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Calvin Henry-Cotnam
 
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Default Plywood decking thickness

Ken ) said...

How thick should plywood decking be assuming you are using joists 16" on
center?

In my earlier years, perhaps 20 years ago, I worked for a builder who
typically used 5/8 floor decking and 1/2" on the roof and exterior walls.
Now I'm faced with an architect that insists 3/4" on the floors is a must
and 5/8" on the roof and walls is needed.

Does the floor decking depend on the finish flooring. The builder I worked
with would always install wide plank 3/4" hardwood flooring. But when I
mentioned that to the architect, he insisted that I should still use 3/4"
decking under it. His concern was regarding the holding capacity of the
nails for the hardwood flooring and that it would avoid squeaky floors
later.


For floors, the subfloor makes a difference on how much it flexes. We used
an engineered system (I-joists) and the span capabilities depended on
the thickness of the subfloor (5/8 versus 3/4) and whether it was just
nailed or glued and nailed. For a firmer floor, we went with the 3/4"
and glued and screwed it in place. Only tongue & groove for the floor.

I used 5/8 T&G for the roof (24" O/C trusses), but the first framer I
hired balked at using this -- they often just use 3/8" plywood with H-clips.
I hired someone else to finish the roof.

My parents' home was build in 1963 and has 1/2" plywood on a stick-framed
structure that is 16" O/C -- there is no way I would use that on 24"!

As for wall sheathing, our code allows 1/4" OSB or plywood -- even 1"
styrofoam sheets can provide the anti-racking support that is needed.
I went with 7/16" OSB, but it starts at the sill plate. This means that
instead of two horizontal runs of sheathing on each 8' wall and short strips
at the end of the floor structure, the first course starts at the sill
and extends about 3' up the first floor walls with another course above it.
The third course of sheathing starts a foot below the top of the first
floor wall and continues to about 2' up the second floor wall. This IS
harder to install, but it provides extra anti-racking strength, plus it
provides a huricane tie between the walls and the floor structure (not
that we are in a huricane zone).

--
Calvin Henry-Cotnam
"I really think Canada should get over to Iraq as quickly as possible"
- Paul Martin - April 30, 2003
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