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Roof stack plan for home with log purlins in Climate Zone 6

user-5594283 | Posted in GBA Pro Help on

Hello Martin and Dana:

I’ve read your Q&A’s for Roof Construction – but I’m Confused and Running out of time before we must proceed with the most effective and energy efficient way of building our Roof in Zone 6 in SE British Columbia. We have one area where the interior finish will not meet with Martin’s Choice of Drywall – so I will proceed to outline our roof stack – starting from the INSIDE(Interior) to OUTSIDE (Exterior). Our roof structure is comprised of 18″ diameter # 1 Grade Douglas Fir Logs – Ridge and Purlin With King Post Log Trusses at the Gable Ends and NO Log Rafters – 14″ I-joists Rafters 24OC. See Below:

1) 2×6″ T&G Ceiling Decking (Either Pine or Douglas Fir) – we will not be using Drywall
2) Certainteed MEMBRANE applied over this decking Tape and Sealed – Doubled up if necessary?
3) 14″ Engineered I-Joist Rafters 24″ OC with Soffit Ventilation in Rafter Bays – Valleys would be
Strapped with Purlins above sheathing to provide ventilation below Standing Seam Metal Roof
4) The Bottom of these will be filled with 4″ of CCPU Foam applied in two layers to allow for curing
5) Balance of Cavity 8″ to be filled with ROXUL Mineral Batt Insulation – Total R Value: approx 60.
6) 2″ Ventilated Air Space – Cold Roof Assembly with Continuous Soffits in each rafter bay
8) Covered with 5/8″ Plywood Sheeting (T&G Sealed and Taped?) or with a 1/8″ Gap? Not Sure???
8) 6′ of Grace Ice and Water Shield HT in the 4 Roof Valleys, Along All Rakes,Eaves, and around the entire base of the Chased Chimney.
9) As per Martin’s Preference – 30 lb Roof Felt (or Titanium 50 Synthetic Roof Underlay?)
10) 1×4 Purlin strapping – following by a 24 gauge standing seam green-colored metal roof

If you feel that this is not appropriate – can you please suggest the BEST Alternative – Non-Vented Roof Approach. I would like to maximise Energy Efficiency, Condensation Problems, Air-Tightness, Thermal Breaks and Roof Life. I might add that we will have 2 Plumbing Roof Vent Stacks for Bathroom and Kitchen + 1 Radon Vent + 2 Stainless Steel Chimneys penetration the roof – all gaps Taped & Sealed with flashing and CCPU Foam which will be enclosed in a Plywood Chase (with a Cricket as recommended by Martin) and later veneered with Cultured Stone. The Metal Roof will be installed by a professional roofer with over 30 years of experience in metal roof installations.

Thank you so much – Mark Kozlowski – Vancouver, BC, Canada

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  1. GBA Editor
    Martin Holladay | | #1

    First, a minor comment -- there is no need to capitalize so many words. The capitalization makes your text hard to read.

    If a roof has valleys, as yours does, it is not a good candidate for a vented roof assembly. The best way to insulate a roof with valleys is with an unvented approach.

    I have no idea whether this house only exists on paper, or whether you have already started building it.

    If you are in the design stage, I would suggest that you aim for an R-49 roof assembly. The insulation should consist of a combination of rigid foam insulation above the roof sheathing and air-permeable insulation under (and in direct contact with) the roof sheathing. Here is a link to an article that explains this approach: How to Install Rigid Foam on Top of Roof Sheathing.

    In Climate Zone 6, the rigid foam layer should have a minimum R-value of R-25. This can be achieved with about 6.5 or 7.5 inches of EPS or about 5 inches of polyiso. The remainder of the R-value (R-24) would be about 6.5 inches thick, so your I-joist rafters should have that depth. Fill the I-joist rafters with fiberglass batts, blown-in fiberglass, mineral wool batts, or dense-packed cellulose, making sure that this layer of air-permeable insulation is in contact with the plywood roof sheathing.

    If your metal roofing is installed on 1x4 strapping, you will still end up with a layer of air between your metal roofing and your roofing underlayment -- and that's a good design.

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