Dimensional Stability of Plywood

[Jeff]

Plywood is supposed to be dimensionally stable, but for general construction use the manufacturers recommend using "H" clamps on the edges and ends of 4' x 8' sheets (like for roofing) in order to allow for expansion. :no:

I'm going to use some birch/maple plywood for a floating frame & panel construct, but I'm wondering about this issue. How important is it? The panels will be 1/2 ply 44"l x 17"w. I don't need to make a tight fit, but I'm wondering about this dimensional stability question for future reference. I have no problem with cutting the groove's width and depth a little bit oversized to allow for expansion. :BangHead:

Thanks for your advice and suggestions folks.

 

[DaveO]

The basic design of Plywood, overlapping layers of veneer with the grain alternated between the layers is supposed to give it great dimensional stability. I generally don't consider plywood to expand/contract. Although I have found it to warp (but that was low grade plywood).
MDF core plywood would probably be the most stable compared to veneer core, but the difference between the two would be so minute.
I have always glued my ply panels in place and have never had any issues with stability. A plus is that also greatly reinforces the door strength allowing less complicated frame joinery.
MTCW,
Dave

 

[Bigdog72]

I believe the "H" clips are an answer to using1/2" plywood or 7/16" OSB fastened to 24" OC trusses. The idea is to place the H clip midway between the open area to add some stability. But the stability is when someone walks across the roof right along those joints and steps between the trusses. The reason for spacing sheet goods in rough construction is for swelling if the material gets wet before it is covered. If your build up will be subject to the elements, take precautions.

 

[Gotcha6]

Geoff is right about the 'H' clips for sheathing & OSB. Codes now require the clips or pulins along the joints. 1/2" Sheathing and 7/16" OSB are both subject to warpage along spans greater than 16". This could lead to a buckled joint showing through the shingles of a roof - or worse - a tearing of the shingles. The old rule of thumb for plywood used to be it should withstand 7 wettings without a delamination. Most homes today are built so fast there's not much chance of the sheathing getting wet. Marine & B-C plywoods are much more durable. They usually have more plies of a more dimensionally stable hardwood core and are less subject to warpage and expasion for this reason.
So if ya doin' a cabinet project, fugeddaboudit. :gar-La;

 

[Gofor]

H clips are also a factor in high wind areas, such as hurricane prone coastal areas, where the shingles get blown off and the plywood wet, but you do not want the wind (especially the vacuum on the down-wind side) tearing off a piece of ply, which starts the whole roof disassembly and ultimately house collapse. H-clips provide additional edge stiffening and support from surrounding panels in these situations.

Bottom line, its a construction/liability issue, not a dimensional issue.

Go

 

[FredP]

plywood is stable within its range. the clips are mainly for strength between the rafters but they are also made to create a gap for expansion. on a roof the plywood is subjected to extremes beyond its stability range. temperatures in the attic can go from well over 110* to well below freezing and humidity changes from 100% to below 40%. even plywood is going to move under these extremes. for the projects we do plywood is very stable. I allow 1/16" or there abouts in frame and panel projects, boxes, ext... never had an issue with it moving enough to matter. I have seen roofs that did not have the clips get rained on before the shingles were applied and the plywood popped the nails up and some were pulled through plywood.after it dried it had to be renailed. it should have been pulled up and redone with the clips. this was before the clips were required. if you get a leak in your roof and do not have clips you could have damage like this occur. It would depend on the severity of the leak of coarse and the length of time and how large an area was wet.

 

[Mike Davis]

I would make allowance for the frame shrinking rather than the plywood.

 

[skysharks]

Jeff, there is a lot of good and correct advice to your question already posted here.
I have to scratch my head:icon_scra and wonder why you would ask about using "H" clips in cabinet construction.
Are your "floating frame and panel construction" project going to be that big? Wow.:eusa_thin
I have built, oh, a few doors, side panels, etc. using this technique and never had a problem.
As other have stated, there is a very negligible dimensional shifting with plywood. It is so small that most of the time it is not even addressed:dontknow:

 

[jerrye]

The ONLY time I've had issues with ply moving is when I made a single cutout door 1 1/2" x 23" out of 1/2" Sandeply for someone that requested the door be made that way, and from that material which they chose. I didn't know better at the time; they didn't know at all; and it warped.

I won't make that mistake again.

 

[Mike Davis]

I made kitchen cabinets with plywood doors for a house we had in Georgia. I used AC plywood and dadoed the edges all around for a slight inset. They are probably still there and that was 20 years ago. Only one door had a very slight warp, really not enough to notice.

I know plywood was better then but really, set in a frame I doubt there will be any issues.

 

[Jeff]

Thanks for your input and advice folks. Seems like our general woodworking projects are ok with plywood and basically no expansion/contraction issues. However, general construction like home building (read roofing) requires the "H" clips for stability/expansion/code reasons.

Two solutions for the same product depending upon the application. ccasion1

 

[JimD]

Homes do use the clips in roofs but in the floor construction, it is normal to glue the plywood to each other and the floor framing with construction adhesive (to prevent squeaks).

For furniture purposes, I believe it is safe to assume MDF, waferboard, and plywood do not move. In the extreme, that is undoubtedly untrue but for typical furniture applications, I have found it to be reliable.

I also glue plywood panels to the frame sometimes. I do it for strength. The only reason I do not always do it is I get glue on the surface of the panel that shows sometimes when I do this. I do it for big doors and not for little.

Jim

 

[Alan in Little Washington]

One more note about "H" clips. Rather than account so much for dimensional stability (expansion and contraction in length, width, thickness) the main reasons they are used are to maintain edge alignment and rigidity where the panels meet should one or both warp like Fred, Gofor, Gotcha6 and others have said. The "H" clips keep the panels aligned should one warp and cause it to out of alignment with the adjoining panel- I've seen this happen on roofs where it results in a noticeable bulge in the shingles. In severe cases it prevents the shingles from laying flat or causes them to separate from the decking. You don't see "H" clips being used with flooring because, in general, T&G panels are used and/or joints must on joists.

I don't know which came first- the requirement to use "H" clips, or an industry desire to use them, but in addition to being code in many circumstances, they allow you to butt joint ply/OSB panels so the joint is other than right on a rafter and the edges are "floating" between the framing- this reduces the need to cut partial panels and reduces waste.