Does putting plywood over studs make it a shear wall?

mudwormOctober 22, 2011

We are remodeling our old house (built in the 40's) and have opened up one side of a long wall in the house. The framing we saw in it (see my old post) was boarder line shoddy. We've since reinforced the heck out of it by adding lots of studs and blocking.

There are two big openings in this wall, one leading to the kitchen and one to the family room. We had seen cracks in the corners of the openings before we took the sheetrock down. To prevent that in the future, besides fixing a sagging foundation pier, we also decided to close the wall up by putting 1/2" plywood sheathing first and then 1/2" sheetrock.

My question is, does adding plywood sheathing on one side make this wall a shear wall? The reason I ask is there are many requirements around building a shear wall (PDF reference). At the minimum, we need to double up studs at every panel edge --every 4 ft -- and add a whole lot more horizontal blocking for the same because the wall is taller than 8 ft. To be honest, we are a bit blocked out at this point.

Okay, deep down, I know we need to/should add all the blocking. For horizontal blocking, can we add it at the same height (e.g. four feet from the roof) all the way across? In other words, is it okay that we have one horizontal seam at the same level along the entire wall? I did not see in that linked PDF about staggering panel seams (like how it's suggested for drywall installation).

Current state of the wall:

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This is how I have in mind to lay out the plywood (from 4'x8' panels). Will make sure there are double studs or flat block backing at every seam.

A 12 inch grid is overlaid on the wall. The brown lines are the edges of panels. Please kindly critique. Thank you!

    Bookmark   October 22, 2011 at 12:02PM
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Oh, just to clarify, the grid is only for illustration purposes; it does not represent any physical framing.

    Bookmark   October 22, 2011 at 12:06PM
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To create a shear wall the type of wood in the framing, as well as the plywood and a nailing schedule, must be designed and followed.

Simply adding plywood will likely increase the shear strength of the wall (compared to drywall) but by how much is not so easily determined.

One of the problems that shows up in older structures is that the wood may not have been graded. This creates uncertainty in the strength of the wood itself and the fasteners driven into the wood.

A common fall back it to use the lowest strength for the wood and the fasteners, but some AHJs are not comfortable with this approach.

    Bookmark   October 22, 2011 at 12:52PM
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For us, we just thought plywood would be an improvement. But I don't know if by adding plywood, we are automatically required to follow all the shear wall requirements (including bringing up to standard the grade of lumber).

Do you think my layout looks okay (i.e. not staggering seams)? I was going to cut the blocks today so tomorrow DH can do the nailing. My wiring is on hold because I need to make sure all the blocking is in place.

    Bookmark   October 22, 2011 at 1:13PM
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You are likely grandfathered to the existing building codes.

In some seismic areas there have been rq1uirments to upgrade though.

Major remodeling can also trigger upgrade requirements.

cal the AHJ and talk to them (especially if you are4 in an earthquake area).

For the mot part simply adding the plywood should not trigger much.

    Bookmark   October 22, 2011 at 3:28PM
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The link below is a fairly concise explanation of what a shear wall is for and how to build one.

Here is a link that might be useful: What is a shear wall?

    Bookmark   October 22, 2011 at 8:12PM
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Thanks handymac. Actually, I found that wiki page very brief. But I got what you were trying to say (or I think).

Brickeyee, thanks for your continuous and tireless mentorship for novices like me. We just wanted to improve the integrity of the structure and were really not going for a full on shear wall. I sure hope the inspector will not assume the latter. Regardless, we did our best to satisfy a shear wall requirements -- we've doubled up studs and provided backing wherever two panels may meet. And we'll follow the nailing schedule as well. The wall looks so much stronger and cleaner than before, which makes us proud.

    Bookmark   October 24, 2011 at 4:24PM
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I sheared a wall in my house and the AHJ actually thought I was nuts to have gone through the exercise, let alone start down the road of real, engineered shear wall construction. BUT, if shear walls are required where you are, a remodel job may trigger shear wall requirements. I've built a shear wall in a house that was sheathed on only one side, per engineering specs. Tie downs are usually required too, BTW. As far as staggering panels, Thor Matteson, in his book, Wood framed Shear Wall Construction, say that a wall with panels lined up might be about 25% weaker but he also says that he'd rather see a good nailing schedule followed than staggered panels. If your joints are staggered, you will be building a superior shear wall. (Chap 3. p. 69.)

