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Reinforcing fink roof truss with sistered 2x12s

Reinforcing fink roof truss with sistered 2x12s

Reinforcing fink roof truss with sistered 2x12s

(OP)
Hi all,
Aero/Mechanical engineer here with a home dilemma. Google results kept sending me here so I figured I'd join. :)

Want to add attic storage. House was built in 1978 with manufactured 2x trusses. House is a colonial (nice simple rectangle). The attic section in questions is about 28' front to back (truss span) and 30' across. Trusses are 24" OC.

Single-W fink trusses made out of 2x4 material all around with metal plates. Obviously factory made, but I can't find a stamp or marking. Attic has an access hatch, but no storage floor. Feels really solid to walk on up there too (did bath fans and alarm / network wiring up there).

Central hallway has partition walls in roughly the middle of the span, including one side which is directly over the bearing walls/beams of the first floor. It is a single top plate and does not run the whole span of the 2nd floor, so it is not likely bearing. The top plates of the walls do tie into the bottom chords and the drywall is directly attached as well.

The dilemma:
Currently only 3.5" of blown in insulation up there. Want 10 to 12". We also want/need storage up there. Mostly light stuff like clothes, toys, old closet doors, spare trim molding, etc. So I'd have to build up above the bottom chord, blow in the insulation, then put a floor down for the storage.

My original plan was a 2x12 "deck frame" running across the joists to make an 8' x 12' platform and use plastic Attic Dek for a floor. That was until I lifted one of those. Each beam there weighs about 100 pounds! So we'd be looking at about 600 pounds of just structure there. With the walls underneath supporting it I'm not as worried, but my gut told me to run the numbers.

An alternate plan would be to sister 2x12's along the bottom chord in the center span area to make the platform about the same size. I'd call the truss company, if I could figure out who they were and if they're still in business. I see it stiffening the tension/bending of the bottom chord, but putting more weight on the truss system as a whole.

Thanks in advance for any help!
Anthony

RE: Reinforcing fink roof truss with sistered 2x12s

arc2v - IMHO, your proposal to use sistered 2x12's has potential; especially if you beef up important connections... say by adding plywood gusset plates using screws. To get a better idea, we need to know the pitch of the roof (e.g. 5:12, etc.)

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Reinforcing fink roof truss with sistered 2x12s

(OP)
It's an 8:12 roof, pretty steep.

Thanks for the help!

Anthony

RE: Reinforcing fink roof truss with sistered 2x12s

Anthony - Steep is good... when it comes to truss structural properties. A steep slope makes the geometry of the members much more favorable for force transfer. I have looked at the forces which your proposed limited attic storage would add - see attached page 1 of 2. IMHO, this project may be technically viable, but just barely.

Suggest you consult a local structural engineer to evaluate the existing trusses for the various load combinations required by today's building code. If the trusses do have some reserve capacity, I have included a conceptual sketch that should accomplish the following:

1. Reduce dead load by using 2x8 stacked on top of truss member 2x4 instead of sistered 2x12.

2. Upgrade tension connectors by adding plywood gusset plates to existing metal connectors.

3. Improve constructability by using the gusset plates to anchor the 2x8 instead of having to field connect 2x12 to the existing truss.

4. Optimize the location of the added load by using plywood spacer plates to ensure the new load is applied at points as close to existing truss joints as possible.

5. Maximize the ability to prefabricate the new components before taking them into the attic (i.e. cut 2x8 to length / install spacers on bottom of 2x8, cut and drill gusset plates, etc.)

6. Reduce cost (2x8 vs. 2x12).

This concept sketch is page 2 of 2.

The important thing is to get the existing trusses evaluated before proceeding - this whole effort may not be acceptable.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Reinforcing fink roof truss with sistered 2x12s

(OP)
Wow. I just expected a "oh no, don't do that" or "yeah, people do that all the time". You actually ran numbers! Thanks!

It looks like these calcs consider the truss alone with point loads on the outside. What about building across the chords, but directly above the partition walls so the load is transferred through the truss, but puts no moment on the truss itself. It would be a smaller storage section, but I'll take that over worrying about the roof integrity.

Friend's dad is an architect; I'll see who he uses as a structural engineer locally. Might get up there and give another go at finding a stamp or other marking on the truss. Are these usually on one particular strut or chord? I'm hoping it's not under that insulation. So dusty.

Thanks again.

RE: Reinforcing fink roof truss with sistered 2x12s

The classical approach to truss analysis that I used for your project is called the "Method Of Joints". It assumes that all forces acting on a truss are applied only at the joints. That's why the sketch shows the loads and reactions external to the truss. The concept that I suggested is intended to mimic the analysis. There are other ways that can be considered, but then a more thorough analysis of the details (likely using software) would be called for.

Get the local structural engineer to investigate your idea to build directly above a (load bearing) wall, that may work out.

P.S. Your project sounds like a challenge... had to run the numbers to see if it was feasible. A few decades ago, I started my career as a Mechanical, working in the Aerospace business.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: Reinforcing fink roof truss with sistered 2x12s

(OP)
Thanks again for the info. The tough part here is that it's just attic storage. Hard to justify major restructuring and thousands of dollars to hold some dusty old boxes and winter clothes.

I would have just put some attic-dek up there and been done with it, but the last two winters here were very cold (Virginia).

I'll see what the PE says.

Thanks,
Anthony

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