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Question DetailsAsked on 7/13/2017

Is synthetic underlayment fire-resistant? Given that the synthetic does not breath, is roof venting a necessity?

We are located in NW PA with lots of snow (and rain it seems these days). Putting new shingle roof on historic 2 1/2 story house made of brick and hardwoods with pretty steep roof built around 1900. There is no insulation between the roof joists. There are 1" to 1.5" spaces between the 3/4" x 10" x 10' planks in the roof. The 2 bedrooms in the semi-finished attic are not heated, but there is 6" fiberglass insulation between the studs in the 4' crawlspaces surrounding these bedrooms. Would like to have option to use those 2 bedrooms as living space in the future. Recommendations?

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Generally, roof wrap needs to meet the requirements of ASTM D108 and UL 780, but is not fire proof - just is not highly flammable, generally tested so it may burn but have limited flame spread. Generally not a big issue because the material under and over it is more combustible than it. Ice and water shield (commonly used on the lower 3-6 feet and around penetrations and under flashing and valleys) is generally much less fire resistant, commonly being made of a tar substance. However, because the roof is considered to be "outside" the living envelope, fire resistance is not a critical factor for small residences (commonly defined as not over 2 or 3 stories nor over 2-8 units, depending on state and local codes.

Generally, there should be no insulation between the exposed attic joists because the space above and under it are normally (unless fully occupied as a cathedral ceiling) outside the conditioned space - basically open to outside air flow. You can find a number of previous questions and answers about attic insulation and living spaces in the attic in the Home > Roofing and Home > Insulation categories under Browse Projects, at lower left. The issue of insulation of the attic rooms (side and ceiling), providing ventilation in the crawlspace area near the eaves, and ventilation over the top of the living space is also addressed there - basically the rooms need to be vapor barriered under the drywall and insulated side and top like an exterior wall, making those walls/ceiling the isolation boundary between "indoors" and "outdoors", treating the rest of the attic space as exterior "unconditioned" space.

You also need to provide good ventilation from the eaves to the ridge vents (assuming conventional ridge-and-slope or gabled roof), with baffles as needed at the eaves and over the edges of the room ceilings to ensure the rafter bays stay open to ventilation from eave to ridge.

Insulation recommendations can be found at EPA and GreenStar websites - generally around R-36 to R-40 for your area, or about twice the batting you have right now, and the interiors of the rooms should have full vapor barrier in the walls and ceilings, under the drywall.

You can put shingles over a planked roof - though if the gaps are 1-1.5" that is a lot more than most manufacturers recommend as the maximum gap - most would recommend overlaying with sheathing, or if the wood strip decking is not in good shape or not uniform thickness, tearing it off down to the rafters and putting on new sheathing from there. Makes for a lot easier and higher quality roofing application, because you will not have places the shingles can sag into, and there will be lot less nail skips. With 10" roof planking (have not seen that for ages - like you say, pre-1930's construction) sometimes to avoid tearing that off, if in good shape and uniform thickness (to avoid roof ripples), ripped same thickness strips are nailed in the gaps (leaving 1/8-1/4" gap on each side and ends depending on temperature at time of application) to support the shingles and reduce the number of nail skips.

Answered 3 years ago by LCD

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