Parallel chord truss vaulted roof
StephenW81
| Posted in General Questions on
For a parallel chord truss vaulted gable roof with an 8/12 slope, a total horizontal span of 27ft and a snow load of 42psf, what would be the truss depth and lumber size required? I found numbers for a 40psf snow load flat parallel chord truss spanning 27ft with 18″ depth and 2X6 top chord and 2X4 bottom chord. Would this also apply for a vaulted roof with the same span, but closer to 32ft total length (16ft for each side of the roof)?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Search and download construction details
Replies
Stephen, do you mean a single truss to clear-span the building, or parallel chord trusses used as rafters? I assume the former; in that case, with parallel chord trusses I've always included an attic space in the top half or third of the span. The bottom chord of the attic portion provides most or all of the resistance to spreading. If you want a fully vaulted ceiling you may need to use deep trusses, or change to scissor trusses.
Draw out what you would like to do, add clear notes, and have your lumberyard rep send the drawings to a truss manufacturer.
Michael, Thank you for your response. I am looking for a vaulted roof design that does not require cross-ties or ridge beam and posts. I have attached an image of what I am talking about. I understand that a truss manufacturer could provide me with specific information on what is possible. However, the actual house build is several years away. I am currently evaluating design options. The dimensions of the truss have an impact on (for instance) whether it can be transported by truck to the building site and the amount of insulation that can be installed in the roof cavity.
Stephen, designing that type of truss design is somewhat complex, compared to something simple like a king post truss, and best left to professionals (with the right software).
The deeper the truss, the less the reaction load at the nodes, where the truss members meet. The tensile strength of lumber isn't usually a limiting factor; the connections are. When the spans between web members, or the eave overhang, get long, they may use lumber wider than 2x4s. I have used quite a few different truss designs but I've never tried to do exactly what you propose, so I'm not sure what is possible. If you find out, please let us know.
Stephan,
Michael has given you good advice.
The limiting dimension to make it work without a ridge-beam is the depth at the peak. If you want parallel chord trusses, that means you end up with a lot more depth the you need at the exterior walls. That's why scissor-trusses are the shape they are. They reflect the minimum depths at each p0int necessary to carry the loads.
I used a vaulted parallel chord truss for a studio over a garage. The pitch was 5/12 and the span was 22'. The depth of the truss was 24". I created a dedicated air space under the roof deck using fiberboard sheathing, had wind wash barriers at the eaves. We then installed 22" of blown insulation.
Malcolm, thank you for your information about the limiting dimension. The implications of your comments may make this option less optimal.
Doug, thank you for sharing your experience. Linear extrapolation (which might not apply here) suggests that for my situation, a depth of ~30" may be required, whereas I was looking for a depth of ~18".
A vaulted roof is not a requirement for the house I am designing. However, due to wildfire risk, I have favored an unvented roof design. If I have an unvented roof, I might as well have a vaulted ceiling vs an unvented attic. House floorplan makes a ridge beam and post support non-optimal, and collar-ties (or rafter-ties if you prefer) would require floorplan adjustment to look nice (to allow equal spacing from walls). House design is obviously a series of compromises. I am in the process of examining design options so that I can make reasonable compromises to ultimately reach a design that works for me.
Stephen, linear extrapolation is not appropriate for this situation--there are many "dials to turn" in designing a truss. More closely spaced web members, stronger connector plates, roof slope and building width all affect the design. Before talking yourself out of what you want, just sketch out what you want and have a truss company tell you what they can do.
Stephen, we have specced a similar truss for a house that is in construction now. For our design, we needed a 32 foot span with a 10/12 pitch and 17' ceiling height.
We wound up with a piggyback truss setup; the base truss is limited to 12' in height, so it can be transported, and the piggyback truss goes the rest of the way. You can see a schematic of it in the dwgs attached.
We plan to deck over the base truss and then run our insulation there (an unvented assembly). The parallel chords are about two feet apart.
The quote for these trusses on a 32 X 48 footprint is $7k, here in Virginia.
HTH,
Mark
Michael, Your recommendation (and Malcom's) to ask a truss company to see what they can do brings up an interesting question. Is it considered good practice to ask a company to spend time designing a product that may never be purchased, and in the best case, will not be purchased for several years? So far, what I have been doing is telling companies that I am in the conceptual design stage and I am considering options for a house that will be built several years from now. As a result, I get very few responses, which is understandable but frustrating. When I do get a response, it is usually from a salesman who can't answer my technical questions, and is in California where I live now vs Oregon where the house will be built. For this reason, GBA has been an excellent resource for me.
Mark, thanks for providing your design and costs; that's very helpful.
For a depth of 24", I believe the following insulation design meets code in Zone 5, but is it advisable for an unvented vaulted roof?: 6" closed cell SPF against the roof sheathing (~R36-40) and 12-14" of Spider blown in fiberglass (~R48-56) with a 4-6" airspace between the fiberglass and air/vapor retarder (e.g. Intello) covered with sheetrock and tongue and groove.
Stephen,
It may not be good practice, but is commonly done. Most truss-plants, like a lot of other trades and suppliers in the building sector, do an awful lot of work without compensation in the hopes of later securing the contract. If they want to charge you for it they will tell you.
Either way, the best approach is to have the design as complete as possible, and limit the variables. so they are only addressing the things that really matter, not a lot of possible alternatives.
