Sizing roof decking

[canopy]

How do you size the thickness of roof decking? In my case I am considering tongue and groove teak decking on 5' rafter spans, 9 in 12 roof pitch, 10lb/sqf roofing tiles. Tropical climate so no snow load.

[Jim_Rogers]

Are you trying to determine roof deck thickness and width or just one?

You do have a good wind load there on top of that hill don't you?

Jim Rogers

[Jim_Rogers]

When I sized your rafters, I used the values for teak of 1950 Fb for Maximum allowable fiber stress in bending.
And 1.5 million in Modulus of elasticity. And also, 245 for shear parallel to grain, which is also known as Maximum allowable horizontal shear.

Using DonP's online calculator from the FF red tool box you can enter this values into the first beam calculator.

To start you need to know the load on the beam in pounds. I assumed you'd use a 1x8 for a roof deck board. So the area one board would hold up would be .67 x 5' (.67 is 8" in feet terms). Giving you a sqft area of 3.33 sqft. Next we multiply that times the load which is the tile roof materials of 10 lbs. The total is then 33.33 lbs of load on the beam.

Next I entered the span of the beam in inches (60) and the size of the beam 8" and 1".

After entering the values stated above I clicked on "show results" and a 1x8 teak board passed, in all three tests.

If you do this yourself you can then enter different widths and thickness to see what will pass and what won't pass.
You will have to adjust the load if you change the width of the decking to reflect the actual new load.

And if you're using tongue and groove then the load is carried by three pieces. One on each side of the one in the middle that you're testing. So that would mean you're going to gain strength with tongue and groove.

Hope that helps and good luck with your project.

Jim Rogers

[canopy]

Thanks, that's very useful. Do you know if 1x6's would do? The rafters are 6" wide and the rafter span is 5' center to center. The rafters were upped to 6x7's due to the increased span after talking to Joe. Architecturally I prefer this wider span as it allows things to fit in better and wider spans I hope will show a little more what a timber frame can do vs stick.

[witterbound]

I'd imagine me would use at least 2x t&g in most homes.

What goes on top of the decking?  3/4 inches for nails or screws isn't much for them to hang on to.

[canopy]

I'd imagine me would use at least 2x t&g in most homes

I believe it. In fact, I have seen references in the US of people using 4x4s for their roof decking! In my case the span is large but a few things may work in my favor. I plan to use "true" 1x6's, not 3/4" thick. Also it is hardwood and finally there is no snow load. I will go thicker or wider if needed.

What goes on top of the decking?

1x3" battens fastened by screws to the decking to support clay tiles on top.

[Jim_Rogers]

..........Do you know if 1x6's would do?

You'll have to follow my description and run the numbers yourself.

Good luck with your project.

Jim Rogers

[canopy]

Yes, 1x6 passes. The calculator is awesome though it does not consider roof pitch and I wonder if that might need factored in.

[Jim_Rogers]

Roof pitch increases the dead load weight when figuring rafter loads, as live load weights are figured level.
When you're figuring a purlin (timber going from rafter to rafter) it is the same weight if it is in the roof slope or flat/level.

I wouldn't use the increased slope weight when figuring purlins or roof decking boards, for that reason.

I maybe wrong, I'm not an engineer, but that is what makes sense to me.

Jim Rogers