywood deck layer to the whole of the roof area; just like at the first floor level this layer extends right to the edges of the inner frame of the wall construction in order to create as effective a structural diaphragm as possible.
The next step was to construct the parapet wall framing. This is constructed from the same heavier 245mm deep section of wall framing we are using around the double height space below, which helps to provide stiffness to the parapet.
The parapet wall detail has been a problematic issue in the structural design: as it is a vertical cantilever it is difficult to adequately restrain it in terms of resistance to wind loading. We have finally resolved this by a combination of methods, one of which is to use plywood sheathing on both the inner and outer faces, the outer layer of which extends downwards and is fastened to the roof rim beam and first floor wall construction below.
The end of the last two weeks saw the arrival of the shaped firring battens which will be fastened to the roof deck and which create the minimum 1 degree fall necessary for the sedum roof. Rather sweetly the smallest pieces arrived with our names written on them!