Warped Plywood

[James Jayko]

I am building a couple of vanities for my bathroom.

Went to the HD, bought a couple of sheets of birch ply, broke them down, put the first vanity together. I almost never work with plywood...apparently the plywood was slightly warped; just enough to make the top edge not line up exactly right. I'd never considered plywood warping; is there a fix for warped plywood? Are some varieties less prone to this than others? Do I need 5 x 5 sheets of European birch? Any guidance to picking plywood? I'm assuming, like I've learned with every other item HD sells, you can get a much better product for the same or less money elsewhere?

[Cary Falk]

I have not had any luck with trying to un-warp plywood. Unfortunately plywood does not fall into "you can get much better product for the same or less elsewhere". I bought some more expensive plywood at a hardwood dealer and it was 10 times worse than anything I bought at Lowes and HD . I have come to the conclusion that you are not going to get quality until you hit close to $100 per 4x8 sheet. I feel plywood is just not worth the hassle anymore.

[Art Mann]

I have noticed that stock plywood from Home Depot and Lowes is often substandard. I seldom buy there any more.

[glenn bradley]

Joinery and construction methods will generally tame wonky ply if things are not too extreme. Once assembled the irregularities really shouldn't be sufficient to force a carcass out of square. If an irregular piece of plywood is used as a reference surface one could certainly build out of square. This opens up all sorts of conversation as to what is 'flat' and what is 'square'. A commercial linen cabinet with 1/8" reveals around drawers and overlay doors can run pretty loose. A humidor with near air tight fixtures, not so much and there is a wide range in between.

I just assume the only 'true' material are the parts I mill myself. I reference any sheet goods that may be involved against that. Although there are certainly methods that use sheet goods as the structural portion of casework (Sommerfeld-type methods), I reserve this for shop fixtures. When I use plywood it is typically for drawer boxes and bottoms (where I use joinery that is self-squaring) or as a 'skin' over, or captured in, a frame. Now having gone and said all that I realize I can't recall the last time I used ply that wasn't Baltic Birch or a close cousin.

Someone who uses sheet goods regularly will probably chime in with some tips on methods. You see some really beautiful work with veneered ply in high end kitchens and baths. It is a skill set I do not posses but, do envy ;-)

[Carlos Alvarez]

Any guidance to picking plywood?

Never buy it at Home Depot. I mean, they sell construction material, not woodworking material. I use their ply for simple/functional things that don't have to look good.

[Lisa Starr]

The "quality" of plywood varies tremendously. Sometimes I can get better ply at one of my local big box stores and sometimes from a "better" lumber yard. If you are building carcases, you can design it to push/pull the plywood parts into submission. I usually break down my sheets for base cabinets with a cut at approx. 24" with a circular saw and known straight edge. If needed, I then cut across those pcs leaving trim stock. Everything is then referenced off that 1st cut for square, straight, parallel cuts on my table saw with the sled. For warping issues,I first clamp the piece in place and make sure you can push/pull the bow out. If so, I then assemble with glue and pocket screws on my flat bench.

[Lee Schierer]

I recently purchased a 4' x 8' sheet of 3/4" birch faced plywood from Home Depot. It was flat when I picked it up at the store and remained flat. However, I cut my pieces almost as soon as I got it home and treated the pieces the sames as I would treat a glued up panel. I made sure that both sides were open to air circulation. None of the pieces were stacked up face to face. I applied equal amounts of finish to all surfaces.

I don't think that modern plywood comes to the stores as dry as the older plywood used to come. If I remember, I will take my moisture meter with me the next time I need plywood and see what the moisture content is near the center of the sheet as compared to the edges.

[Carlos Alvarez]

Quality varies, yes... You make a good point, and maybe some people don't understand plywood gradings. It's complicated, more than you'd think. It's worth an hour of studying to understand it, if you plan to build important pieces with it. I almost never use it for anything visible, but it's a great substrate for certain designs where I use thin pieces of exotics for their look, but not enough to be structural. Not laminates, but maybe 1/8-3/8" thick. So the ply still has to be straight and more importantly, STABLE. I have some very ancient pieces of high-end maple ply that have never warped despite being abused in my general sheet goods pile, and even once having been rained on. I also have cheaper ply that warps in a month or less of just sitting there.

[John TenEyck]

I've bought plenty of acceptable hardwood plywood at HD, and Lowes. It's not consistent, that's the problem, but most of the time it's better than the stock plywood carried my local full service lumber yard. Their birch comes from China and it is absolute junk. HD and Lowes often have Columbia Purebond or some other brand from North America.

Anyway, if it's flat in the store it should stay reasonably flat when you get it home. That's not always true, but mostly, so make sure the sheets you buy are flat. I've bought quite a lot of Baltic birch plywood, too. While it's usually more consistent, especially internally, I've had a few sheets of that twist, too. Even high end plywood at $150 doesn't always stay dead flat. It's a real crap shoot anymore, but I can't say I've ever had more than a couple of sheets that twisted so badly that I couldn't use them for something. If it's flat enough that you can cut it square then the assembly process should pull it square in all directions.

