Tag Archives: plywood sheathing

Plywood – Nominal vs. Actual Thickness

Plywood – Nominal vs. Actual Thickness

My long-time former wife was a R.N. (Registered Nurse) for decades. In medicine, measures are exact – a cc is a cc. Patients do not receive about a dose of something and a pulse of 60 bpm (beats per minute) is not actually 45.

Explaining lumber sizes to her was quite a fete. She really wanted a 2×4 to be two inches by four inches, and when a 2×8 was only 7-1/4 inches deep, rather than 7-1/2” (or eight), she was highly displeased.

Luckily, she and I never got into a debate about how thick plywood sheathing is!

There are several variations available when it comes to plywood thickness, and this can have an effect on your project. Plywood thickness ranges from 1/8 inch up to 1 ¼ inches. However, bear in mind these numbers do not actually represent actual plywood thickness.

Plywood is made by peeling thin sheets of wood from a log, then gluing them together in layers with their grains perpendicular to one another. This cross-grain construction gives plywood its strength and durability. After veneers have been peeled, they are sorted by size and quality. These veneers are then clipped into uniform widths and laid out in a sequence to form a panel.

Next veneers are glued together using a hot press. Glue used can vary depending upon plywood’s intended use. For example, exterior use requires a waterproof glue, while interior plywood can use a less expensive non-waterproof glue. Hot press applies heat and pressure to veneers, bonding them together into a solid panel.

Once bonded, plywood is trimmed to its final size and sanded to remove any rough spots. Final step is to grade based upon appearance and quality. Grades range from A (highest) to D (lowest).

When plywood is sanded, during manufacturing, it can remove up to 1/32 inch of material. So, for example, a plywood sheet with a thickness of 3/8 inch really has a thickness of 11/32 inch.

Standard thicknesses for plywood panels are 1 ¼, 1 1/8, ¾, 5/8, ½, 3/8, ¼, and 1/8 inches. This makes corresponding actual thicknesses of 1 ¼, 1 1/8, 23/32, 19/32, 15/32, 11/32, 1/4, and ⅛, respectively. This takes into account sanding effects throughout plywood manufacturing processes.

An Alternative to a Very Flat Roof Extension

An Alternative to a Very Flat Roof Extension

Reader BOB in SANDPOINT writes:

“Hello we want to add a patio / work on car / whatever, extension off the front of an existing shop with a metal roof – joining the existing metal roof with the metal roof of the extension with a sort of pitch break. The challenge is the 10′ height of the existing roof and only having 18″ drop to the garage entrance. Ideally we would extend out about 14′ but I am thinking the extension roof would not be steep enough. I was planning on installing a pitch break joint where the two roofs meet (but not having an actual difference in pitch – or as little as possible).also, being in Montana we do have significant snow. I know with the pitch issue of only having the 18″ drop – and the target of 14′ I will have to compromise but trying to figure how much and where.”

Mike the Pole Barn Guru says:

Well Bob, you do have some challenges and are not the first person to have them.

Most steel roofing companies will not warranty their product on roof slopes of under a three in 12 slope (3/12). At very least, doing a single sloping roof this flat, in snow country, poses a myriad of challenges – some of them you have already considered. Rafters and purlins for such an extension will require them to be engineered to support the weight of snow sliding off steeper main roof. Ponding is a real concern, as ice and snow will build up and not slide off. You should probably look at using 26 gauge steel with ribs higher than what is on your existing building (meaning steel profiles will not match) and perhaps installing over solid sheathing – either OSB or plywood, with 90# asphalt impregnated (felt) paper or a synthetic water and ice shield below.

If you are thinking this is sounding expensive or perhaps impossible, there is an alternative option.

Your extension roof could be done as a reverse gable – meaning you would not have height issues in front of your existing overhead door opening. This would also allow for snow to slide off to right and left of your extension, rather than plopping off directly into your line of travel in and out. We use reverse gables frequently, in new construction, to protect door openings from rain water or snow slide offs. Here is some extended reading for you: https://www.hansenpolebuildings.com/2015/07/reverse-gable-porch/

Do Screws Back Out of Steel Roofing?

I had a question posed of me recently which included: “Where will the water go when the screws back out of my steel roofing”? While I answered the question at hand, I didn’t actually get into the why this might happen, or the solutions.

How to avoid the potential problem completely……use the right part, properly installed and driven into the correct material. Three easy steps, should not be so difficult.

The part – most commonly used screws are a #9 diameter by one inch long. When we tested steel roofing to determine sheer strength these screws pulled out of the framing under a minimal load (so minimal the steel didn’t even have ripples in it from the applied load). You can read more about our testing here: https://www.hansenpolebuildings.com/2012/08/this-is-a-test-steel-strength/.

Going to a longer part solved the pull out issues in our testing. We also went to a larger diameter part in our testing, the shank below the screw heading being ¼ inch across, while the threads are a #12. The larger diameter screws also have deeper threads, which means they bite and grip the wood more tightly.

Proper installation – screws which are over or under driven, or driven at an angle are prone to a myriad of problems, all which end in leaks.  Over driven screws tend to damage the wood fibers, leaving little solid material to hold the screw. Use a screw gun with a clutch, so screws do not get over driven.

Driving into the right material– what could go wrong? I see folks using OSB or plywood sheathing under roof steel with the idea they can drive the screws into the sheeting and still hold, even when the screw tip misses a purlin. These screws will come back out.

Green lumber (or dried lumber which has been allowed to get wet) will cause screws to be loose as the moisture leaves the lumber once the building is dried inside. Of course green lumber has a myriad of other challenges which can be read about here: https://www.hansenpolebuildings.com/2011/09/499green-lumber-vs-dry-lumber/.

Right part, right screw, right material below – drop the mic and walk off the stage. Three easy steps for proper screw installation and keeping leaks from happening.