When you are doing a roof tear off, it is important to determine if some (or all) of your roof deck needs to be replaced. Most residential roofs are made of wood roof decks. There are exceptions to these like metal “B” Deck or concrete, but these are very infrequent.
This article will assume your roof to be made of wood decking. (The roof deck is what your waterproofing material attaches or adheres to.) To fully understand the process in determing the wood replacement needs, lets look at what can happen in the following situations.
There are three types of wood roof decks.
1. Tongue and Grove
2. Plywood or OSB sheething
3. Plank sheething
Tongue and Grove
This is usually 2×6 with a “tongue” formed on one edge of the board that fits snugly into the “grove” in the board adjacent to it. It is a challenge to replace this type as it is not readily available at most lumber yards. You will likely need more than basic Home Improvement skills to do this type of wood replacement. It may be best to hire a contractor to help with this task.
This type of roof deck is very stout and durable. The only time it needs to be repaired is generally when it has had extensive exposure to moisture.
Plywood or OSB Sheething
If your house was built since the late to mid 70’s, you probably have Plywood or Oriented Strand Board (OSB) also falsely called Wafer Board. Builders started using plywood when it was invented because of its longevity and strength against splitting. OSB became popular in the late 80’s to early 90’s and is now used almost exclusively by builders. It is strong and durable, but costs less than plywood.
Plywood or OSB needs to be replaced when it is:
1. Broken from either a defect in the plywood (sometimes it can break where one of the ply’s have a knot) or just too much pin point pressure.
2. Water damaged and wrotted or delaminated.
3. Too thin. If the plywood thickness is less than ½” or the OSB less than 7/16″, it is going to be weak with the roof rafters spaced the standard 2′ on center.
Plank sheething (usually 1×6 or 1×8) was used as a roof deck before plywood. In the first half of the 20th Century, roofs in Utah were built with wood shingles over planks spaced about 1 ½” apart. Wood Shingle are expensive so when these roofs need to be torn off and replaced, it is cheaper to resheath the entire roof and go over it with asphalt shingles than to go over with new wood shingles.
It is ver important to examine this type of deck carefully. Splits can form in the planks, and they WILL get longer from the heat in the attic. Knots in the planks can cause it to break after the roof is done so if it looks iffy, it is best to replace it. Sometimes it is just best to resheet it to give you the best deck for your new roof. If the planks are more than ¼” apart, it may be best to resheet the roof.
Keep in mind that besides supporting the foot traffic, and snow load that is inherent with almost any roof, the purpose of the roof deck is to hold the fasteners that secure the roof. If the wood is compromised, it may not be able to give the nails or screws enough holding power to allow the roof to last it’s full potential life span.