Rafter Size?

[Dale Hatfield]

 Im about to do some sawing for a friend. That wants to put in timber  rafters on stick frame. Out of yellow poplar. They will span 18 feet with a 10/12 pitch. He wants 4x 6 for the rafters. I dont think thats big enough. Any Idea what size he really needs?

Dale

[Jim_Rogers]

In order to figure the correct rafter size you need to know what the spacing will be as well as the span, and the combined roof load. The combined roof load is the dead load of the roof, materials that will go up there, such as asphalt roofing shingles, tiles, slate, standing seam metal roofing or wood shingles, plus the roof decking materials such as boards or plywood. And the live load which is snow load for your area.
Once you now the combined load and spacing you can then size the beams correctly for the type of wood you want to use.
Don P has created a calculator that can be used to figure rafter sizes. It is under the tools box icon.

Hope this helps.

Jim Rogers

[Raphael]

Does each rafter span 18' or does each pair span 18'?

With rafters it often comes down to an aesthetic balancing act between timber size and rafter spacing (which is limited by your roof sheathing).  Play around with Don's calculator, it'll give you a good idea of the range you need to be looking at.

[Dale Hatfield]

Thanks Guys
I dont know snow load for this area Souther Ohio. Very Little
Dead roof load  3/4   poplar board ceiling ,2x4 purlins, itchy pink, Metal roof.

Building width is 18 feet so the pair will span that with a 10 -12 pitch. set a 4 foot on center.

I done  a quick figger yesterday and I think that,  at that pitch the birdsmouth opening would be greater than that beam will allow.
For some reason i cant don cal page to open get a 404 error
Dale

[Jim_Rogers]

The layout of a birds mouth rafter seat cut should be in the bottom 1/3 to bottom 1/4 of the plumb height of the rafter. Or is the total height of the rafter? hum, I'll have to look that one up......
Or if someone knows the correct rule of thumb please post it.
Thanks.

Jim Rogers

[Raphael]

It'd be the plumb height.
I've heard it said as make no more than the bottom 1/3 of a plumb cut or something like that.

Do you plan on letting in those purlins to make them flush w/ the rafters?

[Don P]

Dale try this link and see if it works;
Rafter span calc

Ohio is mostly 20 psf snow I believe, your local building inspectors will know for sure. The roof you describe would fall under the typical 10 psf dead load assumption.
Other inputs you'll need for the calc;
Span 108"
#2 Yellow Poplar, Fb 700 psi
MOE 1.3 million psi
Shear 145 psi
Nail load on 16d cc sinkers (hand drive 16's) 102 lbs/nail in poplar

You didn't mention a ceiling joist, notice the outward thrust on the wall under full design load is about 800 lbs pushing out on the walls per rafter pair. A ceiling joist is one way of restraining this, the calc is telling how many nails it would take to resist that force at the connection to the rafter and in the splice if there is one. Assuming no storage on the joist a 2x8 in y-p would make the span in one piece.

The sheathing is too light by code FWIW, they call for 1-1/2" thick but at 4' span it only needs to be Fb 400 psi, thats #3 grade. I haven't checked actual strength of what you proposed, just quoting the bible.

Double up studs, at least, under the rafters.

Feel free to pm if I lost you anywhere.

I need a pic of what y'all are saying about birdsmouths  ???

[Dale Hatfield]

Don
Thanks  for the info.  I don't understand sheeting being too weak. It isn't supporting anything other than itself and insulation.(or is it?) The sheeting will run perpendicular to the rafters. Then purlins nailed  same direction  then Steel roof on the purlins.
This is just a rustic cabin off in the woods no code , no water, and a genarator. Just dont want the roof to fall in.
Their will be a loft in one end  collar ties on the other  is my guess.

Thanks
Dale

[Don P]

Gotcha, the purlins and steel are transferring the load to the rafters, the poplar boards are just ceiling panelling. I think that would be fine. Might want to remind him to keep the collars as low as possible if that's how he goes.