We’re not sure how many times we’ve written these words, but they are worth repeating: water is the enemy of wood. Today’s piece is about that very subject once again, this time as it relates to plywood.
In the wild, trees have bark to protect the heartwood, the wood that is in the center of the tree, the wood we look for at the lumber yard. Once the tree has been felled, that protection is gone, and the wood is at the mercy of the elements and inattentive woodworkers who neglect to care for their investment.
Warping, cupping, and rot are the dangers we face when plywood gets wet, and if we are not careful, one or more will ensue. But how long does that take? How much water or moisture will have to insinuate themselves into the plywood before that risk is realized? Is there no cure? If there is, what is it?
What is Plywood?
First, a few basics to understand how and where water can adversely affect plywoods will be helpful. If there is a risk, it’s good to know what it is and where it will occur.
Plywood, as we know, is a manufactured wood product. Layers of it, which we call veneer, are glued together to a dimension, with ¾ “ being the most common. When those layers, or plies, are stacked on each other, every piece is rotated 90 degrees to create cross-grains as the stack grows.
Glue holds them to each other, and the alternating grains produce strength in the stacking in all directions, countering the effects of expansion and contraction from an ever-changing environment of hot/cold and damp/dry. It also lessens splitting when nailed.
Please notice we said layers of wood. Plywood is real wood, usually softwoods, with layers in the core of the sheets of lesser quality and grade than the top layer, the one you will see. Sometimes birch, sometimes oak, the higher grades of plywood sheets, which customarily come in 4’ x 8’ sheets, top (visible) layers make plywoods suitable for projects that will be seen.
Lesser grade plywoods, with defects and flaws in appearance, work well for projects that won’t be seen – subflooring, siding in new construction, and such. They will be structurally sound in these uses but won’t be pretty to see. Prices of plywoods vary accordingly, with higher grades having no flaws, being well sanded, with a birch or oak top ply being the most expensive, and those with many visible flaws and knots, and unsanded, being the least expensive.
Even with those price variants, plywoods are an economical alternative to solid woods. The method of manufacturing with alternating cross-grains produces a strong product. The strength-to-weight ratio produces wood products that are resistant to impact damage and can withstand the expansion/contraction dynamic caused by heat and cold, giving you a wood product that serves well in structural uses like subflooring in the home. The lack of chemical reactivity makes plywood perfect for such uses as concrete forms when pouring building foundations, too.
In short, plywood is a pretty cool manufactured wood product that has a wide variety of uses in the home woodworking shop that, run from cabinetry work to kids’ furniture to shelving (those projects that will be seen), and to new flooring underlayment and the exterior of newly framed walls for home additions, and so much more.
Our most recent piece on plywood used edge banding to hide the plies to give your plywood project the look of solid wood (at substantial savings). You’ll find that piece here.
Is Plywood Ruined When It Gets Wet?
It’s not the wicked witch of the west and does not melt when it gets wet. Yet, plywood is real wood, and so water will cause damage. There’s a “but” that goes with that because water doesn’t need to be fatal to plywood.
Water is not acid, and the damage it can cause is not immediate. Actually, water on plywood can take more than a year to do its worst, so rot sets in, and its structural integrity is damaged beyond repair.
The worst can be avoided if standing water is removed quickly and the plywood is wiped down.
How To Save Wet Wood
Beyond that, here are other steps you can take depending on the extent of the water volume that has affected the plywood sheet(s):
- Excess water. Simply wipe it off. Nothing fancy here or involved – just use a cloth or towel to wipe it off and not let it stand.
- A dry space. This is pretty basic, as well. Move the sheet or sheets to a dry space. Enhancing that space with a dehumidifier will be helpful, and creating a good air flow by opening the windows or doors will also help.
- Support. Airflow will only help if it can circulate completely over, under, and around the plywood sheets. So, offer it some support by using a couple of 2 x 4s to separate the sheets if there is more than just one; cardboard boxes can also help if you don’t have any 2 x 4s.
