In attics built purely for storage purposes you may find 2x6 ceiling joists or in some cases even 2x4 joists.
Attic joists 2x6.
Many a garage has been loaded down for decades the way you suggest and out and out failures are rare in spite of the rather inferior garage construction techniques used 60 80 years ago.
The 2 6 joists will sag no doubt says danh.
2x4 s actual size spaced 16 on center running the width of the building uninterrupted with the ends resting on top of the wall top plate.
Each joist is strong enough to support my weight 175lbs concentrated in a 1 segment of the joist and.
This is made up of ceiling joists typically 2x6 or larger boards.
If your joists are 2x6s or 2x8s they are not adequate for supporting attic flooring in a new living area and the furnishings and humans who will use them.
With a span like that the timber size that you sister in would have to be at least 2x10 s.
Flooring for dead loads may not be sufficient for live loads the loads created by people and the features of living space.
Look this up in a joist span table available in numerous places on the web if you do not believe me.
I d like to double my ceiling joists to make my attic space easier to work in for electrical plumbing and for lightweight storage.
They will probably not collapse and the load will probably not compromise the structure.
Achieving greater r values in attics the higher the r value the better the thermal performance of the insulation.
If the joists are larger they might.
As long as they are not damaged the joists should be strong enough to allow you to move around in the attic for an inspection and to provide storage for typical boxed items.
Although you can construct a garage using thicker stronger joists 2 inch by 6 inch joists are sufficient for most garages even if you intend to build an attic storage area above your garage.
The floor structure of an attic is also the framing for the ceiling beneath it.