Width, pitch, loft size, truss, rafter, so many decisions. Help!

[grover]

I want to do a 24 ft wide 1 1/2 story with a loft.  Originally I got a lumber quote it gave me scissor trusses, 12/12 outside and 7/12 inside pitch.  The loft was figured with room trusses with an 8 ft ceiling.  After some thought I don't think the combo will work together well so back to the drawing board.

I'm thinking about bringing the roof pitch down from 12/12 to 10/12 (just for the ease of building.)  I talked to a truss guy about a beam that would go the length of the cabin which is 28 ft.  He said he could do a truss beam (not really sure what that is called).  The beam would be 3 ft x 28 ft.  If I did 10 ft side walls I think that would give me enough room in the loft to do 8 ft ceiling and have a 12 to 14 ft wide room.

The other option would be to forget about the truss beam and do a beam of maybe a 2 x 10 and use rafter ties.  You still get the cathedral effect and the structural integrity. 

Am I making any sense?

How would you do it?

[mgramann]

While we are doing a full second floor, this is how we are doing it on our 24' wide build.  We are using what I believe are called parallel chord trusses with a "piggyback" on the top.  Experts please forgive me if my terminology is incorrect.

Here is a view from the outside.  It shows the truss without the piggyback on top.


Here is a view from the inside.  It had 9' ceilings-more than enough room for a loft effect.  With 30" kneewalls and the 12/12 pitch, a 6' height is achieved about 42" away from the wall.

[MountainDon]

The structure is 28 feet long, end wall to end wall?? 

Any place in between the two ends to insert a support column?  That would shorten the length of the structural ridge beam and reduce the needed depth of the ridge beam. Structural ridge beam removes the need for any rafter ties at all; there are no outward horizontal forces from those rafters. Frame the roof with 2x12 to allow R38C insulation in the roof rafter bays. Make the side walls tall enough to provide sufficient headroom in the loft area; 10 foot overall should work I think, but I have not drawn or calculated that. Simple enough with some graph paper. If walls in the cathedral ceiling open area are taller than 10 feet some lateral bracing will be needed. That can be worked out. 




Loft floor could be floor trusses or I-joists, or if there was an interior load bearing wall 2x dimensional lumber.

Work out all these details and then you know what the foundation will have to be.

[MountainDon]

The beam would be 3 ft x 28 ft.

that throws me off... 28 ft long okay; 3 feet in what other direction.... must be a typo?   

[Don_P]

Not at all, a girder truss instead of a ridgebeam. Remember that the internal forces in a beam go down dramatically as the beam depth increases (deeper is cheaper). By making a heavy parallel chord truss 28' long and 3' deep they have reduced the forces to the point that 28' is attainable... it's a neat solution. The end reactions are going to be large, 1/4 of the roof load at each end. I like mgramann's trusses as well, no point loads coming down from the ends of a beam, instead many smaller distributed truss loads along the wall.

[MountainDon]

ok... like a deep floor truss, lengthwise..   Need to exercise my imagination more.   

[grover]

Sorry if I didn't explain it well but Don P has the right idea.  I believe he said the beam would be 2 x 6's so 6 inches wide and 3 ft tall spanning 28 ft.  It would be covered by a flat top ceiling.  I didn't want any support columns but if there were it would have to be at the loft opening and I would have to plan the foundation to support that area.  I like mgramann's trusses but the only problem I see is the design automatically raises the roof a couple of feet.  I think a 24 x 28 with 10 ft walls and then another 2 ft of added truss height might look a little out of scale tall.

How does the idea of exposed rafter ties sound.  I'm not opposed to the ties showing as long as they can be made to look a little rustic.  Maybe rough cut lumber.  That way the weight would be more evenly distributed than with a beam.  Even exposed trusses would be great.  See if this link works.  http://www.ryanwholesale.com/0ba239a0.jpg
Does that mean I have to find a 24 ft continuous 2x?

[Don_P]

The 2x6's in the girder truss would be up on edge most likely so 1.5" thick, per ply, sometimes multiple trusses are ganged together into a thicker assembly, like a built up beam. I cross an old truss railroad bridge often. The trusses are about 24' tall and span about 60' and carried trains full of ore. It now does light duty with concrete trucks etc, but it is close to a century old. No members are wider than 8" steel with most of it out of angle iron sections built up by riveting shapes together. We now use deep, heavy I beams to span creeks like that, our cranes are much larger than the old timers had so they used many small parts to build a deep truss rather than one huge beam in simple span. Funny thing is the modern bridges on each side of that truss bridge have been totally reworked while the old bridge which they are spiking keeps humming along  .
The ties in the linked pic are raised as high as they can be... 1/3 of the way up the roof height. That works fine in the greatroom area but wouldn't work in the loft. It's also an engineered solution. Let me know if you want some heavy timber trusses.

[grover]

I really like the look of the trusses in the pic I linked.  I know it would blow my budget out of the water though.  I also really like the look of the rough sawn lumber in a cabin.  I want to do the loft floor with them and leave it exposed over the kitchen area.  I saw a cabin that used rough sawn around the lower part of the walls and drywall on the higher part.  They also covered the seams of the drywall with a 1x4 board which saves time finishing drywall.   

Don P
Let me know where you are located.  If you are close I might be interested in your offer.

[Don_P]

I'm in southwest Virginia near Galax in an area renowned for its' eastern white pine. I can't get into my photobucket album at present, it has some pics of trusses we've done scattered amongst a whole lot of other junk. If you're bored and have high speed I think you can view the entire album here;
http://photobucket.com/images/windyhilll

When I say this is an engineered solution, properly an engineer designs the truss for the job since this is outside of prescriptive. I designed the trusses you see in the album, a structural engineer friend has visited and we've talked through it, incorrect. Correctly you would hire an SE that is registered in your state and we would all co-ordinate on a design that he (or she) could stamp. Then the next hurdle is grading of the timbers, correctly all materials have been graded by an approved independent third party, typically $400-500. Our inspectors have allowed an engineer to do this, although not technically correct that is fairly common. So for wood from my sawmill we go that route. There is also a nearby company that has a grading stamp for use on their log homes. They are not allowed to sell me a timber that we call graded although they have graded it... technical bs. Our inspector has allowed me to use those as graded timbers. He is trying to meet the intent without breaking our backs too hard. I'm just trying to bring you up to speed on the letter of the law. Engineering for stock non prescriptive items is easy, glulams, lvl's, lightweight metal connected trusses from the truss plant. Their in-house engineering staff handles that as part of their service. Custom engineering for heavy timber carries a cost that kicks it out of the cheap route most of the time. Sort of like pier and beam, if done correctly would make conventional methods look much more attractive. Prescriptive methods and generic engineering are the cheapest route. Your mileage may vary considerably.

[grover]

Sounds like the parallel cord trusses might be the best answer.