Garage rafters

proctrc · 06-13-2010, 03:05 PM

I have a 24 (wide) by 28 (length) garage. The rafters are 2x6 on 24 inch centers and the runners going between the side walls are 2x6 24 foot on every other rafter.

Question is: Is it adequate enough to have the 24 foot runners going between the side walls on every other rafter (48 inches between runners) or should the runners be on every rafter? I don't plan on putting anything very heavy in the ceiling. Just a few pieces of plywood and some storage boxes.

Would appreciate some expert advice.

Daniel Holzman · 06-13-2010, 03:23 PM

The "runners" you refer to are probably floor joists, based on the fact that you say they are 24 feet long. Your specific question is if the floor joists are OK spaced every 48 inches, rather than 24 inches.

I am not sure if you are asking this question with respect to code (does my garage meet code?) or are you concerned that the garage may fall down because of inadequate number of joists?

The question of how much load you can put on the floor is a function of the bending capacity of the floor joists, which at 2x6 spaced every 24 inches, spanning 24 feet, is very low. To give you a more precise answer, we need to know the exact dimensions of the 2x6 (are they full size or dressed?), and the maximum allowable fiber bending stress of the floor joists, which may be marked on the wood, or could be estimated if you know the wood species and grade of wood.

Perhaps post some pictures, and try to determine the species of wood. And let us know if this is a code issue, or for your use only.

Yoyizit · 06-13-2010, 06:30 PM

Besides the exact dims you should measure sag while you put a weight at midspan of a rafter. I guess whatever weight causes 1/2" of sag is plenty.
This will give the wood E value.
Measure several and average the values.

proctrc · 06-13-2010, 08:33 PM

The runners are Hemlock Fir. It's a brand new garage so I would hope there wouldn't be any sag yet! I don't have any load on top of them. All I plan to do is put a couple of sheets of 1/2-3/4 plywood so that I can put boxes up there.

I don't want to over engineer the garage but on the other hand I don't want it coming down in a stiff wind.

My question has nothing to do with code. It's more of a question getting opinions on whether I need to add additional runners or I guess more specifically garage attic floor joists.

Scuba_Dave · 06-13-2010, 08:37 PM

Boxes of styrofoam ?
Boxes of metal scrap ?

The weight you will put up there will determine what you need to do

Yoyizit · 06-13-2010, 08:40 PM

Quote:

Originally Posted by proctrc

The runners are Hemlock Fir.

OK, E is 1.6 MegaPSI. Along with the exact dimensions and your OC spacing the dead+live PSF load for a given sag/deflection can then be calculated.

My Excel spreadsheet gives me 1.8 PSF with these values. Figure the dead load of the wood at 35 #/cu.ft.

A concentrated load of 67# at a runner midspan should give a 1.45" midspan deflection, taking into account the [dead load] weight of the wood.