Is it OK to use 5/4 PT decking on 45 degree angle to 16

by Kurt
(Bowling Green, Kentucky)

I'm replacing the surface of a 300 s.f. deck. The previous material was 2x6 PT but built with tongue and groove system that held water and caused lots of decay.

Some decay has occurred on two of the joists which I will replace. It was installed on 45 degree angle to 2x10 joists with 16" spacing.

Is it OK to use 5/4 PT decking on 45 degree angle to 16" spaced joists or should I install at 90 degrees. Using screws to attach decking.

I want to pass building code here in Kentucky.

Editor's Comments:

If this was nominal 2" decking (1.5" actual thickness) this would be any easy question to answer - Yes.

But with 5/4" material it is not always so simple. Presumably this is southern yellow pine which happens to be one of the strongest of all the pine species. So this is helpful.

However after a quick read of the Prescriptive Residential Wood Deck Construction Guide published by the American Wood Council there is a statement on Deck Requirements that states that 2" material may also be used on 45 degree angle to joists but nothing more.

So there is no statement that 5/4" treated pine boards can be used with joists spaced at 16" o.c. Now, keep in mind the Guide does say that if the material is span rated by the manufacturer that can make it permissible.

There are in fact deflection standards in the ASTM testing procedures that composite material manufacturers work to meet. So it seems the same concept applies here.

Does the manufacturer of the decking material place any limits on the maximum span between joists? This could be found by speaking to the folks at the contractor desk of your local building materials store.

The Practical Effect

What you may notice is when joists are spaced at 16" o.c. the actual span distance (hypoteneuse) will be longer and the 5/4" may feel a bit bouncier.

Personally, from my building experience, if I was building this from scratch and planning to use 5/4" boards I would have framed the joists at 12" o.c. just to make darn sure everything felt right.

The other reality is that if this is not being inspected you may feel this is just fine. It certainly is not going to be an immanent danger or risk. You will likely feel a bit of bounce if you really jumped on it.

The Take Away

To be safe and certain you don't have any hangups with an unpleasant building inspector, install the 5/4" decking at 90 degrees to the joists and you will be fine.

5/4" PT is commonly used at 16" joist spacings.

If there is anyone else out there who has some recent experience that can add or improve this reply to Kurt, please chime in.

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