When looking to buy lumber, there are so many options it can be hard to know the difference between all the different woods. For example, plywood comes in different ply amounts, grades, ratings, and sizes. So what is the difference between 3-ply and 4-ply plywood?
3-ply plywood is the standard for internal-use plywood, featuring 3 layers of wood. While 4-ply plywood is available, it is very rare, and 5-ply plywood would be a better choice if you’re looking for increased structural ability compared to a 3-ply.
I’ll take you through all the aspects of plywood, touching on plies, grade, rating, and size. We’ll look at some of the manufacturing techniques and advice regarding plywood, so you know how to pick the best plywood for your needs.
What Is A Ply?
A ply refers to a layer of veneer used to create plywood’s various thicknesses. Veneer has a slightly different meaning in the world of lumber than how it is typically used.
A veneer is a thin layer over the top of a solid material. One of the primary reasons something is classified as a veneer rather than something else is that a veneer is primarily designed to give better aesthetics.
A veneer can be merely plastic, giving the options for paint or other methods of imprinting patterns or coloring it. This means that you can use lower-quality wood that doesn’t look so great but then use a veneer to increase its appeal.
A common veneer technique uses stick-on laminate or similar to give the appearance of textured wood, marble, or any other pattern.
Thus plywood is man-made and not a natural wood product.
Thus, the term plywood always refers to engineered wood products, in that they are composites of various woods rather than being cut straight from logs as planks or sheets.
However, while the term veneer may have a meaning of applying to the top or surface of something only, in the lumber world, any layer in an engineered wood product is referred to as a veneer.
It’s possible to see the individual layers or veneers when you look at certain plywood products. It is evenly used intentionally as an aesthetic device by exposing the veneers in areas like the corners, commonly seen when using a dovetail joint.
By having more plies or veneers, you end up with a thicker and stronger piece of wood.
How Do You Make Plywood?
Plywood consists of at least 3 veneers or plies bonded together with an adhesive.
Through trial and error, it has become clear that each ply should be oriented with its grain running perpendicular or 90 degrees to the adjoining layer. This has been shown to reduce the shrinkage, bowing, and improve the strength of plywood.
Plywood is made out of specially-grown trees that tend to be smaller in diameter than the usual trees to get lumber from. This is not an issue, as the production process involves rotary peeling or sliced veneers resulting in long plies from logs.
Rotary peeling involves a massive lathe into which logs are inserted. The lathe sheers off the plies in long sections that can then be glued together to form plywood.
Another method is veneer slicing, which looks to be slightly less wasteful than rotary peeling. It involves a similar process to traditional lumber manufacture as the veneer slicing machine slices off the plies.
This has been shown to be better for keeping the original look of the wood and a better grain and minimizes sawdust.
What Do The Numbers In Front Of Plywood Types Mean?
The number will be referring to the amount of plies or layers to the wood.
Plywood has an odd number of plies and needs at least three plies. Of course, if the thickness of each ply stays the same and the type of wood is the same, the more plies you have, the more wood is being used.
Plywood with fewer plies is weaker than a plywood with more plies. Depending on what is available, you may find plywood going up to 13 plies.
This means that a higher ply number will indicate more strength for any given thickness, but it will be more expensive.
Although specialty plywood can have any number of plies above three, most plywood is categorized as 3-ply, 5-ply or multi-ply. Plywood that has only one or two layers will not be strong enough and will warp and deform.
Calculations have shown that you need two faces and then a core, bringing the minimum number of plies to three. Due to the grain of the face being better than the inside, an odd number of plies is required to make sure both sides of the plywood have the grain out.
As the grains will intentionally be put to 90 degrees of each other during construction, the grains will reinforce each other over the three or more layers.
What Is 3-ply Plywood?
3-ply is one of the most common types of plywood. It will tend to be thin as it only has three layers, making it generally unsuitable for outdoor or exterior uses. It is one of the most widely-used building materials.
Due to this, 3-ply plywood tends to be employed in more decorative usages than thicker plywood boards, such as furniture.
What Is 5-ply Plywood?
5-ply is fundamentally the same production process as 3-ply plywood, but two additional layers have been added.
This will increase the strength and thickness of the plywood if other factors remain the same.
Given these additional features, 5-ply can be used indoors or outdoors. However, it is still not a wood that should be depended on to give permanent structures a strong frame, so it should not be used to build your home or shelter.
What Is Multi-ply Plywood?
Multi-ply is a catch-all term to cover any type of plywood that has at least seven plies.
Multi-ply is rated to be used in roofing, framing, and other projects requiring structural integrity.
