Modifying roof trusses to support living space

[FS7]

I know this is a common question (or a common project). I am aware of what is involved. I have a 20x20 garage and I plan to convert about a 10x20 space into living space (it borders one of the master bedrooms).

My plan is to run dimensional lumber from each end of the garage to the other, meeting in the middle and resting on the top plate, and sister with additional lumber to form a spliced joist. I will put a board on top of the new joists, running perpendicular, and on top of these will be the vertical supports (purlin props) that attach to the rafters. Further up, collar ties will attach each rafter to the other. This effectively largely converts the existing trusses into attic trusses (in some sense building new trusses along side the existing trusses).

One thing I am not sure of (or if it even matters at all) is whether the new floor joists should be in any way attached to the existing joists (which will at this point effectively only be supporting the weight of the ceiling drywall). The way I see it, it adds load to the new joists, though not significant, while strengthening the existing joists (which don't really need it). Again, I am not sure if it matters, but my instinct is to not connect the two.

[srv52761]

Probably asking the wrong people.  An engineer/architect would be a better bet.

Something to discuss, as I was an energy manager in my previous life... How do you get the required r-value in your ceiling.  Even with foam, you're only  looking at maybe r-30 if your top cord is a 2x6.  (Not sure of your locale)

[FS7]

My father was a civil and structural engineer for most of his life so that part is covered. We've walked through the design.

The last part I was not sure of was the insulation. If the bottom chord is 2x6 (it will either be 2x6 or 2x8) I think it will be sufficient. It's over uninsulated space (the garage) but it is not technically exposed.

What i was more worried about was the portion along the rafters. That portion was the only part that would use the original trusses, which have 2x4 rafters. I could insulate that as any exterior wall, but this will impede airflow up to the ridge vent. I believe simple air chutes will work, but whether standard batt insulation will be sufficient after that is unknown.

[ez-duzit]

My father was a civil and structural engineer for most of his life so that part is covered. We've walked through the design...

This reminds me of a scene in "Groundhog Day" when Bill Murray is complimented on learning to play the piano so quickly and replies that, although he'd never had any piano lessons, his father was a piano tuner.

Your project requires somewhat more than a "walk through the design".

[FS7]

This reminds me of a scene in "Groundhog Day" when Bill Murray is complimented on learning to play the piano so quickly and replies that, although he'd never had any piano lessons, his father was a piano tuner.

Your project requires somewhat more than a "walk through the design".

Perhaps it wasn't clear but we've gone over it in detail. It's not rocket science to somebody who dealt with structural issues like that on a daily basis.

[TDKPE]

This reminds me of a scene in "Groundhog Day" when Bill Murray is complimented on learning to play the piano so quickly and replies that, although he'd never had any piano lessons, his father was a piano tuner mover.

Sorry.  I couldn’t help myself, as that’s one of my all-time favorite movies, and I know that line.  

Along with almost all the other lines.  

I really need to get a life.  

[jamesglenn]

Modifying the roof truss in anyway changes the tension/compression of the various chords. changing them to an attic truss will change the loads on the bottom chords despite just holding the drywall now. It has to be reviewed as a whole system and not new and old. 

I would guess the bottom chord 2x4's are not one piece? You would need to make sure the splice occurs over the new wall.

If I am following you correctly, there will be a new beam or wall centered in the garage to support the new floor joists? These need to be sized for that span, and I would calculate the dead load of the ceiling in it and attach the existing bottom chords as well. You need to calculate the nailing of these per code.

If the wall is existing then the joists need to align with the studs and make sure there is mid wall blocking to stop deflection. The studs of the new wall upstairs need to align with the floor joists to transfer the load to the concrete which needs to be verified can support the new loads being pointed loaded on it.

As for the insulation, you need to review the building code for your locality. Because it is an unheated garage below, you need to meet certain insulation requirements. The roof and walls need to meet these as well.

Assuming you are getting a permit to make these structural modifications which will also require egress review, electrical inspections, etc.; you will need drawings to submit to the building department.

I know many do things without a permit, and I am no fan of big government; but it becomes a liability issue if something happens.  Home owners insurance will forfeit, lawsuits, not able to sell your house one day because a permit was not pulled, along with numerous other potential issues.

[Snipe Hunter]

From the perspective of a home inspector:
When I see any major structural modification, I take note. If it was permitted, I can usually find a county inspection sticker and date on the main service panel. Depends on the county though. It will have plumbing, structural and electric inspection dates and sign off signatures. But it doesn't tell the whole story, it just helps me piece together when modifications were done, it just doesn't spell out where the modifications are in the home.

When I see truss, beam or joist modifications, I recommend the buyer ask the seller to provide documentation of the modifications. That might be the original permit for the modification or it might be the engineered drawings and calculations. They usually don't have or haven't saved this information. The permit office requires an engineered-stamped drawing with the calculations to issue a permit for a structural modification and that will be on file at the permit office. I don't chase permits.

This is just a heads up if you ever intend to sell the house.

If I see a problem with the modification like movement (sagging ceilings, drywall cracks, uneven flooring, skewed door frames etc) I recommend consulting with a structural engineer and 9 times out of 10, the seller winds up paying for it to save the deal. It can easily be $1000 for an engineer to come out and look at it and provide a letter .

[srv52761]

.....   The permit office requires an engineered-stamped drawing with the calculations to issue a permit for a structural modification and that will be on file at the permit office. I don't chase permits.

This is just a heads up if you ever intend to sell the house.

If I see a problem with the modification like movement (sagging ceilings, drywall cracks, uneven flooring, skewed door frames etc) I recommend consulting with a structural engineer and 9 times out of 10, the seller winds up paying for it to save the deal. It can easily be $1000 for an engineer to come out and look at it and provide a letter .

And if they don’t sign off, you’re screwed big time.

When I removed a lally  column in my basement, besides the city inspector, my insurance company required a letter from an engineer.  The letter traveled with the deed when I sold the house.  This was a long time ago and your project is more complicated and I am sure the prices have gone up, but in 1996 it was the best $400 I ever spent.  He speced the size and number of beams, the nailing pattern, the supporting posts, the attaching straps, found the original blueprints and verified the footers under the supporting lally column were sufficient, and supplied documentation for me to present to the AHJ when it was inspected so he could just go through a checklist.  I did all the labor, but with the engineer’s specs, it was just follow the dots and I was done.

[Snipe Hunter]

And if they don’t sign off, you’re screwed big time.

When I removed a lally  column in my basement,

I have one in the middle of a 2 car garage. Dumb. One of these days I'd like it gone. Can't open the car doors.