- Time and patience. Give the drying process time to do its good work. Patience will pay dividends, and by time and patience, we suggest a couple of days, at the least, and maybe 4-5 days, even. Patience will pay off in savings since you may well be able to salvage the pieces.
For minor water exposure, these steps can cure and save the patient. But what if the exposure to water is more substantial?
How To Tell If Your Plywood Is Ruined
Water will find easier entry into plywood at the edges rather than through the top or bottom ply, which is pretty easy to understand. The edge has been cut, and the pores are open and exposed, allowing the water to be wicked into the plies. Glues used in the manufacture of plywood are mostly waterproof, but the wood pores are the open door.
So, damage to plywood will be first noticed on the edges. You’ll notice swelling and discoloration in the plies and along the edges. This is a sign that not only has the water entered but that it has also been there for a while. As we noted, it takes time for rot to be caused in response to water presence.
All may not be lost, though. Depending on how deeply the water has been pulled, edges can be trimmed, whether by your table saw or circular saw and removing a few inches of edge to examine how deeply the water has seeped in can give you a clear idea of how much can be saved and used.
If, after removing an edge, you find only moisture but no swelling or rot, follow the drying steps we’ve outlined earlier, and you will have saved a sheet minus only a few inches on an edge or side of the 4’ x 8’ sheet.
The color and swelling will indicate the unusable portion of a plywood sheet and the deterioration of the wood fiber in the plies. Exposure to water can be dangerous to plywood, but it doesn’t necessarily mean it has been ruined.
Save what you can and find uses for it on another project if you need the full dimension of a sheet on the current one. There will always be projects that need something less than full sheets – jigs, workbenches, shelving, cabinets for the kitchen or workshop, desks, and more.
Is Warping Fatal to Plywood When It Gets Wet?
We’ve already offered thoughts and tips on saving wet plywood, and now we come to possibly even saving plywood that has been warped by water.
Warping occurs when water affects one side of the plywood sheet more than the other side. Water weakens the fibers and makes them more pliable, and the wetter site will bow out in the warp. The side that has been less affected by the water will not be as pliable, and so that side will “cup.” To use fancier words, the wetter side will be the convex side, bowing outward; the less wet side will be the concave side and will cup inward.
Not all is lost, though, so do not discard warped plywood.
Dry it first. Follow the steps noted earlier and give the sheet a good drying out. Time and patience will serve you well.
Nature provides. Spray warm water on the concave side to make it more pliable. Lay the plywood sheet concave side down on a concrete patio or in your driveway, some surface outdoors that is flat and solid. Let the sun do its thing to shine on and warm the plywood, and the concave side’s pliability will allow the sheet to flatten and be dried by the sun’s rays. A day or two later, the plywood will have flattened and be usable. Yes, we know that the sun’s UV rays can be harmful to wood, but not in just a couple of days. There’s no need to worry about that.
Weight helps, too. Perhaps you don’t have that flat surface in the sunshine at your home. No worries. Warm water on the concave side to make it more pliable, weight added to the convex (humped) top to apply pressure downward, and the same thing can be achieved. As the warm water applied dries out, the concave side will once again become firm, and the sheet will be straightened.
We’re sure you can devise alternatives to this process based on what is handy in your shop. Perhaps the use of clamps, rather than a weighty object, can help flatten the plywood sheet, something that has occurred to us, also.
Just in case you think this is just fluff to fill a page with words, we found a lot of support in videos, as you might imagine. This first one is very simple and straightforward in addressing warping in a plywood sheet using just the sun and the warmth of a concrete patio beneath.
The 2-minute tip guy uses a combination of water, clamps, and weight to achieve the same save of his plywood sheet.
As you have read and now can see, saving wet plywood is neither difficult nor complicated. The greatest cost to you will be your time and your patience.
Warped plywood is not useless plywood. Don’t be discouraged. The effort is worth it, saving you money and keeping your plywood inventory healthy.