What Do Plywood Ratings Mean?
Plywood is rated according to the level of gluing, type of glue, and use case of the plywood.
At a minimum, you can expect plywood to be rated as exterior, exposure 1, exposure 2, interior, and structural 1.
Exterior means that the plywood can be used outdoors and is relatively resistant to damage from the weather. Exterior plywood has been waterproofed to an extent and is excellent for permanent outdoor structures like a shed.
Exposure 1 means limited waterproofing is present. This means that it can be used to construct certain objects and can resist low levels of elemental exposure but is unsuitable long term for such applications.
Exposure 2 is even less resistant to moisture than exposure 1. Exposure 2 rated plywood cannot be used for anything apart from internal use.
Interior-rated plywood has no waterproofing and should not be exposed to moisture. This means that untreated plywood rated to interior is best left in a dry area, including cleaning up spills as quickly as possible.
The last type, structural 1, is meant for retrofitting or upgrading earthquake resistance. These panels provide a certain level of seismic resistance and so are more expensive. It isn’t necessary to use them unless specifically for earthquake reinforcement protection.
What Are Plywood Grades?
Plywood grades come in four types, represented by the letters A, B, C, and D.
Grades are a reflection of the superficial or cosmetic qualities of the wood. Plywood rating refers to the quality and appearance of veneers, whereas plywood rating is meant to inform you about the structural integrity and use cases.
A-grade plywood is the best quality plywood. It will have a smooth, sanded surface with a visible grain but without knots.
Any defects are filled in with synthetic filler, meaning the veneer can be easily painted without much preparation work. A-grade plywood is for use cases that emphasize the look of the wood over its functional properties, meaning it is excellent for furniture.
B-grade is similar to A-grade plywood but will have defects. These defects must be minor and should measure no longer than 1-inch across.
C-grade and D-grade are unsanded and have several minor defects and knots. These will require some repair work, with C-grade featuring knots up to 1 1/2-inches across and D-grade up to 2 1/2-inches.
C and D-grade plywood will often feature uneven discoloration, sanding defects, or otherwise obvious aesthetic issues.
C-grade plywood is often used where it can’t be seen or its appearance is excessive, such as in subfloors or garages. D-grade is much the same but with more significant defects.
Sometimes plywood falls between two standards; for example, you may find ratings with mixed classifications, such as AB. This just means that one part or side of the wood meets the A grade, whereas the other side is only worthy of a B.
What Size Does Plywood Come In?
Plywood will generally come in 4 foot or 8-foot pieces, with thickness measuring either ½ inch or ¾ inch. The thickness of the pieces will not matter a whole lot, as structural integrity will come down to ply number more than anything.
Plywood, like all lumber, has come in standard sizes since 1924. There is sometimes a bit of confusion around this topic as if you measure lumber in a setting like a big box hardware store; you may find that the dimensions of the wood do not meet up to the advertised width and length.
The reason for the difference in sizes is due to several factors, including drying and the production process. Lumber will tend to shrink as it dries out in the journey from the lumber mill to the store.
Further, the demand for better-looking wood leads to additional planing and shaving of the wood to remove surface defects like knots. The introduction of kiln drying to speed up the drying time also causes shrinkage.
Standard sizes are useful as it makes it easier to transport, as well as helping builders, carpenters, cabinetmakers, and yourself to make use of wood with minimal waste. Pay attention to the various sizes when thinking up your next project.
If you know what size wood comes in, this helps when you are designing something. For example, you can make a tabletop the size of a standard 4-foot piece of plywood, meaning you don’t need to cut it to size and create wasteful offcuts.
What Types Of Wood Does Plywood Come In?
While there may be 3-ply or multi-ply plywood, the wood that the plies are made of will be different and appropriately labeled.
Plywood will be divided into softwood or hardwood types. Softwood plywood is typically cedar, redwood, or pine. It may seem counterintuitive, but softwood does not necessarily mean that it is unsuitable for projects that need structural integrity.
Softwood plywood can be used in frames, shelving, and many other varieties.
Hardwoods found in plywood will most commonly be birch, oak, maple, walnut, or poplar. These are great for furniture, cases, and other applications that require very strong frames.
In the end, the difference between what makes a hardwood versus a softwood is slightly arbitrary and reflects slightly archaic thinking.
The terminology is thought to have come about due to the relative variability of wood properties. This is because different trees have different moisture contents and growth rates.
This meant that certain types of trees, referred to now as deciduous trees, tended to wear out saws and blades, causing them to be called hardwood trees. Deciduous trees drop their leaves before winter in order to maintain more water, which makes them heavier.
