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FAQ: Roofing Problems

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My roof needs to be redone, what should I use as sheathing?
F.K. writes:
My roof needs to be redone. The original sheathing is tongue and groove, which needs to be replaced. Should I have this replaced with plywood or with tongue and groove again? Which is better? What are the advantages and disadvantages of each? Should I use something else instead (such as oriented stranboard)? If so, why? I live in River Edge, New Jersey (in the New York City metropolitan area). I have gotten conflicting recommendations from roofing contractors and I really don't know what would be best. Thank you for your help.

Paul Fisette answers:
I would not redo the roof deck with boards. It takes more time, more money, and is less structurally resistant to racking. To use plywood or OSB you need to specify the appropriate grade for the spacing that exists between framing members. For example if the grade stamp on a piece of plywood says 32/16 this means that the sheet can be used on roofs where roof rafters are spaced up to 32 inches apart and on floors where floor joists are spaced up to 16 inches apart. I do not think that plywood or OSB is a good idea when roof rafters are spaced more than 24 inches on center however. Panel clips should be used for 1/2" plywood that is used on roofs with 24" on center framing spacing too. I think that plywood and OSB are equals when used on the roof as long as they are kept dry. Because roofs are prone to leak over the course of time, I am inclined to use plywood on roofs rather than osb (just a personal preference.) OSB is a lot less expensive and if you are going to have a dry roof, it is a good product.

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What are H-clips used for in installing roof sheathing?
D.Q. writes:
What is the purpose of plywood H clips? Do the clips have to be of an approved design ,and if so , who would approve them?

Paul Fisette answers:
The purpose of plywood H-clips is to reduce the effective spacing of the roof rafters and stiffen the roof deck (I have never seen them used for floor sheathing and only occasionally for wall sheathing). In other words, if you use 1/2" plywood for roof sheathing and you space the trusses or rafters 24" on center, this may be allowed by code and by the performance standard on the grade stamp --- BUT the roof sheathing will be springy. When you place the H-clip at the halfway point (12") between the rafters, the plywood stiffens. As a result the roof shingles nail easier and the roof will remain straighter over time (no sags with accumulated load. I do think that there are H-clip recommendations for some subfloor applications too, but I have not run into the occasion to use them. I do not know about a standard for clips. I have never heard of one. Teco is one of the manufacturers. You could contact them, or APA the Engineered Wood Association(who does the grade-stamping of plywood) 206-565-6600 to check this out.

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What should I do about ice dams?
Jay writes:
The roof (4/12 slope) on my house was professionally replaced in Sept. 94. They removed the existing shingles and gutters. Installed Grace Ice & Water Shield for the fist six feet of roof edge along the gutter, six feet wide for the entire length of the valley, and eighteen inches wide along the rake. Double felting the rest of the roof with 15# felt. Installed Elk Prestique II 40 year laminated fiberglass roof shingles and eighty-four feet of CertainTeed Shingle Vent II Ridge Ventilation. They also installed Rollex vented Soffit & Fascia. Removing all plywood along the gutters and installing two foot by four foot vent chutes between the trusses to allow for air flow from the soffit to the ridge. My problem is this; ice dams are forming and the snow is at least 18 deep through out the roof. The ridge vent is covered with snow. I have a wood burning fire place with a masonry chimney that is used to heat the house. The location of the chimney in the roof is close to the valley. The snow is melted to the shingles in a six foot radius of the chimney. What can I do to prevent the ice dams. They are pulling the gutters away from the fascia and I am concerned that they may be causing shingle damage. I have no leaks.

Paul Fisette answers:
I am glad to hear you do not have leaks. The work you have done was designed NOT to eliminate the formation of ice dams, but rather to eliminate the damage that results when ice dams form. Ice dams form because heat is lost from the house at a level that elevates part of the roof to temperatures above freezing, while allowing other parts of the roof to stay cold, below freezing. For a full explanation of how they are formed and prevented see the article about ice dams on our home page. Click here for "Preventing Ice Dams".

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