How to reinforce 2×4 attic joists – basic questions
Hello,
I am in need of storage space and plan on using my garage attic area for storage. I recently had an access hole cut into the garage ceiling to check out the attic area. The home was built in 1968. The attic joists are only 2x4s and are 27 inch on center (i double checked). I talked to a structural engineer and he advised to sister the existing 2×4 joists. My ultimate plan is to sister the joists to make a sturdier attic ceiling, blow in insulation on the roof, and put plywood across the joists for a storage platform for my boxes (christmas decorations, etc.). And then ultimately put in an attic ladder. Here are my basic questions:
1) The garage is a 2 car garage- 20 feet wide x 20 feet. The roof is a gable roof. All the 2×4 joists run in one direction across the garage with one wood peice running perpendicular to the joists in the center of the garage. I do not know what to call this middle/center peice of wood. If I sister the joists, do I also need to add more support in the center? Do I need to add other perpendicular peices of wood in the middle? Sort of like sister the middle peice?
2) Is using 2×8 20 foot long ceiling joists overkill? Can I get away with 2×6 20 foot long peices attached next to the existing 2x4s?
OR, should I put in 2×4 joists every 13.5 inches? (In between the existing joists?)
3) What thickness of plywood would you recommend? I was thinking of using 5/8 inch plywood across my joists as a storage platform since the joists are 27 inches apart. The 5/8 would be sturdier but heavier on my ceiling. Is there another option?
4) To add extra 20 foot long joists, will my contractor have to rip out my garage ceiling? Or is there a way to just take out a small section of ceiling and then repair that peice instead of taking out the whole ceiling?
Please know that I will be hiring a contractor to do the work. I am just educating myself on a project so I don’t go in blind.
Thanks in advance for your help!
Replies
20-foot-long 2x4 joists would not hold up a ceiling, so obviously that piece in the middle is structural. Is it supported on posts, hung from the peak, or what? And how big (tall and thick) is it? (Since the middle piece is almost certaily supporting the 2x4s, the new pieces only need to be 10 feet long, lapped at the center.)
It would be silly to install 2x4 pieces. They should at least be 2x6 (which are more than twice as strong as 2x4s). And maybe they should be 2x8, depending on the expected load.
I'm not sure what code would require for this (if you're going to have it inspected), but for lightweight storage you could probably get away with using 3/8" plywood (though it would be a little bouncy with the 27-inch span). 5/8 should be plenty.
do you have a supporting wall in the garage? sounds like a strong back beam to me. if this is the case then you will have a much large project then you expect.
no, 2x4s wont be large enough.
if you have a supporting wall breaking the 2x4x20 span then you could use 2x6s and have limited storage.
What was your P.E.'s response?
Shelly01 wrote:
I talked to a structural engineer and he advised to sister the existing 2x4 joists.
What did the structural engineer of record for your project advise you to do as fare as sistering your 2x4 joists and sheathing your new attic floor?
A 2x8 will not clear span 20'. So no, it's not overkill.
I guess you could call it underkill ???