What is the best, OSB or Plywood?
All wood floors have one thing in common. They are only as good as the subfloor beneath them. Everything in your home interacts with your subfloor so making the right choice has a long term effect. Just because subfloors are not visible that doesn’t make them not important. Without a good solid flat subfloor with great fastener holding power even the best hardwood flooring will not perform as they should. Like all wood products moisture is its worst enemy. When a roofless, partially built structure takes on moisture what effect does that have on the subfloor? After gathering information online I was left very confused as each product made good points on why it was better than its competitor whether it compared OSB against plywood or OSB against OSB. In OSB we found there are 3 price ranges and qualities so we chose what we considered good, better and best. In the plywood we chose 5/8” and ¾”.
OSB is manufactured from heat cured adhesives and rectangular shaped wood strands arranged in cross oriented layers. The end product is an engineered wood panel that shares many of the same strength and performance characteristics of plywood. While OSB was developed quite recently, it became more popular than plywood in North America by the year 2000. Today, nearly 3 times as much OSB is produced in North America than plywood. Manufacturers have a Good, Better and Best quality available. This is based on how much resin is integrated into the wood to resist water absorption. Less resin means less expensive but less moisture resistance. The higher the price the more moisture resistant the product. The moisture resistance is measured with No Sand Warranties measured in days. The less expensive having no warranty, the middle material having a 180-200 day No Sand warranty, and the best material having a 365-500 day, to a lifetime No Sand warranty. The Limited Warrantees are very long and hard to understand. What the limited warranty does not cover. “Sustained cascading or pooling of water, immersion in water or other abnormal exposure to moisture, or exposure to moisture avoidable by good and customary maintenance practices”. Also, product must be stored before and after installation according to APA standards which in my translation means it can’t get wet.
Testing OSB and Plywood
Our test procedure was very simple.
- We purchased 1 - 4’ x 8’ sheet of each product and cut them into pieces 2’ x 4’.
- Each piece was weighed, moisture content tested, and thickness measured with calipers. Each piece was numbered and weighed writing information on the piece with black permanent marker.
- 2 pieces of each product were submerged into a tank filled with water with sticks separating them so they would absorb the water equally.
- Samples taken out after 22 hours, weighed and measured and returned to tank.
- Samples taken out after 68 hours, weighed and measured and left out of tank.
- Wet subfloors nailed and stapled and then exposed to 22 C degrees and 20% humidity until the product had reached 6-8%. Once all products reached 6-8% nails and staples removed and holding power rated for each product. We measured the moisture content after 10 days/17 days and 30 days.
- Wet pieces ripped and cut into blocks to expose centre pieces. After 24 hours measured and stacked 6 pcs high. Dry pieces cut into same size blocks and put beside wet pieces and pictures taken.
- Stacks of 6 pieces left to dry for 10 additional days then re-measured and moisture content taken.
- Dry samples not exposed to water nailed and stapled. Remove some nails and staples to test holding power.
- OSB swells when exposed to moisture. Once swelled, it does not shrink back.
- Plywood picks up moisture quickly, but loses the moisture quickly. When plywood absorbs moisture, the swell is very limited. 5 times less than OSB.
- Additionally, plywood dries back to the original thickness
- Not all OSB is the same. There is a huge difference in moisture resistance between the lower, medium, and high quality OSB.
- OSB swells more on the edges than on the inside.
When exposed to moisture, the OSB expanded significantly and when it was re-dried it did not shrink back to it’s original thickness. Plywood expanded minimally in thickness, and when dried out, it returned to the normal thickness.
The plywood tested has significantly more fastener holding power than OSB. Although the Advantech OSB had similiar nail holding power to the 5/8” plywood. The OSB swelled much more at the joints than it did in the center.Overall, we found plywood outperformed OSB and is a better subfloor option. We also found a significant difference in the performance between the different OSB options