John

[Carlos Alvarez]

I think a lot of the post-purchase warping may have to do with location. I'm in AZ, and most commercial wood seems to arrive here overly humid. Construction grade stuff from HD/Lowe's is so wet that it often clogs up Forstner bits. So as it acclimates to our dry environment, it warps.

[Edwin Santos]

Quality varies, yes... You make a good point, and maybe some people don't understand plywood gradings. It's complicated, more than you'd think. It's worth an hour of studying to understand it, if you plan to build important pieces with it.

I think this is very true. Not only are the gradings and codes a little complicated but some wood dealers use different nomenclature than others, and then there are always new products showing up on the scene that create new options. One such example from my supplier is what they call Gold Ply which is a true plywood product, free of voids, with a thin layer of MDF sandwiched in the middle. It's good stuff because you have the structural integrity of the cross layers of wood, and the MDF encourages it all to stay flat. In most domestic A grade veneer facings, it runs me about $95-105/sheet. As a substrate, I still find true Baltic Birch hard to beat in terms of price and quality if you have no objection to 5x5 sheets and the metric thicknesses. In my neighborhood, 18mm (3/4") runs about $30, 1/2" about $18 and 1/4" $10 I recall. Depending on how you store it, Baltic Birch will warp too but like Glenn points out, usually the construction of your project will resolve it.

Is there any way to pull the warp out of your project with a structural member like a rail placed where you need it?
Edwin

[Carlos Alvarez]

Wow, Gold Ply sounds so useful. In addition to flatness, MDF has excellent vibration/sound damping. I've done a ply-MDF-ply sandwich for things that I want to be solid and dead as far as vibration or sound.

[John TenEyck]

I think a lot of the post-purchase warping may have to do with location. I'm in AZ, and most commercial wood seems to arrive here overly humid. Construction grade stuff from HD/Lowe's is so wet that it often clogs up Forstner bits. So as it acclimates to our dry environment, it warps.

That could be true, but there is a big difference in specs. between construction grade wood and hardwood plywood. I've never found a wet sheet of hardwood plywood at HD, Lowes, or anywhere else. The stuff at HD and Lowes is indoors and always dry, at least where I live in NY. I'm not saying their stuff is always good; far from it. I've walked away many times after looking at what they have to offer. Of the two big box stores in my area, Lowe's is consistently better in the past year or two, broader selection and in stock, and better quality.

John

[Edwin Santos]

Wow, Gold Ply sounds so useful. In addition to flatness, MDF has excellent vibration/sound damping. I've done a ply-MDF-ply sandwich for things that I want to be solid and dead as far as vibration or sound.

Carlos, you'll find GoldPly at the Phoenix branch of Peterman Lumber. They also stock a decent selection of straight MDF core sheet goods in various veneers but probably not in the thickness you created in the sandwich glue up you're describing.

[Jim Morgan]

... As a substrate, I still find true Baltic Birch hard to beat in terms of price and quality if you have no objection to 5x5 sheets and the metric thicknesses. In my neighborhood, 18mm (3/4") runs about $30, 1/2" about $18 and 1/4" $10 I recall...
Edwin

Wow, I wish I lived in your neighborhood. Around here, a 5x5 sheet of 18mm B/BB runs north of $60.

[Cary Falk]

Wow, I wish I lived in your neighborhood. Around here, a 5x5 sheet of 18mm B/BB runs north of $60.


Same here.

[Edwin Santos]

Well the price does jump around a bit, kind of like the stock market so it can depend on the day you're buying it.
But your point stands because I've never seen 18mm over $40 around here. In fact, the last price I was quoted last year for 5x5 18mm clearcoated was $45. I think the grade is B/BB but I'm not 100%.
I sometimes wonder if these prices might go way up with all the talk of tariffs and trade wars. I suppose we'll see.
Edwin

[John TenEyck]

Holy cow, that's cheap. I paid nearly $80/sheet for 18 mm, after shipping and tax. Last time it was about $65. As you said, it varies quite a lot, but never as low as you are able to get it, even if I bought a full pallet.

John

[Ben Zara]

Look for a place that stores the plywood properly.

A lot of places store the sheets with two or three supports perpendicular to the length of the sheet. That may be fine for solid wood but inevitably the sheets toward the bottom of the stack will have dips where they are not supported because the weight of the sheets is forcing the plywood to flex below into the huge gaps between the supports.

Lumber yards do this to make loading with a forklift easy but it screws up the plywood.

Another option which is time consuming and annoying would be to glue together 3/8 plywood that has opposite bow to straighten the sheets. I've done that in small pieces and it sort of works.