I have submitted an online request for a quote to Oregon Truss. According to their website it takes ~2 weeks for a quote. I have attached a file with the information I provided. We will see what they say if they get back to me.
Stephen, I consider it extremely important to be honest and ethical, and I don't like to waste others' time, so I appreciate your concern about asking for a truss design. However, ignoring your timeline, you are designing a house that you intend to build, and the design is dependent on the roof framing options. So you can't complete your design until you know whether you can use your roof design. As long as you intend to buy trusses from this company when the time comes, I don't see an ethical problem with not disclosing your intended timeline. Timelines change all the time, anyway. But do what feels right to you.
As for your assembly, you're talking about a flash-and-batt approach. Your proportions are about right; prescriptively in zone 5 you should have at least 41% of the R-value in the foam layer to keep moisture accumulation to a minimum. A variable permeance membrane at the interior (Intello or similar) is not required, as long as you have standard latex paint (i.e., a class 3 vapor retarder) on drywall. You might consider including one or the other, if using T+G boards. Intello is a lot easier to install than drywall and paint. Siga Majrex is another good option. Certainteed Membrain is a third option, though I think it would get beat up when installing the T+G.
I'm very interested to hear what you hear back. I'm looking for nearly the same info, future home in process of planning - zone 5 in western Massachusetts, 26' outside wall to outside wall span (12" thick double stud exterior walls with cellulose insulation), 9/12 desired pitch, metal roof (OK with grand rib or similar if standing seam boots me out of budget), solar array on south face only. I have no hard limits on truss depth. I'm interested in filling the roof cavity with cellulose with an air gap and venting at bottom face of the roof deck, unless someone can tell me why that would not be a good idea.
I similarly don't want to pester truss folks with questions while I'm this far out, so I would very much appreciate any info you would be willing to share, Stephen!
Jryznic, in looking to build a house just like you described, also in Massachusetts. I'd appreciate any update to what you ended up doing.
Jryznic, I never received a response from Oregon Truss. I am still looking for a truss manufacturer who will be willing to provide a quote.
Hey all, sorry to resurrect a dormant thread, but curious if there's been any progress or any news from industry professionals on real world design limits of parallel chord scissor trusses.
I have a request for a quote out to several truss builders asking for a quote on a very similar truss (24" heel height, 26' out to out span, 10:12 pitch). If this truss is feasible without putting a lot of lateral pressure on the top of the outside wall, it could be a wonderful solution for 1.5 story plans. Plenty of room for insulation in a 2' deep truss. I'll report back if I hear anything, just curious.
"If this truss is feasible without putting a lot of lateral pressure on the top of the outside wall"
I may be misunderstanding your quote, but trusses have to be designed so that they do not exert any lateral forces on exterior walls. it isn't something you can split between the truss and some other form of lateral restraint.
This is Mark Harrison again. We have built the roodf I described above, and here is what I learned.
I found that the 2' to 3' thick cavity in the parallel chord trusses didn't really buy me that much. To build an unvented assembly in Zone 4, I needed the outer 1/3 of my insulation to be vapor impermeable (i.e., some sort of foam). That meant either a layer of foamboard on top of the trusses or blown-in foam under the decking (see Martin's illuminating article "How To Build an Insulated Cathedral Ceiling").
I wound up with 10" of blown in foam (4" closed, 6" open) under the decking (because my 10/12 roof was too steep for foamboard). This was a good way to complete the airseal on the house (0.44 ACH @ 50 Pascals on our first blower-door test, woo-hoo!) but it is an expensive way to insulate. Most of the void space in my trusses remains unfilled, because if we filled it with closed cell foam and blown-in cellulose in the appropriate proportions, we would wind up with an R-100 roof at excessive cost.
If your climate is colder than Zone 4, or your trusses are less deep than mine (about 30"), then packing your truss with insulation might then be cost-effective for you.
One last point: We ran a continuous layer of closed cell foam under the decking, burying the rafter chords of the trusses. This gave us a continuous air barrier that rolled down to the top of the stud wall. The foam sealed around the shafts of our Sun Tubes and other roof penetrations. This simple (though expensive) air barrier was a big advantage because we were working with crews who did not understand air barrier detailing.
Mark,
Thank you for the update. The empty cavity space was behind my specs for maximum 24" depth. From a fire standpoint, I would prefer to have the cavity completely filled with insulation. Does your statement about the 2' to 3' cavity not buying your much mean that you would have had the option of a reduced parallel chord depth, or was the depth needed to meet structural requirements, or compared to some other roof design?
For my own update, I still have not been able to obtain a quote, but I haven't searched exhaustively given I am still several years away from a build. However, from Mark's experience it appears doable, although perhaps not the best choice.
I know this is old, but I’m also curious about this as well:
“Does your statement about the 2' to 3' cavity not buying your much mean that you would have had the option of a reduced parallel chord depth, or was the depth needed to meet structural requirements, or compared to some other roof design?”
The depth of the truss was needed for structural reasons. But I did not need it for my insulation plan.
At one point, I thought I would put a bunch of fluffy insulation into that space. But I needed 4 in ches of impermeable insulation, and once that was in place,it was not cost effective to use
fluffy insulation for the rest.
Thanks Mark!
If you have Google Sketchup Medeek Design has a great truss resource along with other products.
http://design.medeek.com/resources/medeektrussplugin.html