Hybrid roof frame design: trusses and sticks - Contractor Talk - Professional Construction and Remodeling Forum

UpNorth · 08-18-2009, 07:04 AM

Stymied by a floor plan program that did not seem to allow any interior post-downs to support a stickframed roof for a modest cross-gable, I threw the sticks out and tried a design that relied on trusses.

Ever thrown up an arrangement like this?

A rendering of the building, 37' x 44' in plan, is shown. Roof pitch is 9:12. Total loading, snow and dead, amounts to a little over 75 psf.

UpNorth · 08-18-2009, 07:07 AM

Here is what the sticked roof might have looked like, if I had an inside-walls arrangement that could have contained the posts for getting the loadpaths somewhat centralized. I did not.

The posts you see get the loads into some upstairs walls, but there is no place to go from there. A truss scheme became necessary.

UpNorth · 08-18-2009, 07:11 AM

The solution using trusses is possible because of the large amount of overhead between the vaulted (or flat) ceilings of the central and under-gable rooms.

Across the short span of 37 feet go three-ply girder trusses, spaced in about 12 feet from each end. They look pretty much like this.

UpNorth · 08-18-2009, 07:14 AM

With the big girders up and braced, the center is filled with spanners that have scissors bottoms where they go over vaulted ceilings. The xray view attached here shows what is going on.

UpNorth · 08-18-2009, 07:17 AM

The overall truss scheme is shown in this pic, and you can see how the 12x12 valley area in each corner is stickframed.

Sticks are 2x12 rafters on 24 centers, with the valley being a two ply sandwich of 11-7/8" LVLs.

UpNorth · 08-18-2009, 07:19 AM

What do you think will take longer, setting and bracing the cruciform arrangement of trusses, or stickframing the corners?

Willie T · 08-18-2009, 08:04 AM

Time-wise, trusses are the way to go. You have to handle and install both sides of a conventional roof. With the trusses, you also have places to run ducts and other utilities.

I also feel a 9:12 vault might be a little too much visually.

Willie T · 08-18-2009, 08:18 AM

Boy, I'd love to see that in a skp. file with all layers turned on. Nice job!

Birch · 08-18-2009, 08:26 AM

I have done a couple similar roofs over properly engineered slab foundations. (Down South) I prefer to use trusses for double roofs (as you show) if possible, and conventional frame the rest, especially with really long unsupported ridge lines. Hybrid roof framing in these scenarios is not unusual for me.

Note; we did not have the girders since there were interior load bearing walls to set the scissor trusses on. Your girder trusses appear to be transferring the roof loads to the perimeter bearings as if you want the entire roof clear-spanned. If you can eliminate the huge girders, or at least, if possible, shorten them to bear on the interior, that would be something I would consider.

There are other framing configurations possible too. The foundation engineering is required to know what else you might consider. Of course, we’d like to see the roof engineered first and the foundations engineered underneath accordingly.

JamesKB2 · 08-18-2009, 10:36 AM

Structural ridges?

If you laid two structural ridges into the main roof section, with each ridge's height at the dormer's ridge height, and then piggy backed higher to get your main ridge height, you MIGHT clear span the main ridge line. Your main ridge rafters would die into the structural ridges, (there would be two, one on each side) and then carry on the plane with short rafters laid on top.

You could also make the dormer's ridge structural too. You then have two "T" shaped ridge formations.

I don't know how beefy that LVL would have to be, but I think it could be done (quad 14"er's...just a guess) for 44' clear span. Otherwise, you would just need one support column under each intersect point at the "T".

Scratch the 44' span. You're spanning the 37' with a main ridge. (Mixed that up!) So, one column at 18.5' at the T. Totally doable.

UpNorth · 08-18-2009, 04:00 PM

Thanks for your comments, James.

I have a solution, and it is as I showed. I rejected everything "stick" because I realized I had no post-down opportunities in the center area that could result in load paths straight down to bearing.

Why should I reconsider anything in that central cruciform area that's sticked, seeing as how, a.) it cannot be adequately supported, b.) there is a great solution and it involves trusses, and c.) the trusses eliminate the need for most secondary ceiling framing?

JamesKB2 · 08-18-2009, 04:32 PM

No problem!

I got the impression that you were partial to stick, so I offered a work around. If you've found a solution, great!

I guess your post was more about "what do you think?' than "what can I do?"

Sorry about that.

And yes, I've done hybrid roof builds. They work out fine.

Warren · 08-18-2009, 05:06 PM

Virtually every new home roof we do is a hybrid. Trusses are nice for big spans. We recently did a 15000 sq ft 3 story that had a couple of 4 ply, bolted attic girders. Whole 3rd floor was attic trusses that hung from the two girders. I actually think a trussed roof performs better when long spans are involved. Installation is easier too. That 37 ft span you show on your plan would probably result in a steel I beam ridge. Wouldn't that be fun?

**framerman** · 08-18-2009, 06:49 PM

I designed and built one myself. I would not do it with trusses. I think you'd lose too much headroom from your perspective. I can't find the pdf files I had ATM. It's been a few years. I'll keep looking for them.

You have an upper and lower ridge there. What I did was run full length ridge beams on both. Ran it right through (or under) the other one.

ClemS · 08-18-2009, 08:27 PM

i'm with framerman on this one. i have "0" faith in trusses. and a truss girder, honestly, i think is a joke

. i don't care what the software tells you, trusses move under load. a lot.

i'm a firm proponent of the good ol' I-Beam. nothing beats steel for loading. i've also had good results with paralams and built-up lvl's on shorter spans.

JamesKB2 · 08-18-2009, 09:01 PM

Quote:

Originally Posted by framerman

I designed and built one myself. I would not do it with trusses. I think you'd lose too much headroom from your perspective. I can't find the pdf files I had ATM. It's been a few years. I'll keep looking for them.

You have an upper and lower ridge there. What I did was run full length ridge beams on both. Ran it right through (or under) the other one.

That's a better solution than mine sir!

I don't know why I didn't see that.

Timuhler · 08-19-2009, 09:57 AM

Quote:

Originally Posted by UpNorth

Here is what the sticked roof might have looked like, if I had an inside-walls arrangement that could have contained the posts for getting the loadpaths somewhat centralized. I did not.

The posts you see get the loads into some upstairs walls, but there is no place to go from there. A truss scheme became necessary.

Gene,

You know me so you know I'm partial to stick framing a roof like that. I would love to see what that main ridge calcs out to be to make the span, and then supporting valleys to the ridge.

Any idea on what the main ridge and the valleys would size out to be? I skimmed the thread so if I missed it, my fault. I just got home from Europe and had a long day of travel yesterday and very little sleep :-)

By the way, you should see the beams in Switzerland up in the Alps, those houses are stout and some are more than 300 years old. Beautiful work. Every job has a small tower crane next to it for all the heavy materials.