OSB sheathing, or "Oriented Strand Board" is the most common form of roof decking. It is what many refer to as "particle board" though this is not technically accurate. It was used on almost every new home built from the 70's on. While it is touted as being more structurally sound than CDX plywood, you can watch any roofing crew tearing off an old roof and see that OSB shreds like cardboard once exposed to moisture. OSB comes in various sizes and thicknesses, but the most common for steep slope roofing is 4' x 8' sheets that are 5/8" thick.
Things to Know About OSB Decking
As with any roof deck, OSB decking serves as the foundation for a roof. Though not technically part of the roofing assembly, it is critical to a roofing system as it is what all components are fastened to. Georgia does not require much in the way of fastening OSB decking. Florida, and many coastal states require hurricane clips to further reinforce the roof deck from high wind conditions. That said, rarely have we ever seen a roof deck blow off due to high wind conditions with the exception of tornadoes.
One of the most common mistakes roofers make when inspecting roofs, is not fulling vetting the roof decking. With the exception of evaluating warping and ability to hold weight, it is simply impossible to evaluate issues with roof decking from on top of the roof. The ability to hold weight does not mean a roof deck should not be replaced. Any wood decking older than 20 years needs to be inspected from under the roofing assembly. The reason for this is two fold:
- As wood grows old, it becomes brittle. If you ever get the opportunity to hang out under a 20+ year old roof deck while the roof is being replaced, you will likely be in a constant barrage of wood splinters. This is because the new nails are causing the old brittle decking to shatter.
- Project Creep: Or what many refer to as hidden costs. Often times, a professional roof inspector can conduct a visual inspection of the underside of a roof and determine to what degree the decking will need to be replaced. This is impossible to estimate from the top of the roof. If your contract doesn't specifically state that the roofer must replace all "rotten" decking as part of the contract price, make sure you understand how much each piece of decking will cost to replace on a per sheet basis. While it is impossible to get an exact count of how much decking needs to be replaced, a professional roofer can get pretty close.