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What is Warping in Plywood?

Despite the high strength in plywood, it could bend or twist and thus become difficult to perform its purpose if wrongly handled. Warping in plywood is the deformity that occurs when the moisture contents of various wood areas change unequally.

It is the uneven ending or twisting due to stresses or unequal shrinkage. Warping can also occur in dry plywood when it intermittently takes up moisture or allows it to its equilibrium state unevenly, either too quickly or too slowly.

Types of Plywood Warping and Causes
Knowing the types of plywood warping causes is the first step in preventing warping in plywood. There are two types of warping in plywood; twisting and cupping.

Twisting is the type of warping where one corner is out of the plane of the other three. When put on a flush surface, three corners of the panel can comfortably rest on it, but the fourth appears to be off the surface when the other three are touching.

Twisting occurs where the plywood acts like a sponge, enlarging as it takes in more water when placed in a high humidity area. Later it contracts and stiffens as it dries out while adjusting to reduced humidity, and this causes twisting and warping to great extents.

Cupping happens when the plywood end closest to moisture absorbs too much of it, causing it to expand than the rest of the material, and this causes unevenness and warping in the plywood.

Cupping is where when a panel is put on a flush surface, the four corners touch the flat surface, but the middle part of the panel is lifted from the flush surface.

Twisting and cupping in plywood are caused by varied factors and should be examined differently, even though sometimes they may co-occur.

How to Prevent Warping in Plywood
Various factors affect warping in plywood, and some are simpler to deal with than others. Distinct wood warping types also appear to be caused by distinct factors.

Moisture Content
The amount of moisture in various plies should be acclimated just before gluing to ensure the standard moisture content when the glue sets are almost uniform to the standard moisture content the stock may be anticipated to attain in service.

The stresses developed in plywood vary in the amount of moisture from the current one when the glue sets. Therefore, the less the extent of these conversions, the less the strains, the less the tendency of warping.

If you are a manufacturer working with plywood, you should closely monitor the relative humidity of your production and storage areas and the plywood material’s moisture content.

Plywood Grain and Sawing Methods
All plywood materials are made of wood fiber sheets bonded with resin to form a composite material. It is a shift of the grain referred to as cross-graining. It has some notable advantages; it lowers the wood’s splitting tendency when fastened at the edges; it lowers enlargement and contraction, provides improved dimensional strength, and makes the panel’s strength coherent around all directions.

Typically, there is an odd ply number to stabilize the sheet, and this lowers warping. Since plywood is glued with grains moving against one another and an odd number of combined parts, it has high hardness straight to the surface ply’s grain direction. Plywood material cut directly from the tree’s heart is more robust against warping and shrinking.

Sawing methods used in lumber processing also influence warping. Understanding this in a better way can assist you in reducing the risks of manufacturing warped plywood products.

Poor variation in saw speed and saw maintenance can produce thinner boards in the ends more than in the middle, leading to bow-type warping. It is also not advisable to saw lumber at an angle to the grain.

Proper Storage Methods to Prevent Warping
How you pile and store plywood is the primary determinant for its adjustments as it adapts to equilibrium moisture content. The planks and the markers put amidst them should be of equal thickness, mainly when laid in the same layer. They should be placed to rest on a flush foundation.

Make a lumber platform and place the plywood on it instead of laying them directly on the ground. Do not just support it at the two ends because the more support you give plywood, the better. 4/5 stringers running across the plywood’s width give better support to the plywood sheets.

If you do not have enough room to place the plywood flush, you still have the option of storing it standing. Build a rack that will support plywood as it leans against the wall so that you can prevent it from warping.

Provided the plywood is uniformly supported across the sheets, its weight will not draw it into a warp. You can use the space under the triangle of the rack you build to keep other items.

If your ceiling is high enough, build a similar rack that lets you stand the plywood on end. It would save space and, at the same time, provide the support required to protect your plywood from warping.

You can make a rack that hangs off the joists of your ceiling if you want your plywood to lay horizontally, although it is a difficult option. However, whenever you need to put your plywood in the rack or extract it, you will need help.

It is essential to allow the plywood sheets to have proper ventilation by distancing the material so that all surfaces are exposed to the air. Store the plywood in a clean, shaded, cool, and dry place.

If the area you are using to store your plywood has high humidity of about 80 percent, wrap it well in a water-resistant material. Doing so will help prevent the plywood from taking in too much moisture contents from the atmospheric air.

Note that plywood will most likely warp when it is in direct contact with cement or concrete blocks. Therefore, avoid storing plywood in direct contact with the ground or up against a bare wall.

Proper Curing of Plywood to Prevent Warping
Curing plywood helps limit warping like kiln-drying, which is used in manufacturing as it helps the operator have more power over the degree and speed of drying. However, all kiln-dried plywood will adjust to the relative humidity of its surrounding.

Kiln drying is preferred more over more natural passive drying because it prevents the activity of insects and sets the sap in resinous softwoods, which is usually essential for plywood used for construction purposes.

Wood experts recommend certain practices that can help prevent warping, like; do not over-dry lumber as it can crack, split and end grain check. The other one is, avoid drying the wood too slowly as that can exacerbate any bowing or any other kind of warping.

Lastly, do not allow partly dry lumber to reacquire moisture quickly. You can also seal the ends of the plywood to help avoid warping caused by unbalanced drying.

Seal the Edges
Sealing the end grain of plywood is a method that works for plywood and other wood products. Plywood and all other wood products take in water 12 times quicker via the grain’s end than via the grain’s side. Apply varnish, shellac, or paint to the end grain to give it a seal that will help to lower the wood’s capability to take in water significantly.

