New roof – critique wanted
Apologies for the long-winded post! I am going to be placing a new roof on my parents house, and I’d like a double check on a few items, while I still have plenty of influence on the design. This will likely be the last roof either of them see before the home is passed to myself or sibling, or sold. Cost is a concern, but quality install and materials can take precedence, as there will be no labor costs associated with the project, other than my/others time.
The gable roof is 4:12, sheathed with 1″x12″ boards during construction in 1982. The current roof (2nd) is in its final stages, with some leaks in long straight lines developing somewhat uniformly over the whole house. The roof has vented soffits, with vents on the gable ends instead of ridge. The state of the insulation is unknown, with no direct access to the attic space – this is a 2nd story addition to a previously ranch floor plan. The house is in climate zone 4, but very close to the border of zone 5.
Plan for the roof:
1. Strip roof to original planks
2. Install a new layer of 7/16″ OSB sheathing (a)
3. Seal the seams with 3M 90 adhesive and ZIP tape
4. Two strips of 36″ Grace Select I&W Shield from the eaves and rake
5. New drip edge on the perimeter, new flashing around chimney
6. GAF Feltbuster synthetic underlayment (b)
7. GAF Starter shingle course
8. GAF Timberline HDZ arch. shingles (c)
9. GAF Cobra ridge vent
10. GAF Timbertex ridge cap
Plan for the attic:
1. Remove any water damaged insulation (d)
2. Install rafter vents, if not present
3. Air seal around fixtures and other obvious leaks
4. Install a bathroom ventilation fan with insulated ducting to roof
5. Add blow-in insulation to achieve something around R50
6. Block existing gable vents
(a) I’m assuming the existing planks have shrunk in their width, creating gaps that allow the long straight leak lines to develop. Combined with the multitude of existing nail holes, I’ve planned to create a new, flat, sealed surface with the OSB. Is this a reasonable design goal? Also, should a 3″ fastener be used, or is a regular 2-1/2″ ring shank sufficient?
(b) The roof isn’t considered low slope, but is on the edge. Is a standard overlap sufficient, or should I follow the low slope advice and perform a half overlap on the underlayment for extra protection? Should I cap nail only to the new layer of OSB sheathing?
(c) Should I strive to nail through both layers of sheathing material? If so, that would require a 1-1/2″ nail. Otherwise, will a 1″ or 1-1/4″ nail be sufficient? Will bounce be a concern between the layers of dissimilar material?
(d) Being that the type, condition, and quantity of insulation is unknown, should I be prepared to remove all of the insulation if necessary, and start from scratch? Any other gotchas on houses from this era?
Attached: supporting pictures
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