Site-built trusses- always recommend a structural engineer?

[Kenton H. Shepard, CMI]

Marcel, I've built in California and Colorado and any new house in either state has to be built from a set of plans stamped by an engineer, meaning he's responsible for anything that fails, including the roof. So in that respect I'd call conventional roofs engineered roofs. A little different meaning than engineered lumber or engineered trusses, since they're both built off site under controlled conditions.

I'm not familiar with the term conventional field built truss. I guess you might call some of the big beam trusses engineered field built trusses though, since they're built onsite to a design speced by an engineer.

I gotta tell you. I often wonder how much misunderstanding comes from using different names for the same thing in different parts of the country.

Kent

[Robert J. OConnor]

Originally Posted by kshepard

I'm getting confused here ... 1. Roof systems need to be engineered whether they're conventional or truss ...

Model codes have provisions for whats called "prescriptive" framing designs for simple structures. You just select framing members (including conventional rafters/joists) from tables ... no calculations or whats called an "engineered design" needed. In some area of the country, buildings with these prescriptive designs do not need plans sealed by a PE/RA. But model codes always require an engineered design for trusses, in addition to any local requirements for sealed plans.

Originally Posted by kshepard

2. History of problem with wood trusses? The only problems I've seen have been created by people who installed them wrong or altered them.

Definitely for older metal plate connected trusses, which are well known to engineers familiar with them. Even issues with the strength of the connector plates which only became apparent after trusses were in-service for a while and subjected to heavy loading or real world unbalanced loading. Attached is just one example paper (although a little technical), and ASCE alone has a significant number of papers on the problems.

I think the bottom line for home inspectors is just to be aware that wood trusses require special designs, that there have been in-service problems (whatever the cause), and to keep your eyes open when inspecting these components. Hope that helps clarify things a bit.

JMO & 2-nickels ...

[Kenton H. Shepard, CMI]

Originally Posted by roconnor

.
I think the bottom line for home inspectors is just to be aware that wood trusses require special designs, that there have been in-service problems (whatever the cause), and to keep your eyes open when inspecting these components. Hope that helps clarify things a bit.

Yep, well put. Sounds like the bottom line to me.

I'm curious though, is there a max. sq footage for the presriptive designs?

[Marcel R. Cyr]

Kenten;

Robert O'Conner could not have better explained what I was trying to say.

Marcel

[Marcel R. Cyr]

Kenton;

Square footage would not apply, (span) is the key word. Although I have personnally built Prescriptive designed rafters as described by Robert O'Conner, to span 65 feet, today would require a PE design. I think it needs to be kept to the confines and limitations described in the IRC tables.

Marcel

[Kenton H. Shepard, CMI]

Thanks marcel.

[Robert J. OConnor]

Originally Posted by kshepard

I'm curious though, is there a max. sq footage for the presriptive designs?

Originally Posted by mcyr

Square footage would not apply ... I think it needs to be kept to the confines and limitations described in the IRC tables.

I agree with Marcel that it's really not about area. In practical terms it's really limited to buildings about 35 feet wide (40 foot max, but that requires other than typical materials), with conventional framing and connections, that are not irregular in shape or in a high wind/seismic zone.

...

[Joseph Tribuzio]

What do you guys think of this one?

[Kenton H. Shepard, CMI]

Wow... old thread. Here's one where I get to answer me... six years later!

If it looks like somone has tried to copy manufactured trusses, using 2x4 or 2x6 and plywood or OSB gussets, recommend an engineer.
Post and beam trusses are often assembled onsite to drawings supplied by an architect.

[Michael Larson]

Originally Posted by kshepard

Wow... old thread. Here's one where I get to answer me... six years later!

If it looks like somone has tried to copy manufactured trusses, using 2x4 or 2x6 and plywood or OSB gussets, recommend an engineer.
Post and beam trusses are often assembled onsite to drawings supplied by an architect.

FYI
My dad and I built a garage back in the mid seventies exactly as you describe from drawings provided by the package provider.

They were very similar to the photos here and the plans specified how many nails held the gussets in place.

I probably would not call for a structural engineer without evidence of a problem after several decades.

