Most of us see site-built trusses from time to time. Since they are seldom designed by an engineer and usually installed on structures built without a permit, should we always recommend evaluation by a structural engineer? Or only when they don’t look good. What if they look way overbuilt? What about recommending a qualified contractor? -Kent
There’s really no such thing as a site built truss, if its site built it is not an acceptable truss. If it looks like a truss (somewhat flimsy) then it is a real problem.
A timber truss can be site built but then you have 12x12’s and larger usually.
If it looks good, and this requires a lot of knowledge about lumber, spans and fasteners, then it is your call but almost always will need at least a contractor or engineer to verify.
I saw some fake trusses on an old carport the other day that were sagging substantially. I was surprised because they were very heavy duty and well built. The clue was the span, it was just too much for the design.
When building heavy duty, the weight of the wood will work against your goal severly too, this is why they are engineered.
have a picture?
I’m just talking in general here, Bruce.
By “site-built” I mean roof systems built to roughly the same design as engineered trusses but with no engineering, just the knowledge of the person designing them on the job.
I’ve built them. I’ve seen a lot that looked fine and had been there a long time and survived some heavy loads with no problem.
I just want to know if other inspectors think there’s a rule of thumb regarding how to report them, or if it depends on the situation.
Site built trusses should be examined by a Professional Engineer.
They would not be accepted here, Ohio, without an engineer. OBC R300.1 Design.
Call that one out.
The basic difference between a truss and long rafter construction is that the rafter is supported mid span by a wall or beam using a brace. The basic/typical truss requires no support except at the ends. You might be looking at rafter construction with load bearing points that are fine. Some framers install the bracing at angles where it might resemble a truss. With insulation around these it might be hard to tell.
I can’t imagine why anyone would bother to build their own trursses or stick biuld the roof system when you can buy trusses cheaper than your material cost of materials for job built trusses or stick built.
Furthermore, I would question the design if the trusses where built by today’s Contractors. Most of them are not even old enough to know the difference between a Fink truss and a Howe truss the way they were originally built and split ring connections would blow their minds.
Report what you see and question what you see. That is your protection. Recommend further evaluation.
All the site-built trusses I’ve seen have been on older homes, except for some gambrel roofs on sheds. But hey! I’m 54, 30 years a carpenter and I don’t know the difference between Fink truss and a Howe truss, Marcel. I do remember seeing some split rings when I first got into the trades.
Most of the modern stick-built roofs I see are hip sections. -Kent
Unless the seller can provide an engineering report on the “site built” trusses, I would always recommend an evaluation of the trusses by a professional engineer.
Yep, I’m familiar with all those, Larry. I’ve set most of them except for the gambrel. I’m guessing Fink and Howe are a couple of the early truss manufacturers.
We were using trusses wneh I got in in '71 but even then they had standard gangnails rather than split rings.
Good illustrations, thanks. -Kent
Thanks Larry for that link. I actually put it in my favorites, My books are getting old and these shapes bring back old memories that does not seem to have changed much over the years going from hand built to factory built trusses.
The Howe and Fink trusses were the usual design used for on-site hand built trussess with plywood gussets in lieu of metal stamped gussuet. No nail guns at the time. It &ucked. The first Howe truss built on site design I was associated with in building was in 1965 under my fathers expertise which spaned 60’. I thought that was something else at that young age.
I exeeded that span myself in 1999 on building a truss package to support a temporary plastic roof with up to 4" of snow load that could be raised with a crane in 16’ modules and tied back together for every floor being built. The shelter would span 65’ and rest on perimeter staging and was raised for each floor level. Being of all reiforced masonry, we managed to erect a complete floor including precast planks, every seven working days. Six stories. The trusses were built with 2x4’s top chords and double 2x4 bottom chords. These were built the old fashion way. Plywood gusset connections. I did confer with the Project Engineer to get a few pointer, but was left up to me to design it. The Engineer preferred the Howe over the Fink design for this accomplishment.
A few saftey factors and precautions were taken into account.
Using current construction standards as a guide, roof framing should be conventional lumber rafters with rafter ties (ceiling joists) spanning across the roof at the wall top plate level … otherwise it’s an engineered design.
If you see a roof truss I would at least point out that they require special design which could not be verified, that they can be very sensitive to loading conditions and possible modifications, and that a complete inspection and evaluation is beyond the scope of a home inspection.
Wood roof trusses have had a history of problems, particularly under heavy loading conditions that only occur periodically and where there have been modifications. Approach the roof truss framing as you would other building component, and give your clients some additional basic information about trusses and potential problems. Without getting too deep, get familiar with basic truss types, connections, and where members should be. Look closely for movements and signs of trouble, particularly at joints and members near the middle (which also might have been unknowingly removed to make additional attic storage space or install equipment).
From just a visual inspection if you see any apparent defects, or if something doesn’t look right or just rubs you the wrong way (you get a gut feeling that something is wrong, but can’t put your finger on it … which comes with experience), then bail to a structural engineer. Just “Observe & Report” … thats the job …
I agree with you a 100%, this is waaaaaaaay beyond the SOP.
As one would say it, talk alot, but write what you see and reccomend further evaluation or consultation with P.E.'s. If something does not look right, note it and report it. That’s all.
We sometimes get carried away with technical terms, but that is alright too. That is how everyone can share knowledge and learn more about what is on the other side of the door. We all do that time and time again, just to keep up with certain things.
The Home Inspection Business is all about share view points on the Visual Points of what the Home Inspections is all about. Let us all keep it that way and sometimes some of us will share the Engineering background and Construction Experience factor to help all our constituents .
Bob, I think you are one to do just that, but in the mean time, I think we all confuse some or elaborate to much like I do. In any case, I believe we are doing good to some, and that is what counts.
Thanks for all the shared information that you provide.
I’m getting confused here.
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.
History of problem with wood trusses? The only problems I’ve seen have been created by people who installed them wrong or altered them.
What’s way beyond the SOP’s?
Write what you see. Absolutely right. We don’t have to be engineers. We just have to know when to call 'em.
Most small houses built do not have design drawings for framing a roof.
Conventional framing is still allowed and defined in the IRC. Simulating the truss design has occurred , still is and always will, in the residential Market.
Most conventional field built trusses, utilize the same conventional span sized lumber as it would be for a conventional stick built roof. The only difference would be assimilation of the truss design without the engineered panel point connections, wind lift designs and engineering needed and required for Manufactured trusses.
Tyring to comprehend the design calculations or attempt to build a truss simulating the engineered truss is waaaaay beyond the normal inspection of an HI. Modified Manufactured trusses should not be altered and if so, should be recommending a PE for further evaluation.
Member sizing of field built trusses that seem to copy or assimulate Manufactured trusses, should be called out also.
Any unorthadox framing which differs substantially from the common truss designs standard, whether conventional or Manufactured should be noted and addressed.
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.
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.
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 …
Yep, well put. Sounds like the bottom line to me.
I’m curious though, is there a max. sq footage for the presriptive designs?
Robert O’Conner could not have better explained what I was trying to say.
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.