Architect in need of help w/ room addition framing

[architect]

Hello forum,
I'd like to start off by saying how thankful I am that a forum like this exists. I've been doing commercial architecture since I graduated college in 04 but have been laid off for the last 8 months. I have a friend that wants to add an addition on to his house. Although I have limited knowledge in residential construction methods, I took on the project as something to keep me motivated.
I'm running into some trouble with a framing design. Please take a look at the pictures below for an idea of what I'm trying to do. I'd like to keep the new ceiling as open as possible.
1) Should I try to fit a scissor truss centered on the room?
2) Will this new load be too much onto the existing structure?

Any opinions or suggestions are completely welcome.
Thanks

[maj]

I'm running into some trouble with a framing design. Please take a look at the pictures below for an idea of what I'm trying to do. I'd like to keep the new ceiling as open as possible.
1) Should I try to fit a scissor truss centered on the room?
2) Will this new load be too much onto the existing structure?


Good morning Ryan,


Are you after an actual vaulted ceiling or a mono pitch ceiling like the cross section shows? A scissor truss will give you the vaulted look in the addition. As a general rule of thumb, the bottom chord pitch of a scissors truss can only be half of the top chord pitch. IOW, it looks to be about a 4/12 pitch on the roof you'll be tieing into. So the interior pitch of the bottom chord vault would only be about a 2/12 pitch. On steeper roof pitches, they can get the interior vault steeper.

If you're after the mono slope vault that is depicted in your rendering, you have 3 options..........
1) dimensional lumber...... IMO, a 17' span is too much for dimensional lumber, but only an engineer can spec. what size lumber to use for your particular region.
2) Parallel chord trusses..... would work good, but would probably be to big for what you're proposing. For that span, a parallel chord truss might be 18" - 20" tall. I'm afraid that would be to much for this instance.
3) Wood I-joist....... I would opt for this option if it were me. I've used I-joists for rafters quit a bit on some of my projects in years past. There's a learning curve, but all the details are clearly specified in the design information supplied with the I-joist companies. You could use the I-joists for rafters just over the living space (for the vault), then use dimensional lumber (2x6) for the remaing rafter forming the peak (attic area).


As far as an answer to your 2nd question....... Only a PE can answer that question for you. Depends on existing soil conditions, the existing footing & foundation size, existing roof design, etc.....

[RTF]

Not to bust your chops but if you are an architect you should have structural engineer friends that can help you figure this out.

[homebild]

Not to bust RTF's chops, but in most states, architects and structural engineers are equally qualified and equally certified to handle load calcs....

This novice architect has to this forum demonstrating a humilty that is rare yet notable for his profession. Asking those of us with actual structural/framing/building experience to help..

I'm moved enough by his honesty and sincerity to offer my help.

I think, therefore, that "scissors trusses" can easily work since the loads will only be on the bearing walls and their respective foundations.

As long as the footings, foundations and bearing walls are well sized, scissor trusses would be the way to go.

And as a building offical, I would approve the plans as drawn.

[architect]

Wow... excellent feedback Homebild and Maj!

Homebild, you hit the nail on the head. I came in here to get suggestions from people that have actually built these things with their own two hands. We run into so many problems designing structures how we "think" these things should be built when in all honesty, the guy putting this together has a much better idea of what will get the job done correctly without blowing the budget. Like I said, I have very limited experience with residential buildings.

Maj, you definitely know your framing. I was open to both ideas, those being a mono pitch or vaulted ceiling. It really comes down to two important factors influencing my decision - ceiling height and budget. I'd like to do a scissor truss yet still retain a maximum ceiling height. With the relatively low height of the existing roof, I'm forced to keep the top plate at a low height if I plan to retain the same height of the new fascia.
Besides ceiling height, the other problem I'm running into with a scissor truss is the fact that I will not be able to rest the lowest point of the truss on the existing top plate. I'd be bearing weight on top of the existing framing and plywood. If you can picture it, this would throw off the look of my vaulted ceiling.
I'm starting to lean toward a mono pitch. Using a 2x10 rafter, the highest point is only 11'-9".
I found a wood rafter calculator online here...
http://www.awc.org/calculators/span/calc/timbercalcstyle.asp
I have a few questions regarding the information I'm entering.
It would be nice if I could get away with 2x8 framing but according to the calculator, I would need a a structural rating lumber of 2250f (MSR) with rafters spaced at 16" o.c.
Will this drastically increase the cost?
Can you give me a price comparison of dimensional lumber vs. i-joists?

Thanks again for all your help!

[maj]

First off, MSR lumber is usually only purchased and used by truss manufacturers and they buy it by the truckloads. The price is not that big of a factor between MSR & regular dimensional lumber commonly stocked at lumberyards. Maybe 10 - 15% more. But getting your hands on such a small quantity may be difficult....... at least around here. I work at a lumberyard, and we do not stock MSR lumber. It's just not commonly used for hand framing.


Properly sized I-joists compared to properly sized hem/fir dimensional lumber is approximately 20-30% more.