Q&A / 

Sheathing

DEAR TIM: I'm trying to educate myself about new construction as I'm thinking of building a new home. I see carpenters nail on large sheets of wood onto the wall studs. What's the purpose of this funny looking material that appears to be made from scattered chips of wood? It seems that it could rot easily if not protected. I asked a worker on a jobsite what the material is and he said sheathing. Is that correct? Tony T., Lebanon, OH

DEAR TONY: The material you probably saw was OSB sheathing. The OSB stands for oriented strand board. This building product has now become the mainstay material to cover both exterior wall studs and roof framing members. Years ago, when I was still a pup carpenter, plywood was the king of both wall sheathing and roof sheathing. OSB sheathing has now pushed plywood to the back buildings of the lumber yards.

This house under construction is using a newer sheathing that's designed to repel water. PHOTO CREDIT:  Tim Carter

Go back 70 years or more, and solid wood was the sheathing that was used on houses. It was tongue and groove and often installed diagonally on walls and floors. To save money on building, the carpenters first used this wood material to build the forms for early poured-concrete foundations.

Once the concrete was set and hard, the carpenters carefully took apart the forms and reused the 1x6 lumber on the floors and walls of the houses. If you look at older homes and see this material, you can often see the drips and stains from the concrete.

The fiberboard sheathing you see on the new homes you inspect serves several purposes. First and foremost it's a structural material. When the sheathing is nailed to wall studs according to the written specifications of the manufacturer and the building code's minimum requirements, it prevents the walls from racking side to side.

The wall sheathing allows a worker to drive a nail anywhere on the wall allowing him to secure a finished product. But keep in mind it's always better to nail into a vertical wall stud as well as the wood sheathing.

Understand that a frame wall can easily collapse like a cardboard box that you flatten once the top and bottom flaps of cardboard are wide open. When a cardboard box is closed and taped it's very difficult to flatten it.

Imagine if you built a wall using 2x4s and didn't cover it with anything. If you nailed the bottom plate to the floor after standing the wall up and then went to the end of the wall and pushed it forward, you could easily flatten the wall. Can you think what might happen to a house without wall sheathing if a strong wind pushed against it? Wall sheathing prevents houses from falling over.

You're correct in noticing that the wood sheathing can rot if exposed to water. This is true for any framing lumber that's not treated with a chemical, coating or covered with a waterproof material. Carpenters of old knew this and covered the side walls of houses with asphalt-saturated felt paper. They also put this felt on roofs under slate, tile and shingles to create a secondary barrier to water.

Today there are modern water and air-infiltration barriers that builders can apply over the OSB wall sheathing. These products are somewhat cumbersome to apply and require tape at the seams where the wrap overlaps itself.

You can purchase wall and roof sheathing that eliminates the modern wraps. This OSB sheathing comes from the factory with a plastic coating that repels water. You tape the seams while the wall is being constructed flat on the ground. This new sheathing material can save money and time because you effectively skip a step in the current building process.

This new sheathing material may become the new gold standard in the progression of wall sheathings because of its innovation. Builders are constantly looking for products that save time and money, but not at the expense of quality. The jury is still out on the true lifespan of this new product, but if the coating doesn't delaminate and the tape adhesive can last 100 years or more, it could be a real winner.

As Kenny Chesney said in one of his songs, "Only time will tell, but it ain't talkin'." Our kids or grandkids will discover the results.

Water is indeed the enemy of wood framing. It's imperative that you or your builder make sure that the wood can stay dry or dry out rapidly if water somehow gets behind the exterior finished surface of your home.

The courthouses of America are filled with lawsuits about rotting homes of all types. The science and skills required to keep homes dry seems to escape many rookie builders. This is indeed a problem in the industry as there doesn't seem to be any formal education system in place to pass down the time-tested secrets and methods that are known to keep exterior walls dry.

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2 Responses to Sheathing

  1. We are going to rebuild our home after hurricane Sandy. I have read before you stated that plywood was stronger than OSB. Is that still the case or wood you go with the new osb that is out?

    • Bill, OSB is plenty strong. It's the new standard. The lumber companies are phasing out plywood as a sheathing product. Heck, they phased it out 10 years ago as OSB is a better use of resources and it's a good structural material.

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