What affects the strength of plywood?
Not all plywood has the same amount of strength and weight it can hold, so when you begin a project it's really important that you have a complete understanding of the structure you're going for and the exact measurements that you'll need.
Plywood is used for a multitude of things, it's used in sheds, scaffolding, and fencing, and its wide use in general load bearing means that it's something that you'll need to research.
Some types can hold five pounds, and others fifty, so it's a big difference to learning!
Whilst some is very thick and there isn't too much to it, it is actually very deceptive and has a large general load, it's very robust.
It's made from layers of wood grain and chips bonded together under glue with really high pressure. It's also so strong as it's layered up, with each sheet perpendicular to the other.
One reason plywood is stronger than other woods is that each sheet has an alternating grain pattern, making it much more robust. This is also why it's stronger relative to its size and thickness than other types of wood.
The plywood itself also has different types, with varying levels of strength because of the thickness, flexibility, species, glue, and layers that give it its qualities.
Let's go a little bit more in-depth on how each quality affects its strength.
Thickness
It's a pretty reasonable assumption that the thicker the plywood is, then the stronger the piece of wood will be. The thickness relies upon maximum load-bearing space too, a number called the span rating.
There are some maths you can do to figure out the maximum live load for each thickness and span rating, a calculation that essentially lets you optimise the strength. If it is 19/32 inches thick, then a span rating of 24 oc is required for the maximum live load, which will be 270 pounds. 23/32 inches with a span rating of 20 oc creates 240 pounds maximum live load, and 7/8 inches of thickness needs a huge 32 oc to reach its maximum live load of 295 pounds.
Flexibility
Flexibility is usually referred to as shear strength, and it stops your sheet of plywood from cracking or breaking when it has a lot of weight on it, or if there are problems with the structure, flexibility is really important for the general strength of the wood.
Species
There is a wide array of this wood, but only two are recognised as actual species of hardwood. The two types are softwood and hardwood, but this just refers to the source of the wood that was used when making the plywood, so this could be hundreds of types of trees under these species.
Maple and oak are some good examples of hardwood, whilst cedars and pine trees are softwood. The general rule of thumb is that hardwood is the stronger and more durable species, but this isn't always true and there are a couple of exceptions.
Glue
Some manufacturers use exterior glue on the surface which can help to make softwood a little bit stronger than its cousin, it has a massive impact.
Glue blocks moisture out that would usually weaken the wood, making it a lot more durable over time.
Layering
The final factor that affects the strength of the wood is the way that it is layered, namely the number of layers.
The weakest plywood is known as shop-grade and is created with less than four layers to be three-quarters of an inch thick.
A piece of plywood that has between four and seven layers will be a lot more versatile and has a decent level of strength. When you go above seven layers, it is called multi-ply, and it is the strongest wood that you can get.
What are the different types of plywood?
We've already spoken about the species of wood and the difference between hardwood and softwood and how they affect the strength of plywood.
The main thing that categorises the wood into hardwood or softwood is the specific subtype of wood, so here is a list, so you can begin to understand what constitutes what and what type it is.
Hardwood Plywood
- Alder
- Ash
- Bamboo
- Beech
- Birch
- Cedar
- Cherry
- Fir
- Mahogany Khaya
- Mahogany Sapele
- Maple
- Oak Red
- Oak White
- Palmwood
- Pine Knot
- Teak
- Walnut
- Wenge
- Zebrawood
Softwood Plywood
- Cedar
- Douglas Fir
- Marine Plywood
- MDO
- Pine
- Spruce
- Redwood
This is understandably a lot of options and extremely overwhelming when you need to look at what you want to buy for your project, so we will go into a little bit more detail now.
Which type of plywood should you be using?
Hardwood and softwood both have different uses so let's single it down to one species first. Hardwood is for lots of home projects like furniture or shelving, whilst softwood is for some big construction projects like support beams, etc.
This might be a little bit controversial to what we already discussed, as hardwood is usually stronger than softwood, so it should serve to support more.
As we already mentioned, however, softwood can be made stronger than hardwood with an exterior glue that can keep the moisture out and make it stronger as well as last longer.
The best options for structural support are marine ply and MDO, but there are far more options for hardwood shelving or furniture. It might even end up being a conversation on aesthetics, a light option, or a dark option.
Some nice light options are maple, fir, ash, or pine knotty.
Alternatively, some dark options are mahogany khaya, teak, walnut, or wedge.
There are lots of factors that make up the strength of plywood and what dictate how much it can hold. A little bit of research on plywood can help you to figure out what you specifically need before you start with your project, so be sure to understand your needs perfectly!
How do you calculate the load capacity of plywood?
The load capacity of the plywood is a calculation of the span rating and the thickness, and then you get a figure called the maximum live load. A rule of thumb to remember is that if the thickness increases but the span rating decreases, then it will support less.
How much weight can a 1/2 inch plywood hold?
Whilst there are lots of other factors, half an inch of plywood can support thirty-five pounds of pressure. This will have lots of other factors however like the size of the wood compared to the thickness that will affect it, so thirty-five is the maximum strength.
Is plywood strong enough to walk on?
Whether plywood will be strong enough to walk or not will depend entirely on the maximum live load. This will depend on the weight of the person too, a heavier person will need much stronger plywood with better materials.
