I would like to add storage space into the attic area of my garage. My garage is 20' wide with 2"x6" rafters 24" OC.  The roof angle is 5/12. All word work is exposed.  There are no trusses. The walls are made of 2"x4" studs 16" on center.

Over one portion of the garage the previous owner put three chords that span the entire width of the garage. Each chord is comprised of two 2"x6" pieces connected end-to-end with a 4' piece of sistered 2"x6". The chords rest (are nailed) on top of my garage wall top plates. The spacing of the chords is roughly 48" OC.  That means the chords cover an area of roughly 8'x20'.

I want to place some OSB on top of the chords to store stuff (roughly 150-250 lbs of random stuff). But, I am doubtful of the chord design (quite frankly, I don't even see what purpose the chords are performing right now).

What options do I have to utilize the roughly 20' wide by 8' long by roughly 4' high (roof angle is 5/12) space in my attic that the chords span?

I have some ideas but I'd rather pose the question than pose my guesses.

Thanks for your help.
 

Doesn't sound like a great idea.  This type of roof was never meant to carry much in the way of ceiling loads --- maybe drywall.

Will it hold 150-250 lbs - probably.  If you can get up there and walk around SAFELY - that will give you your answer.  But over time, you will start to see the 2x6 "creep" down.  Wood does that over time.

Suggest you hire a local structural engineer and he will be in a much better position to explain what he sees and what he thinks will happen....

Good Luck..

The existing bottom chords resist the outward thrust of the rafters on the walls.  They ARE NOT designed for gravity loads, DO NOT place loads on them.

A structural engineer, or a framing contractor, will look at the span length and the loading requirement (minimum attic floor load, NOT the small load you want to impose) and size joists for the span.  A quick Google will find span tables for the wood available (probably SYP - southern yellow pine, or SPF - spuce-pine-fir).  You should expect something like 2x10's at 16 inch spacing.

Also, run a couple of strongbacks across the top of the joists from end to end to keep them from rolling over, or plywood the attic floor from end to end down the middle.

If it is inaccessible, and you just want to place a few pieces of pipe or lumber on the "attic floor", then you might be able to place a few additional 2x8's to hold the load, but I'm not going to tell you that in my capacity as an engineer, since things can easily go wrong.