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Plywood bowing?

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Forum topic by ToddJB posted 526 days ago 941 views 0 times favorited 27 replies Add to Favorites Watch
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ToddJB

846 posts in 636 days


526 days ago

This might be a bit of an unorthodox question, but I need to hang a piece of drywall on a ceiling. And I have a 29in x 8ft gap I need to cover. I have no room for cross braces so my idea was to cover the gap with a piece of plywood and attach the drywall to it. My question is, since every 10th of an inch counts here what is the thinnest thickness of ply I could go without any sagging? Bonus question, does the orientation of the plywood change its flex strength?

Any info is greatly appreciated.

-- I came - I sawed - I over-built


27 replies so far

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bondogaposis

2239 posts in 857 days


#1 posted 526 days ago

I am having a great deal of difficulty envisioning your problem from the description. Got a picture? Why not attach the sheet rock directly to the ceiling joists? Why would you need plywood?

-- Bondo Gaposis

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ToddJB

846 posts in 636 days


#2 posted 526 days ago

It’s pretty difficult to explain. I’m finishing my basement. This section I am talking about is to cover my trunk line. The trunk line butts directly up against the main support beam of the house. Directly under that beam is where multiple doorways will go. So I need every bit of space I can get. I do not have room to put a braces under the trunk line so I was planning on affixing the plywood under the trunk line by using L brakets on the support beam side and then to the framing on the other side of the trunkline. Then attaching the drywall to that.

I am under the impression that 29” is too far of a gap for drywall by itself. Also, I do not think attaching the drywall directly to L brackets would be a strong hold.

-- I came - I sawed - I over-built

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derosa

1482 posts in 1342 days


#3 posted 525 days ago

I’d just put up the ply at the same thickness as the rest or the sheetrock and plaster as normal. You could spend more time feathering the joint to eliminate the effect of the piece hanging lower then you may on smoothing the ply with filler to match if you are painting it.

-- --Rev. Russ in NY-- A posse ad esse

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ToddJB

846 posts in 636 days


#4 posted 525 days ago

No feathering would be needed. The plywood covers the total area at that height.

My question really isn’t how to do this, but more what’s the thinnest product I can get away with without sag. So it would need to support itself and the weight of the drywall.

-- I came - I sawed - I over-built

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404 - Not Found

2544 posts in 1475 days


#5 posted 525 days ago

Is there space to either put a 29” x 8’ piece of 3/4” ply pocket screwed so it’s flush to the two joists that are there? or could you add in another joist to reduce the width?

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ToddJB

846 posts in 636 days


#6 posted 525 days ago

Cannot add any joists or braces, the trunk line takes up the whole space.

-- I came - I sawed - I over-built

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404 - Not Found

2544 posts in 1475 days


#7 posted 525 days ago

Can you post a picture? It would help.

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Rex B

304 posts in 756 days


#8 posted 525 days ago

I can’t help with your problem, but I can answer the bonus question. It depends on the number of plies in your plywood, but in general plywood is stiffer parallel to the grain (of the veneer plies) than perpendicular to the grain. As the number of plies is increased, the stiffnesses parallel and perpendicular to the grain become more equal.

So the conclusion: while it is true that plywood has a higher ratio of cross-grain strength to along-grain strength than solid wood, it is still stiffest along the grain of the veneer plies.

There are some great articles from the US Forest service regarding this, just google it.

-- Rex

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Mark Smith

466 posts in 546 days


#9 posted 525 days ago

You could add stiffiners to the plywood. For example, maybe attach a couple of lengths of angle iron or steel to the top of the plywood that would help prevent it from sagging over time.

-- Mark Smith, Tracy, CA., http://www.markscustomwoodcrafts.com

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FirehouseWoodworking

619 posts in 1779 days


#10 posted 525 days ago

Todd,

Is there any way to glue the drywall up to the main bearing beam with construction adhesive and bypass the plywood all together? This would also save you 3/4” of headroom for the doors.

Short of going with some very expensive 3/4-inch Baltic birch, Finn Form, etc., i don’t think you will get away from the sagging issue.

Another alternative to consider would be to just use a hardwood plywood to finish off the opening and fore go the drywall. You could fasten it up with construction adhesive and self-tapping screws to the beam and perhaps some well positioned blocking. Finish it to match the room’s trim. Just a thought. Good luck.

Cheers!

-- Dave; Lansing, Kansas

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oldnovice

3280 posts in 1874 days


#11 posted 525 days ago

Use MDO since it takes paint very well and can almost look like sheet rock.
To stop any bowing how about gluing some stiffeners to the back side of the MDO?

-- "I never met a board I didn't like!"

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ToddJB

846 posts in 636 days


#12 posted 525 days ago

Thanks all, I will see if I can get some pics up a little later

-- I came - I sawed - I over-built

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ToddJB

846 posts in 636 days


#13 posted 510 days ago

Sorry it took forever to get this pic up. So here is what I’m dealing with.

-- I came - I sawed - I over-built

View DS's profile

DS

2086 posts in 926 days


#14 posted 510 days ago

This may not be the correct way, but I’ve seen drywall guys screw the sheetrock directly into the metal ductwork with no framing whatsoever.

-- "Hard work is not defined by the difficulty of the task as much as a person's desire to perform it.", DS251

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ToddJB

846 posts in 636 days


#15 posted 510 days ago

Hmmm… That is interesting. I am not confident though that it would hold.

-- I came - I sawed - I over-built

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