Puzzled by roof that looks like a wall
Hi all, so I see this (attached) style of roof somewhat frequently in contemporary architecture. I have to admit that I like it. As we all probably know, little to no roof overhang is quite fashionable right now, and has been for awhile. This design sort of takes that idea to the extreme. This is very much contrary to Martin’s roofing preferences so I’m especially curious about his opinion on whether this can work long term.
I’m wondering:
1. What do you think is going on here? It seems to be charred cedar boards for siding as well as roof cladding. In this case the cedar boards on the roof would have to purely decorative, or perhaps as a UV barrier to protect the water barrier underneath. If that is the case, could blueskin be adequate as a water barrier under the cedar? Maybe they have hidden an EPDM sheet? Torch down membrane? I suppose each project could be different, but I’m wondering if anyone has encountered this first hand? I did come close to this design at the Fogo Island Inn in Newfoundland a couple years ago, and it seemed to me like a blueskin roof membrane with a painted spruce deck above it. Could that have been all there was to it?
2. Does this seem like a good idea?
Thanks!
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Replies
Hi Tim -
I don't think that it could be more appropriate that the source of this design is "cabinporn."
Letting all of your roof bulk water load run right down the face of your wall is asking for trouble.
Yes, this design would be less disastrous in some more forgiving climates than others, but clicking on the photo tells us that it is a prefab cabin in the UK. Not exactly a forgiving climate.
Going to the Koto Cabins website, it's also telling that one of the principals has his head down...(https://kotodesign.co.uk/about-koto-cabins/). There is no mention anywhere on the website regarding building science, moisture management, durability.
To offer a design like this without addressing the obvious concerns of how moisture is managed at roof-wall transitions is simply scary.
Peter
Thanks Peter. Your comments align with my suspicions. I suppose there could be an elaborate drainage system hiding underneath the sleek cladding, but it seems cumbersome to design that way.
The Fogo Island Inn was designed by Todd Saunders, who has become somewhat of a “starchitect” - at least in the Scandinavian school of design. I actually emailed him after my visit to ask about the mechanics of the roof, but didn’t get a reply. Could water management be each architect’s own trade secret in this style, or is this a really irresponsible design that looks good, for awhile... ?
That picture looks like a photoshop creation to me. It's not clear whether they've actually built anything yet, or whether these designs are fully engineered, or just artist concepts.
Since this is a prefab structure, they could have done interesting things we aren’t aware of. Maybe the building is made of interlocking plastic panels that form both the structure and water proofing and the entire exterior is just an elaborate facade with no other purpose. Factory built assemblies can do things that aren’t practical for onsite custom builds.
Bill
While I have no knowledge of how this particular cabin is designed, I think the correct approach to building a roof that looks like this is to build it like you would build a roof deck, since that's essentially what it is, even if the wood boards aren't necessarily designed to be walked on regularly.
Here's a BSC article about roof decks: https://buildingscience.com/documents/insights/bsi-051-decks-roofs-you-can-walk-on
The BSC article mainly shows examples of tile decks, but you can support the wood cladding/decking on PT sleepers and still use basically the same approach.
I have seen many, many wooden roofs like this in European architecture magazines. I think they are fairly common there and the building standards are higher than ours so they are probably doing it right. I have searched the web to find out what the substrate is under these roofs and had no luck. Matt Risinger is doing a series on building in Switzerland right now and I wish he would answer this question. However most of the wooden roofs have a good pitch. Here is another example: http://www.b2ai.com/en/projects/detail/barn-house-roeselare
Lisa
I had a pre fab building that the plywood sheathing was sprayed with chopped strand fiberglass & resin in the factory -think upside down boat. No overhangs like shown. It managed to crack along the low slope ridge line & with freeze thaw the water came out the wall in a second giant crack. We resided with metal siding & an overhang.
Kebony promotes the wooden roof in it's brochures: https://kebony.com/us/timber-roof-cladding/
But I think larch is more common. There are no over-hangs.... Lisa
I agree with Brendan Albano, and the BSC article he linked to. There is no reason why a properly detailed building with no roof overhang can't work, but it is important to detail it properly. I've often designed or built projects in locations where roof overhangs are not a benefit, but a detriment, as they catch wind and rain blowing in upside-down.
What are those details? With a low-slope roof like the one you show, you're pretty much stuck with foam in the assembly--either rigid foam above the roof sheathing, which is preferred, or spray foam in the rafter cavities. If you had a roof slope you could vent the roof.
At the walls, provide a robust rain screen gap, vented at top and bottom; use a top-notch WRB and make sure it is fully waterproof before any cladding is installed (which is always good practice, but more important when there is no overhang) and have the roof dripedge run over the top of the siding. The installers of membrane roofs (typically EPDM in my area) usually use a commercial-style dripedge that is taller than ideal, aesthetically; for a sleeker look try to get them to use a dripedge that is not as tall. The siding will take a beating, as it does at the bottom of walls that have overhangs, so consider a durable material such as unfinished cedar, coated steel or any number of synthetic materials. And don't expect the siding to last forever.