Home addition, Partial Demo of Existing Gable Roof, New Roof Rafters

[dukengnr79]

Hi all,

I've attached a sketch of a minor dilemma I'm facing while doing a "courtesy" read on an architect friend's home remodeling project. I'm a P.E. structural engineer, but the extent of my experience in wood design was in studying for the P.E. exam...

1.) The proposed design calls for partial demolition of half the existing gable frame [is it good enough to just add notes indicating temporary bracing is the contractor's responsibility?]
2.) Those existing rafters (only 6" deep at 16"... ) are being shown as spliced with an additional 6" rafter for about 2/3 of it's length to extend up to the new roof ridge height. Problem?
3.) Is it common to have the bottom of the roof, where the rafter sits on the wall at a different elevation in the back of the house vs. the front? More specifically, in order to resist the horiz. thrust at the back of the house, I'd have to use ceiling joists and tie into those existing 6" "doubled up" rafters on the front.
4.) Any particular reason you'd avoid framing differing depth rafters at the ridge board, assuming the ridge board matches the depth of the deeper rafter (14" depth on the back side, 6" depth on the front side..)
5.) Anything else?

I haven't crunched any numbers on this yet, was mostly just looking for a sense from the community here on a scale of 1 = no big deal to 10 = run away from this a fast as I can.

Thanks

 

[Once20036]

I also don't have much experience with wood, but I`ll throw in my two cents.
Who created the drawings? The contractor? The architect? A structural engineer?

1) I would leave temporary bracing to the contractor.
2) Feels a little off to me. If the original roof used 2x6s, they may not have enough reserve capacity to span 20% further.
3) My house bears at a different elevation front and back. I haven't dug into the framing details, but I assume the ceiling joists tie in, as you describe.
4) Intuitively I don't see an issue, but I`ll defer to the folks with more wood experience.

On a scale from 1-10, this rings in around a 4 or 5 to me. It might be no big deal, but I wouldn't recommend a friend or family member proceed without digging deeper into item #2.

 

[BSVBD]

We all have "courtesies" to attend to. You need to make the call as to whether this courtesy is critical to your personal and/or professional relationship and reputation. From my experience, regardless of how messy things appears, there is typically a way to make it work. In my 20 years, the only time I've said no, was when a local city inspector, suspicious after his own, initial inspection, asked our firm to inspect a home-builders' wood framing in progress. I DID inspect, but, I did NOT approve. The reason I bring up that situation is because it reminds me of the splice condition that your sketch is proposing. See #2 below.

1.) General contractor must provide temporary support of existing structure.
2.) Do NOT simply splice the 6" rafters. In the end, will a 6" rafter work? The ceiling joists will NOT provide enough support to consider the 6" rafter to cantilever to reduce its' span, unless you design some elaborate truss type connection which does not appear feasible. My advice - Size the rafter, then, either, remove the right side of the existing roof also and put in new rafters or put new, full-length rafters along side the existing. People who have enough money to hire either an architectural or engineering consultant for their residence, certainly have enough money to go the extra mile to have their home designed safely.
3.) It certainly is common for differing elevations. What you have is similar to a "salt-box" house. Not understanding the whole picture of this house, the ceiling joists seem to be a reasonable tension tie / collar tie solution. Consider this - are you creating an attic potential on the right side of the house that may add additional load that the original design was not designed for? We're not talking extremely high loads, but, again, without the full picture, don't neglect something simple!
4.) No reason to avoid framing differing depth rafters at the ridge board.
5.) Anything else? - We don't have the full picture. It appears you may be considering removing the left, orange wall? That doesn't look good. Do you have a support solution? You didn't inquire of that situation so I'm assuming you are aware of all other, undiscussed, uninquired about, structural necessities. Don't neglect anything!

1-10? 1? 2? No BIG deal. Just, assure the client that they want a stable, safe house. Don't skimp on that splice. Do it right!