[Doug Edwards, CMI]

If I called for an engineer every time I inspected stick built roofing frames, trusses they would not be able to keep up with the work load. I would guess roughly half the homes here particularly down town and East Hill area are 50 yrs and older. Most have survived dozens of major hurricanes and severe tropical storms over those years and show no signs of going quietly. Tight as a drum and not a hurricane clip in sight. Most are made from old heart pine and cypress. Some of the beams and timbers are as straight as a rifle shot and the quality of the workmen in those days still shows through. I did a house over 100 yrs old last year down in the historical district with 6 x 6 beams underneath where they did not use any fasteners. All mortise and tendons with the sharpest joinery you ever saw. No bowing, cracks, checking or even blemishes. I approach each house individually, try not to assume anything about it and just report in as simple terms as I can what I observe. I never editorialize the report findings. I have found many new homes with engineered trusses that needed to be bulldozed and started over. I have found trusses that may have been engineered but the quality of the wood was so inferior it should never have been used for anything except for firewood. Just my two cents.

[Kenton H. Shepard, CMI]

Roof framing! Just one more system in which it's difficult to say anything generic that's true everywhere. I don't see that many site built trusses here and the ones I do see are often pretty bad.

Some of the old homes here (100 yrs or so) have great work and some don't.

I think if you are in an area where there were good craftsmen, people who did shoddy work had trouble finding work. I'm guessing it's truer in the
Eastern US in which "old" is a lot older than places farther west. 100 years ago there weren't any 100 year-old homes in Colorado, but a lot in cities on the east coast.

I really like looking at the older stuff because you have to get into the mind of the builder and see how they made decisions and solved problems. I took a course in CO called "Stepping into in the Boots of the Builder" about assessment and stabilization of historic wood structures. Out here, it was barns. Along the edge of the plains, every 30 miles or so there was another sawmill and local builders who worked their area. You can recognize individual building methods and tool marks if you get to see anough places.
Nice break from the same old thing.

[Kenton H. Shepard, CMI]

Originally Posted by dedwards

If I called for an engineer every time I inspected stick built roofing frames, trusses they would not be able to keep up with the work load. I would guess roughly half the homes here particularly down town and East Hill area are 50 yrs and older. Most have survived dozens of major hurricanes and severe tropical storms over those years and show no signs of going quietly. Tight as a drum and not a hurricane clip in sight. Most are made from old heart pine and cypress. Some of the beams and timbers are as straight as a rifle shot and the quality of the workmen in those days still shows through. I did a house over 100 yrs old last year down in the historical district with 6 x 6 beams underneath where they did not use any fasteners. All mortise and tendons with the sharpest joinery you ever saw. No bowing, cracks, checking or even blemishes. I approach each house individually, try not to assume anything about it and just report in as simple terms as I can what I observe. I never editorialize the report findings. I have found many new homes with engineered trusses that needed to be bulldozed and started over. I have found trusses that may have been engineered but the quality of the wood was so inferior it should never have been used for anything except for firewood. Just my two cents.

Doug, are these conventionally-framed roofs built using 2x4 (common here too) or attempts to build roof trusses onsite?

[Doug Edwards, CMI]

Heavens no! These old conventionally built roofs, frames and such hardly ever have a 2x4 in any of it and for that matter no nominal sized lumber. At one time Pensacola was a major export port of lumber (lots of old growth pine, etc.). They shipped lumber all over the world out of here. Now days, only wood grown locally is for paper. Shame too. There were some massive logs brought down the rivers from the northern parts of the counties. There are a few guys that still harvest some of the old logs out of the rivers and bayous. The smallest I think I have ever seen is true 2 x 6s but most are larger beams and timbers. That old wood is almost black from age and hard as iron. You can not pull a nail out of it with a crowbar, you just end up giving yourself a charliehorse or a hernia.

[Mark S. Tyson]

Originally Posted by kshepard

I'm getting confused here.

1. Roof systems need to be engineered whether they're conventional or truss. Site-built trusses are usually built on site by someone who doesn't get the design signed off by an engineer, just creates his own design.
Good point about noting sensitivity to alterations in the report. Maybe if they're told in the report they won't hack out braces to create storage space after they move in.

2. History of problem with wood trusses? The only problems I've seen have been created by people who installed them wrong or altered them.

3. What's way beyond the SOP's?

4. Write what you see. Absolutely right. We don't have to be engineers. We just have to know when to call 'em.

The building code does not specifically forbid the use of site built trusses provided that they are designed by accepted engineering practice and a design drawing meeting the the 18 requirements of IRC chapter 8 section 802.10.1 is submitted too, and approved by the Building official prior to installation. With that said here in Florida by State statute the design must be approved by the engineer of record before submitting to the Building Official