It Looks Flat, But It’s Not
We’ve obviously shied away from a traditionally pitched roof, however one common misconception about the house is that the roof is flat. While it definitely appears that way from the ground, it actually slopes away from the structure at around 2 degrees. It doesn’t seem like much, but it’s enough to keep the water moving and prevent it from pooling.
The slope is actually quite pronounced when you’re looking at the lower roof from inside the house (mainly because it simply slopes from one side to another). However, the upper roof is a different story altogether. The roof has 2 peaks (on both sids of the clerestory) with each sloping away in 3 different directions. I’m not 100% sure how the framers figure out the various angles (and it’s actually hurting my brain just thinking about it), but I went up on the roof this afternoon and was amazed how well it was assembled. We’ve had quite a lot of rain over the last few days, but other than a little frost (and the snow that was falling from the sky) it was essentially bone dry up there. Of course the plywood sheathing isn’t exactly watertight and I assume a lot of water simply washed through the cracks, but there definitely wasn’t any pooling up there. I’d say that’s a good sign, no?
We had originally considered a product called PosiSlope, which is described as a “factory pre-designed tapered roof insulation and drainage system”. It’s essentially a large foam puzzle that gets assembled on the roof, with pieces custom made based on your drawings. Compared to strapping out the roof and sloping by hand, it’s assembled and installed in a fraction of the time, plus it has an inherent R-value… which is an obvious plus. Of course, it’s also happened to exceed our budget by just a few dollars. Sigh.
The next step is waterproofing the roof, although I don’t know the exact approach we’re going to take yet. I assume it’ll be a torch-down solution of some kind, but I could be wrong. We’re also planning on covering the entire clerestory (top and sides) with Ice & Water Shield to ensure it’s completely watertight. Well, as watertight as it can be… I suppose nothing is 100%. Regardless, I’m becoming somewhat paranoid about the clerestory leaking and I want to do everything I can to ensure it stays nice and dry.
Post Tags: framing, roofing, waterproofing
Jeremy,
In our case it was cheaper to go with the tapered insulation. If you aren’t using pre-fabed trusses, you must consider the extra labor and cutting to obtain the slope in the framing. We did use SIPs panels for the roof (very high insulation value built right into the “framing system”), so laying the panels flat, and then using tapered insulation primarily for obtaining slope turned out to be the best solution. Tapered insulation that provides all of your insulation value would likely be more expensive. I would recommend using fully adhered vs. mechanically fastened membrane.
— posted on November 17, 2008I’m interested to know how you are draining your roof now? In the photos there appears to be a parapet around the perimeter of the roof where your drawings show the roof slope draining directly into the eaves.
— posted on November 17, 2008We took into account the extra labour costs and it still netted out to be much cheaper! That said, SIPs probably would have been the ideal solution for the roof, as the insulation costs look like they’re going to be higher than I had expected.
As for draining the roof, we’re going to be installing scuppers at the corners (basically flush with the structure). The scuppers will be connected to downspouts which will run down the sides of the house.
— posted on November 17, 2008I’m curious to know what the cost / SF was that was quoted to you for the sloped Insulation. I have a project that I’m designing right now in Los Angeles with a “flat roof” and would really consider using something like that.
— posted on November 17, 2008You might want to consider Blueskin, made by Bacor, instead of ice & water shield. It’s a superior product, but for the small area you have won’t be too much of a premium.
— posted on November 18, 2008Blueskin on the clerestory only that is!
— posted on November 18, 2008Funny you should mention that… we were talking about using Blueskin… although I’m going to assume it’s actually blue?!
We need to use something that’s back (or very dark) because of the way we’re going to clad the clerestory. We’re going to make that area look like a “lantern” of sorts – that is, it’ll have cladding that will partially cover the windows. This will provide some shading (to reduce the amount of thermal gain inside). Regardless, you’ll see parts of the waterproofing product so we need to ensure it blends in
— posted on November 18, 2008Hmmmm, you shouldn’t have to see any of the waterproofing product. Why would you want to? The waterproofing product goes on the sheathing and then the cladding goes on top of it and covers it. You then caulk between the cladding and the window. The waterproofing protects the sheathing and the structure from water infiltration in the event water gets behind the cladding (and someday it will)(Blueskin is sky blue with white letters on it).
— posted on November 18, 2008Congratulation on getting your roof on before snow…..It is snowing here in Burlington out my window…. Good timing.
— posted on November 19, 2008Scott,
It sounds like they are creating a rainscreen system for the clerestory so you would end up seeing the membrane between the reveals of the cladding – hence the reason you would want it to be black so that it dissapears in the shadows. VaproShield might be a good option as they make it in black.
— posted on November 24, 2008Ryan – Indeed, that’s exactly what we’re planning on doing up there… I hadn’t heard of VaproShield before, but I’ll pass it on to our contractor. Thanks for sharing!
— posted on November 24, 2008Jeremy,
We originally had planned to do a rainscreen system on the exterior of our house as well, but opted against for a couple of complex reasons I will spare you the explanation of. I will tell you that I did the research on weather-proofing materials and you are definitely going to want to use VaporShield over the ice and water substrates. the VaporShield not only comes in black but more importantly it has a UV protection. IF you use the Ice and Water stuff it will break down from the exposure to the sunlight that it will receive as a result of the spacing on your rainscreen. If you read the product specs on the Ice and Water material the UV exposure is only a few months before it begins to break down.
— posted on December 2, 2008I would second the vote for VaporShield. It’s bomber stuff durability-wise, and it breathes. If we had done a rainscreen it’s the membrane we would have used. It’s not cheap, but if you want to feel good at night after a driving rain storm, this would be the protection you want.
— posted on December 2, 2008Thanks guys – I’ll inquire into the VaporShield!
— posted on December 2, 2008