    Bookmark   October 26, 2011 at 9:35PM
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Hi Bill, when I mentioned to an inspector that we are putting plywood over the studs, he did not say that make sure it's built to shear wall standard. But of course, it all depends on the field inspector. We've done plenty of blocking and stud doubling. Will deal with it if the field inspector makes more requirements.

With 15/32 OSB panels, the current plan is we will use 10d common 2 1/4 inch length nails, 3 inch spacing on edges and 4 in field. That'll be lots of nailing.

    Bookmark   October 27, 2011 at 7:21PM
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Yea, lots of nailing. Get a palm nailer. There are requirements about how far in the nail head should be set too.

I'd make absolutely sure that an engineered shear wall is not required by your AHJ or you'll just have to tear it all out. If you're just doing this for fun and your own peace of mind then I suppose you can do whatever you want and the result will still be a stronger house. Those OSB panels you mention, are they rated for shear, struct 1? In my area now, 3/4" struct plywood is required. Your blocking at panel edges should be at least 3" wide - I used 4x4's for mine. You might pre-drill for those 10d's, which need to be 7/8" from the edge I think.

There's lots of rules and your local AJH may have their own, so, like I say, if a shear wall is required, make sure you have all the rules - you might even be required to have an engineer's stamp.

    Bookmark   October 27, 2011 at 8:30PM
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A shear wall takes a horizontal load (wind or siesmic) from the floor above and transfers it to the floor below and through that floor plane into the foundations, so it is important that the wall be properly attached to both floors and the floor framing be blocked.

The narrow plywood panels adjacent to the openings will not provide much lateral resistance. I would put three 4x8 (or 4x9) panels vertically in the middle area and nail them at 4" o.c. at the perimeter and 8" o.c. at the intermediate framing. The rest of the drywall could be shimmed or have a double layer; there's no reason to waste plywood. The top and bottom plates at the panels should be lag screwed into solid blocking in the floor framing.

    Bookmark   November 2, 2011 at 5:07PM
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In my experience, the only time an interior shear wall would be required in a private residence is when the exterior walls along a side of the house has too many openings (not enough braced wall to meet the building code requirement) or a second floor exterior wall is not aligned over the first floor exterior wall that sits on the foundation (discontinuous load path from upper walls and roof to foundation).

When any of those conditions has occurred, an engineer was required to design the shear wall.

I can tell you for sure that an engineer would not approve of the proposed panel layout. Rather than using such excessive nailing you should arrange the panels to create the fewest panel joints. The more joints, the longer the nailed edges and the weaker the shear wall.

    Bookmark   November 7, 2011 at 4:02PM
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"In my experience, the only time an interior shear wall would be required in a private residence is when the exterior walls along a side of the house has too many openings..."

Houses in earthquake zones often require interior shear walls, together with a tighter sill plate anchoring schedule.

    Bookmark   November 8, 2011 at 9:04AM
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Since an engineer is not required for this renovation I had assumed the house was not located in an earthquake zone.

    Bookmark   November 8, 2011 at 9:49AM
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Staggering of horizontal panels is for strength perpendicular to the wall. This wall is being designed for strength parallel to the wall and therefore the weak point is the nailing so decreasing the length of joints is the goal.

If you simplify the panel layout as shown on the sketch, it will reduce the length of nailed edges by a third which will greatly increase the resistance to lateral forces for less panel cost. The small panels will add little or nothing to the resistance so an added layer of drywall is fine.

The nailing doesn't have to be 10d. 8d @ 3" o.c. along edges and 8" o.c. at intermediate supports is fine. 4" o.c. and 12" o.c. would probably be more than enough as well.

    Bookmark   November 9, 2011 at 9:47AM
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