After almost a month of dealing with business in New York, I'm back on the roof here in London. It's been good to have some time away, but the goal for these next few weeks will be to finish the roof and finally get the scaffolding taken down.
The first thing I'd like to get done is the roofing on the flat portion of the new dormer's roof. I'm doing this first to minimize the exposure to the weather of the new (waterproof) WBP plywood substrate which had been put down before I went back to NYC.
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| June 7th - the plywood substrate |
The flat roof brings up an interesting discussion on the technology differences between flat roofing (which is a modern concept) and sloped roofing (which is the traditional way of building). Flat roofing only really started coming into use in the 1920's with the advent of modernism. Flat roofs have always been difficult to maintain, as they often leak - one early well-known use of a flat roof was by
Le Corbusier in the
Villa Savoye, which apparently started leaking as soon as the family moved in. Part of reason for this is that without a significant slope, water is not immediately drained from the roof surface, as it is in a traditional sloped roof - flat roofing relies on the chemical nature of the membranes and very slight, 2-3 degree slope to avoid water penetration.
Fortunately the technology has improved dramatically since then - these days, flat roofing products are petroleum-based membranes that are waterproof, like asphalt or modified bitumen. But they are sticky, smelly, and generally nasty! You're basically sticking together several layers of this (toxic) stuff to create the waterproof surface. Whereas the slates on the sloped roof rely on gravity to pull water down onto overlapping layers of natural (or artificial) stone that create an impermeable layer with very simple, but highly effective technique.
From a design standpoint, I much prefer flat roofs because of the freedom they provide in developing the form of a building. But from a hands-on construction standpoint, I find the slates a much more elegant and environmentally-friendly system. The biggest irony is that modern flat roofing membranes can last upto 30 years if properly installed and maintained, whereas slates roofs can last upwards of 300. So much for modern technology...
However, in this case we didn't actually have a choice: we had to use a flat roof on our dormer because otherwise we would not have been able to stand up inside, due to the height and design restrictions imposed by the Planning Dept (which are more onerous because we're in a historical conservation district). As a result, we needed both a sloping slate roof, and a flat modified bitumen roof. Going into this, I had not really considered the issues and detailing involved with having both systems adjacent to each other - particularly the flashing that spans both systems.
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| Working out the details in my sketchbook |
After much thought, sketching and research, I developed several details how to do this (after not finding any details anywhere). With the new details in hand, it was time to start the flat roofing:
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| June 7th - one of the new flashing details. The plywood overhangs the last slate, and lead covers the joint. The top of the lead is held by a batten that will be covered by the waterproof membrane |
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| June 9th - Applying the primer for the two layer roofing system. |
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| Fully Primed |
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| Rolling out the 1st layer, which has a sticky bottom side that adheres to the primer. |
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| One roll adhered and starting the second |
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| The protective paper removed from the 2nd roll, ready to be attached to the primed plywood. |
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| Running the 1st layer up the brick wall. |
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| A roll of the cap-sheet (the top layer) |
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| The cap sheet wraps around the batten to provide a drip-edge for the flat roof, and to make the edge waterproof. |
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| With the edge finished, the 1st roll of cap-sheet is laid-out |
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| Half the cap sheet installed - it also has a protective paper that is removed to expose a sticky surface that adheres to the bottom layer. |
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| Doing the 2nd roll. |
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| The details along the wall are tricky and time consuming. |
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| Wall details finished - they will be covered with a lead flashing. |
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| The finished flat roof! |
