Loft Floor Joist ?

[peteh2833]

I'm building the 20x30 1 1/2 story with a full 2nd floor loft. I was going to use a beam down the center of the of the loft to support the joists and cut the span in half to 10'. My ? is what size joists can I use? i want to go to a local mill and get some made. I was thinking 2x8's but if I were to use 4x material what size? I can't find any span tables for 4x material. I was going to use 24" or 32" on center and possible 2x6 TG on the floor if it isn't too expensive. Thanks

[John_C]

I ran it through the calculator assuming #2 Hem-Fir  as a midle of the road wood strength wise.  Doug Fir or S. Yellow Pine would be stronger.

If you went with 4x8  you could space them 48" o.c.    2x6  T&G should be fine for that spacing in a loft..   As always YMMV   here is the link so you can play with the numbers

http://www.awc.org/calculators/span/calc/timbercalcstyle.asp

[John Raabe]

Good work John C.

That should work quite well.

As a rough rule of thumb you can use a standard joist calculator and use a 4x member at twice the spacing of the 2x member. So if the chart says a 2x8 will span 10' at 24" o/c then a 4x8 could be expected to do the same 10' span at 48" o/c. This is fairly conservative since the width of the 4x (3.5") is more than twice the width of the 2x (1.5").

2x6 T&G decking can span 4' comfortably. I have gone as far as 6'-6" o/c for the beams but we had to screw down a 3/4" plywood subfloor to get the deflection down to what felt solid. I would limit the decking span to 5'.

[peteh2833]

Thanks for the info. I'll be going to the mill next week sometime to see what i can come up with. Do you think 4x6x10's will work for floor joists on 32"-36" centers to span and attach to a 6x12 beam running down the center??? Thanks Pete

[John Raabe]

By the same analysis as above a 4x6 will make it for a 32" spacing. (A DF 2x6 will span 10'-9" @ 16" o/c on my chart.) Have the main beam sized locally and make sure the engineer determines the size of the footer you will need as well.

[peteh2833]

Ok. Thanks. The main beam is steel under the sub floor and the loft beam will only carry the loft floor. If Douglas Fir isn't available is it ok to use other Pine that is available in Western Pa???

[John Raabe]

Southern Pine is about the same strength as DF of the same grade.

[peteh2833]

I think the only thing available here in Pa that is locally milled is White Pine and Hemlock.

[glenn kangiser]

If using local milled material sometimes you can go thicker to get the strength while keeping the 11 1/2 dimension (for 2x12) to meet the size of standard lumber without throwing things off on depth.

[peteh2833]

Ok I'll keep that in mind. I'm looking at going with 4x6 joists on 30" centers and attached to a 6x12 center beam so the span is only 10 feet.