Note that capillary action will make the wood take in more sealant when sealing end grains. Apply dense coats until it gets to a point where the sealant has soaked into the end grain, and you no longer see pits in the wood.

How to Check for Warping
As a manufacturer, you can follow the following steps to check for warping in plywood to avoid delivering poor plywood materials to customers or straightening some plywood with warps to cut on losses. Follow the steps below;

Place a plywood piece on a flush surface.
Examine along the wood’s edge to affirm it is even along the entire length.
If the end or one part of the board is lifted above the others, there are high chances that the board is warped.
Every side of a board should be examined in this manner to ensure its shape is uniform.

Is There Plywood that Does Not Warp?
Usually, no plywood does not warp; however, there is a type of plywood that is more stable to withstand the high humidity conditions that cause the plywood to warp in most cases.

As seen earlier, the most stable plywood is marine plywood, as it is the toughest and strongest of all plywood in the market. It is glued together using high-quality glue to make the plies moisture resistant and structurally solid.

Also, some more wood varieties are more solid and unlikely to warp, like;

Fir- Seasoned fir is very strong once its moisture content gets balanced with the relative moisture of the atmospheric air. When the wood is at equilibrium or seasoned, fir undergoes minimal shrinking or warping. While other types of plywood continue to shrink and swell after seasoning, fir stays incredibly stable once it dries. It is also moderately resistant to damage from rot, making it a good choice for outdoor decks and fences. It is perfect plywood for building structures that need to withstand moisture and weather exposure.
Cedar- It is among the heaviest wood species, which assists stop splitting caused by changes in moisture.
Redwood- It has a similarly straight grain pattern, and a natural chemical inside that safeguards it against moisture penetration.
Besides considering wood species, you also need to think about the type of glue used to bond plywood veneers together.

Exterior graded plywood is unique because it is made in a different way from standard plywood. First, its layers are bonded together using waterproof glue, which makes it perfect for outdoor use. It also has high durability and sturdiness, making it ideal for indoor use, primarily in high humidity rooms like the bathroom and the kitchen.

Therefore, Douglas fir plywood that has been glued together with exterior glue is a fairly warp-resistant combination.

How to Prevent Plywood from Warping When Painting
When plywood absorbs moisture in the air, it warps; also, wood with too much moisture will warp as it evaporates. If the plywood is dried well, paint, varnish, shellac, and others will keep it from warping.

Some painted plywood projects can still warp even after construction is done, and most regularly, two things that might be working together may be the cause of this warping; lack of structural support and moisture.

When you paint one side of the plywood, you seal it from taking in moisture. If the other side remains unpainted, it can still take in moisture. The unequal absorption of moisture will cause warping in plywood, as said earlier.

It is very usual to experience this problem in garages and laundry rooms because both have high humidity. The easier solution here is also to paint the backside of the panel.

How to Straighten a Warp Plywood
We have already discussed what causes plywood to warp and how to prevent it. In addition, someone might want to flatten a warped piece of plywood for specific reasons.

To do that, you can flatten them with hot water and a warm environment which might take several hours but once finished, the plywood will be rejuvenated and ready for use.

You may follow the following steps to flatten warped plywood;

Step 1

Spray hot water uniformly onto the concave area of the warped plywood. The concave area is the interior of the hump formed by the warp.

Step 2

Set the plywood sheet onto a flat surface that has enough exposure to warm air. The convex or humped side should face up, and the side you sprayed with water should face down.

Step 3

Set a heavy object or weight on top of the hump to help it flatten out quicker.

Step 4

Let the plywood dry in the sun, checking its progress every 5-10 minutes. At the same time, moisture is being absorbed into the concave side. In case it warps on the other side, reverse the process. Once it is flat, place the plywood in a cooler spot such as a garage or basement.

Plywood is most likely to warp when it is used for homemade cabinet doors without any door frame. Preferably, any project of this kind should always use some frame on the doors. If you have any plywood doors that are already warped, you can solve the problem by attaching some battens to the backside of the doors.

These could be camouflaged as shallow shelves and, at the same time, improve the storage space. Before attaching these battens, ensure the hinges can handle that weight.

How to Fix Warped Plywood Shelves
When plywood is used to make shelves, they are very likely to warp, and the problem is the books’ weight or other items laid on the shelves. If the density of the plywood used is not adequate to support the weight of the books placed on the shelves, they will eventually warp.

The warp might only be seen when the shelves are loaded, or it might have affected them such that it remains even when they are not loaded.

The best way to solve this is to reinforce the shelves to have adequate strength to bear that amount of weight across the stretch between supports. Well-built commercial shelves efficiently do this by attaching a lip hanging down from the front edge of the shelf. The lip works critically the same way as an I-beam, making the shelf inherently denser and better supporting the weight.

You can attach a lip to your shelf easily by simply ripping a 1″ piece of hardwood 1″x4″ plank and attaching it under the front edge of the shelf, with the 1″ dimension scaled vertically. As you attach the two pieces together, begin from the middle and work your way outwards to ensure you do not leave any spaces amidst the two pieces.

If you do not have any hardwood available to do it, you can still do it using 1″ steel angle iron or 1″ square steel tubing as they would provide even more strength.

If you have ever dealt with plywood, you know it needs proper handling to prevent it from warping. However, some plywood types like marine grade are strong enough to withstand very wet and humid conditions and can happily stay submerged in water without absorbing moisture.

You can go for marine grade to avoid warping problems, but if your budget does not fit, you can still do a great project with the standard plywood by first treating it to boost its durability and prevent it from warping.

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