I need to suspend something from a 1.5-inch-thick rafter in my unfinished attic. If I put a 3/8" eye bolt through the wood, then connect a chain leading to the object, what would be the maximum weight for the object? What if I put two eye bolts, say a foot apart, then ran the chain from one bolt to the object, then up to the second bolt, would that hold twice the weight?
IANAE, but I think this would depend alot on the length of the rafter and where on the rafter you plan to put the bolt(s). What are you trying to hang?
Also, I don’t think you’ll be able to hang twice the weight with two hooks the way you mention, but you may be able to if you put the hooks on separate rafters.
Write back with the rafter size and clear span and you might get a somewhat meaningfull answer. The roof pitch would be useful also.
Putting a 2X4 long enough to cross 3 rafters placed at 90º to the rafters with the attach point right next to the center rafter but not through it should hold most house hold items. If you are trying to hang ‘silly’ weight, you’ll probably damage the house.
*Use large washers on the eye bolt to spread the load on the cross piece.
If it is a “heavy” bag or some such, you need to take into account the swinging weight and vibrations.
More info is needed.
YMMV
Yes. Vibrations and swinging can increase peak load considerably.
Can you link to a picture of your attic and describe what you want to hang?
Structurally, there are better ways to do it. If you put a 3/8" hole in a 1 1/2" board (I’m assuming edgewise,) you’ll weaken it. Going through sideways will be stronger. There are also steel strap fixtures commonly available that are even stronger. Talk to somebody at a lumberyard or home improvement store, and they’ll sell you the right thingie.
In addition, some added struts from the rafters down to the ceiling joists will help spead the load to more structual members.