Rafter ties / ceiling joists sizing

[Thenuker]

Hello everyone,

I'm currently building an attached garage 12x20x 16' tall, the first 4' is concrete stub wall with grade coving most with the rest of the wall 12' 2x6 on 16oc stick framed. I'm to the point that after I get the second top plate put on my walls I'm looking ahead to the rafter ties that will run the 20' from front to back. I will put plywood down on top of these on 24" centers so I have a platform to install the 2x8 rafters up to the ridge board. The roof will be a gable 6/12 front to back along the 20'.

I'm trying to decide if I can run the rafter ties as 2x6 that the plywood platform will sit on. This will not be used for storage and will not have any drywall on the ceiling as I will be spray foaming the rafters and roof sheeting.

Thanks for any clarity/help.

Jordan

 

[BN4537]

How far will these 2x6s be spanning? Are they parallel to the rafters? Have you considered that you need an approved thermal barrier or possibly an ignition barrier to cover the spray foam per R316.4 and/or R316.5.3?

 

[bill1952]

Just looking at the tables in the IRC, no way 2x6 rafter tie on 24" centers spans 20', and I would worry the temporary work platform with you and rafters piled on it would fail. Did I completely misunderstand the situation?

rafter ties that will run the 20' from front to back

 

[SCBO1]

If I'm getting the gist of this project right? Using the 2x6 ties with plywwod attached, but not for storage? If the ceiling joist tie is used as the ceiling joist.

IRC Table R802.5.1 (1) Ceiling joist spans
2x6 with uninhabitable attics without storage on 24" O/C
FAIL with no center support (bearing wall)
Some 2x8's may work without center support if the grade is available.

 

[bill1952]

My conclusion of using prescriptive requirements of IRC. Probably 2x10. But fine to use for temporary support during construction. I'd still be careful about stacking all the rafters up there and with me working.

 

[Mark K]

I suggest that if you consulted with an engineer sanity would prevail. I assume that you will be doing the work yourself. If you continue to proceed with what you propose you should have an ambulance on standby.

The IRC was intended to address a limited situation that did not provide for temporary loads during construction.

I suggest that contractors would find a way to install the rafters without plywood on the rafter ties. The rafter ties are generally assumed to also act as ceiling joists that definitely do not span 20 feet.

While the building department personnel generally have immunity but when they go beyond that role and advise on things that are not in the code, such as contractor means and methods of construction, they may find themselves potentially liable when there are problems. Thus I suggest that the role of this forum is to discuss building code issues and not provide consulting services to builders.

 

[bill1952]

Using ties that meet code - probably 2x10 - you could put a temporary support - a wall - across the 12' at mid span. That would temporarily make that a pretty strong platform for use during construction of the roof.

 

[Thenuker]

Okay so what I'm hearing is my idea is going to get my killed LOL. So if I stick an LVL in the middle span blocked onto solid 2x6 to the top of the concrete I can do 2x6 at 10ft on each side of the wall resting on the beam? That is no problem at all. I appreciate all of the replies.

Jordan

 

[bill1952]

I think so. Looks like any species 2x6 no. 2 grade or better will work for 10' span on 24" centers, 10 dl, 20 ll, "uninhabital attics limited storage" by IRC 2018. If not 20' pieces, I'd get 12' and overlap them. lvl should be selected and supported for 300 plf I believe.