Hi all,

Let this be my intro post. I'm a small-time general contractor up here, I specialize in high-end trim carpentry, built-ins, coffered ceilings and such things. That said, I'm restoring an old house for my wife and I (not for immediate resale purpose!), we are starting a family and with a kid in our near future I expect to be in this house for a dozen years at least. It's in an extremely desirable location and of course like most old houses it's been very much neglected.

I have several challenging issues to deal with but at the moment with winter approaching I need to focus on the roofing challenges. I've stripped the house to its bare bones (double masonry walls and rough sawn timber joists and rafters). There are leaks in the envelope, the roof is shot (I knew that when we bought) and I'm grasping at straws trying to decide what the best system will be.

Facts:

- House built in the late 1800's
- Rough sawn rafters and original deck boards in fantastic shape
- Mixed low-pitch / gable end roof with dormers
- Location: Montreal, QC (Canada). It's -35C in the winter and +35C in the summer with 100% humidity being rather common (urban area)

Design Brief:

- On low-pitch portion of roof we want to keep timber rafters and roof deck exposed from the inside.
- Low-pitch roof can grow in thickness from the outside since new eaves / parapets are being done anyways.
- Gable ends with dormer windows cannot afford to gain any thickness outside, house is protected by architectural patrimony rules.
- Same with the dormers, cannot afford to alter exterior profile / dimensions
- Dormers will have their sides glad in aluminum / hardy concrete board / wood (undecided)
- There are skylights going into the low-pitch roof. They will be curb-mounted.
- All flashing / eaves / gutters are being redone in new aluminum. Anything goes.


Challenges:

- Insulating and finishing the low-pitch roof entirely from the outside. At this point I'm thinking of stripping any exterior roofing from the deck, roughing in the curbs for the skylights very tall to accommodate any future roof thickness, then applying something like an 8" thick "half-SIP" and finishing the top layer however the manufacturer recommends (double deck with furring or directly on the SIP, whatever they recommend)

- Insulating and finishing the gable ends and dormers. Again I would strip any roofing from the outside and, sort out any deck repairs and do this part with Grace Ice & Water shield topped with a slate or imitation slate product. For insulation I would spray isocyanate to fill the rafter bays, under the small dormer roofs and the sides of the dormer's framing members. New windows (hybrid extruded aluminum / wood casement, low-e and argon filled) will go in the dormers, foamed in place of course.

.... Do I have it right? I'm in contact with a local SIP manufacturer but haven't spoken in detail to them since I found this forum and started reading it to educate myself.

Photos and 3D views of the project:

Any input much appreciated!




     

     

If the structural roof deck & rafters of the low pitch area are in good shape you don't really need to go with a full or half- SIP custom solution. Standard plywood sized half-SIP-like nailbase iso panels from Hunter or Atlas et all can be applied, and deliver about R6 for every 25mm of thickness, lowering your total thickness requirements by about 1/3 from what you'd need with a EPS half-SIP. Using vented nailbase versions adds about 25mm of thickness, but protects the panel from water damage in the event of a leak. (Going with a furring mounted secondary deck works too, and may be less expensive.) Rather than a single panel providing the entire R, stacking the nailbase panels atop a layer of fiber-faced iso panels staggering & taping/sealing the seams gives it SIP-like air-tightness.

Ice & Water Shield is a strong vapor-barrier, so if you put that on the exterior and spray iso on the interior you may end up trapping moisture in the roof deck. Using semi-permeable roofing felt would be a better choice, unless you work out a scheme to have a vented (10mm minimum) rainscreen-style gap below the roof deck to give moisture an evaporative escape path. Ice & Water Shield can be used in the valleys, but putting it over the entire dormer or roof would be a mistake unless you vent underneath the roof deck.

Pay special attention to preserving or improving the top & bottom venting of the exterior brick wythe (assuming it's a cavity wall), or you can end up with rot issues where the structural timbers are in contact with the masonry. Many are vented into attics, and may need some help if you're changing the attic venting structure.

Thanks for the feedback Dana, much appreciated! I used this information to help me in deciding to do half the work myslef (stripping and preparing a new roof deck) and have the rest of the layers installed by professionals. I'm down to the original 130 year-old 1 1/4" tongue and groove fir plank roof deck right now, it is in great condition and we're going to keep the deck AND the structural beams visible from inside the house in the end.

To help with the structural integrity of the deck I'm adding a layer of OSB which will provide a good foundation for the rest of the roofing system. There will be short fasteners from the OSB to the deck planks but all other layers will be bonded. I'm going with a commercial roofing contractor who is accustomed to doing very large bonded roofs with insulation above the deck in our area. We're going with the following system along the lines of your recommendation using local products they work with daily:

1. On top of the new OSB deck surface, a seam-taped and bonded Permstop 40-60-40 vapor barrier
2. 4'x4' ISO panels, bonded and seam taped
3. 1/2" fireproof Soprabase FR bonded fiberboard with edges heat-welded
4. Top layer is a heat-welded Sopralene Flam 250 GR bonded membrane