Can anyone comment on the overall comparative cost of pressed trusses at 2ft centers, with OSB sheeting (like Mytek for instance), compared with a roof constructed in SIP with Glulam purlins.

Assuming both can be manufactured off-site both can deal with complicated forms (with the right engineering) and I assume the former should be cheaper than the latter. But what sort of cost saving should I see and with the additional insulation stage of the trusses.

I don't want a ventilated roof void as I want the insulation in the plane of the sheeting.

Two factors to consider:
Do you want a vaulted ceiling with exposed beams? If so, then the SIPS roof would be a natural choice. If you intend a flat ceiling, then the choice is not so obvious.
How complicated is the roof line of the home? This is a major factor for me. The more complicated the roof system,
the more likely I am to use trusses, and spray foam insulation on the underside of the roof sheathing. While SIPS can be cut to the more complicated roofs, the cost factors increase with the complexity of the roof.

lets say for discussion purposes that the roof is a "top hat" ie. that it is a simple hipped roof on all four corners.

Well, you picked the perfect "either/or". How big is the footprint of the house?

say 40 x 25 ft with a pitch of 25 degrees

I suspect that your proposed roof would be less expensive if built with trusses. The rectangular footprint presents some loading issues for the SIPS, as well as the relatively low pitch. Depending on your SIPS mfg's engineering, some type of external beamed support would be necessary depending on the location of any interior support walls.
The 25' span means that the trusses can be fairly light weight 2x4 construction, and a straight forward installation process. Depending on the final details of the plans, I would most likely recommend the trusses.

Jazzdude, So, before you ditch SIPs in favor of trusses, key question (Wes alluded to this issue in one of his earlier posts on this thread) Are you interested in having any vaulted ceilings in your home?
If so, SIPs probably make more sense than trusses. While its easier to fabricate, and install SIPs in a simple gable end design, we've done numerous vaulted hip roofs with both recessed, and exposed, support beams.

SIPs also meet your requirements, as stated in your original post "I don't want a vented roof void as I want the insulation in the plane of the sheeting"

Another benefit of SIPs, in both vaulted ceiling, and wall, applications, is that you have the "sheeting" on the interior side of the panels. This feature typically results in easier drywall installation, with less waste to dispose of.

Good luck with your project, whatever products that you decide to use.

Why not use the roof truss system and the SIPs. You can move your trusses to 24" OC and replace a few to offset the cost. You can then use 4' x 8' panels, which are cheaper than the other sizes, and put the system together for a comparable price.

The truss system, gives you design flexibility in the ceiling for mechanical, electrical, et. al. Maybe I am just partial to the SIPs but we use a truss/SIP system all the time.

location is an important factor - are you in a high wind zone?

if I understand you correctly, you're saying put up trusses, but with wider spacing, then put sip panels on top of the trusses? 

I guess in that case, the space between the sip & the drywall on the ceiling would be left uninsulated?  I have not heard of that option being used.  It seems that you're duplicating some of the structural elements, but if you can eliminate other things that would be needed (ridge beam, for example) it could be workable.

One question - have you actually done this, or is this a theoretical solution?

We've installed SIPs over Timber frame trusses spaced at between 4' to 24' OC, depending upon the live load. However, I don't see any advantage to this approach unless you want that vaulted ceiling/ timber frame "look", and then the trusses still wouldn't be spaced at 24" OC, unless you're dealing with extremely high roof loads.

One source for information regarding SIP spans and spline requirements, assembly details, etc. is www.r-control.com click on "library" and then "technical". You'll have to log in, but its a very simple process.

I have ganged 3 trusses together with bolts, and put them on 8' centers, then spanned across them with the SIPs.  Then, if you want a timberframe look, you can wrap the trusses with cedar etc..... or use some 2X to frame between the trusses for a flat ceiling.  The only other thing I make sure is that there is a solid post under each of the trusses going all the way to the floor for better support.  Putting the trusses on 8' centers works great especially if you are using 4' or 8' wide panels.
I have some pictures if you want to see them.

www.marksonbuildersoftexas.com

I am aware of the hybrid truss/SIP approach.

As a SIP can usually span 12' along its length so with a network of large trusses and purlins the SIP roof works fine.

My original question was to replace this entirely for the all lightweight truss / OSB / plank insulation approach. How much cheaper should this be also taking site labour into account.

jerkylips,
We use post frame 'super' trusses designed to be spaced 4' or 8' on center with SIPS regularly. And there can be a cost savings compared to longer span SIPS and beams.

I've also used steel trusses for a flatter roof, which were about 1/3 the cost of wood trusses.

On my build I used trusses on 24" centers with 8" SIP panels on those. My roof is a 6/12 pitch with a hip design with a small gable on the ends. Not sure what the technical name is. I also ran a supply and return into the attic space. I obviously sealed the roof in order to condition the space. I am three years in with no problems. My house is 81' by 28'. When I had the trusses made, I asked them to narrow the ends as much as possible so to reduce the gap I needed to close up. Anyway, I live in a high lake effect snow area and this design gives me awesome load capabilities as well as a conditioned attic. I did not need any kind of vaulted ceiling, so I would have had to fur out a place to hang drywall anyway. I also had a tray ceiling design in the master.
I priced this all out and the truss + SIPs wasn't that much more.
Note: The biggest problem was hitting a 1.5" truss with a 10" screw through an 8" panel. WOW! btw, most of the larger panels were 8' wide by 17' long.

My roof will have a 10:12 slope, span of 28' and is only 40' long. The Soutn end will have a cathedral ceiling with a loft over the kitchen and stairs (behind the Masonry Stove) to the loft and Attic Room or 2nd level. Looks like the SIP + truss system would work best for my design. The Perlins would be over the walls of the Attic Room/loft. I'm looking for a sample of a similar design and a means of determining the size and spacing of the trusses. I'd like to get at least 4' o.c. If I'm bearing on SIP 6" walls, will I need Post every 4'. Can the load transfer to the I-bm floor trusses on CMU/CIP walls of the basement or do I need to rethink this design from the top down?

Posted By Tarr Run on 20 Aug 2013 10:57 AM
My roof will have a 10:12 slope, span of 28' and is only 40' long. The Soutn end will have a cathedral ceiling with a loft over the kitchen and stairs (behind the Masonry Stove) to the loft and Attic Room or 2nd level. Looks like the SIP + truss system would work best for my design. The Perlins would be over the walls of the Attic Room/loft. I'm looking for a sample of a similar design and a means of determining the size and spacing of the trusses. I'd like to get at least 4' o.c. If I'm bearing on SIP 6" walls, will I need Post every 4'. Can the load transfer to the I-bm floor trusses on CMU/CIP walls of the basement or do I need to rethink this design from the top down?

That's a question for a structural engineer. In the end you will have to get this design engineered as it is not your basic slam/bam shoe box design.

That's a pretty insane roof slope. Where are you located that you require a 10:12 pitch? I assume it's a high snow load climate area.

The design is derived from "Canyon View" of www.ameri-loghomes.com but won't be a "log" home. The site is in the WV N. Panhandle on the 40th parallel. I have no intentions of attempting to present this to a builder without a Structural Engineers review. I have been a formwork designer for 25 years in the concrete formwork business and have some familiarity with beam analysis. The 10:12 pitch accomodates an attic loft. I've determined that a ridge beam and two perlins along the loft walls should accomodate the SIPs with I-bm splines. My objective is to come up with a preliminary design which the Str. Engr. will only have to analyze, tweek and certify. I use Autocad and the internet is a storehouse of knowledge.