New Truss Roof Over Existing Ceiling Structure?

[TOMinSATX]

Hello All,
I’m a remodeling contractor in San Antonio, Texas who specializes in handicap modifications and adaptation. I’ve been working on a pro-bono project off and on for the last year for a family who needs (and wants) several changes to their late 1940’s home on a limited VA funding. They’ve finally got the approvals for the money (plus some of their own) so now we can move onto getting others involved.
While moving interior walls and other such items is not a problem, I’ve been asked to design a new roof for them to allow attic storage, new mechanicals installation, as well as additional insulation space. The key problem is to retain the existing ceiling (and its structure). Several contractors I’ve talked to, in general would prefer using manufactured trusses to built up. As I understand it, the only thing that the local permit people want engineered is the foundation.
I’ve attached a drawing with what I’m proposing; my “job” is to get this designed as close to correct as possible to reduce time and costs down the line. I expect the licensed contractor who will tackle the addition and roofing portion of the job to get the truss mfg to work up the final solution.
My main question is; can we run the new trusses over the top of the existing ceiling joists? Everything I’ve found says that the truss has to bear directly on the top plate. I’m hoping that we can block between the existing joists, and run the trusses over them on install CLR 2x's as you would for a piggy-back installation. Any input or suggestions would be appreciated.

 

[ToadJones]

I have built new houses similar to what you show here.

Second story floor joists with a rim joist, then I installed a 2x on top of the joists on which either rafter bird's mouth or attic trusses rested.

I think a rim joist (which is probably not there now) will be necessary for the lateral stability of the exst. joist

I always made sure the trusses or rafters lined up with the joist below.

 

[ToadJones]

Tying things down for wind is something you may need to look at with some Simpson hardware.

MSquared, this is in your wheelhouse, no?

 

[msquared48]

I agree with Toad here. Span the new trusses to the outside walls, and berar on a new double top plate placed above the end of the ceiling joists. As the structure probably has a lathe and plaster ceiling, you need a gap for deflection of the truss so that the ceiling will not be impacted.

The foundation width wiill have to be verified for bearing.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[ToadJones]

Something dawned on me here....
I know your detail doesn't show it as such but the existing ceiling joists might be cut on the ends to mach the roof slope. If that is the case, I'm not sure the detail will work very well without some re-work

 

[TOMinSATX]

Thanks for both of your input and suggestions.

Oddly enough, the ceilings are 5/8” sheetrock (old sheetrock). While I haven’t seen if the rafters were bird mouth cut, I’d have to guess and say yes the ceiling joists are trimmed at their tops, which is one of the reasons I was headed in the 2x6 blocking direction, rather than a rim joist, not to mention the extra work of trimming each of the joists back. Cross checking my measurements inside the house against the exterior indicate that there isn’t much off the top, if any. I may get the owner to let me remove one of the venting blocks and see how things were done (add item #54 to my list).

The foundation drawings on the ancient blueprint study plan set the owner loaned me, shows 10” walls supporting the 5” rebar reinforced floor over compacted base. We’ve got an abundant supply of limestone around here which is used in just about every house, because that’s what most of them are setting on to begin with. Since the load increase is minimal, and there are no signs of foundation problems, I’d have to guess that the loading will be within limits. We have to get a foundation engineers seal on the addition, we can check with him on this matter as well (item #55 on my list).

Now I need to throw some confusion into the mix…
Since the attic space will be for storage (Lord knows what) my original thought was to have the bottom chord fastened to the CLRs running underneath with brackets and vertical nails, with them in turn vertically nailed to the top of the CJs. (Drawing “A”) This would distribute the load to several joists at a time when the ½” plywood flooring was installed over the insulation and on top of the bottom chord.

At this stage, my design only has a single 2x6 plate on top of the CJs. Do we need to raise it another 1.5” and build the floor on top of that and let it float? The one hallway wall (the 12-8 span) runs the full length of this section and is load bearing (purlin struts lead to it). So should we at least brace over that wall like we’re going to do on the perimeter (between the btm chord & CJ)?

If I understand the two of you correctly, “B” might be closer to the answer…

Drawings attached, and thanks again for your time.

 

[Ron]

One thing you need to remember is that this plan is likely going to require a structural engineer to validate. Check with your local building official.

 

[MiketheEngineer]

All good ideas and done quite frequently. Many years ago I spent about 3 months designing pitched trusses to go over about 200-300 flat roofs at one or military academies - they like airplanes.

Why - BECAUSE the flat roofs LEAKED!!

Get with a truss mfg early on and they can/should be most helpful!! If not - find someone else. They are all hurting for work and will bend over backwards to get a job!!

 

[TekEngr]

i m trying to design a PEB roof system on existing RCC column but i m confuse what i should do i should design the roof system as fixed or as a pinned support on staad pro softwear.
and which part of the roof is more critical, knee side or ridge side.

 

[ToadJones]

shahg123-
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