Site Built Truss Wording

**Robert Foster** · 07-29-2010, 08:36 PM

I ran across some site built trusses today. The roof deck deflection was pretty extreme IMO and the 2x4 top chords, as measured along the roof deck were spanning between 8 and 9 + feet...spacing at 16 o.c. Googling some rafter span tables and assuming southern pine/fir and 10 psf dead load and 20 psf live load I think puts those chords at or exceeding the allowable span. Is the below rough draft of a comment called for ? ...suggestions/editing/education are all welcome.

The roof of this home is constructed with what appear to be site built trusses. I observed greater than typical deflection of the roof deck while walking the roof. I also observed spans of the 2x4 top chords used in these trusses which are near to or exceeding the typical allowable span for 2x4 rafters. Recommend further evaluation of these trusses by a structural engineer.

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**Michael Thomas** · 07-30-2010, 03:26 AM

1) When reporting this sort of "deflection", I would say something Ike:

"The roof is "bouncy" (has excessive deflection) when walked on".

IME that way there is a better chance the comment will be understood by non-technical readers.

2) I would not speculate on the unsupported top-chord length: the trusses are site-built, in your opinion there was excessive deflection when you walked the roof, depending on how much deflection its either an SE to design repairs, or an SE to evaluate and design repairs as required.

Also, I add boilerplate (as suggested by someone here) to the effect that after the SE has evaluated the repair, that the property owners retain one copy of the Structural Engineer's acceptance letter for their records, and staple another in a plastic slip sheet to a truss near the access.

**Dom D'Agostino** · 07-30-2010, 07:14 AM

Rafter spans don't translate to truss spans. I've seen plenty of 30 ft top chords (or longer) with no issues.

See example here

**Robert Foster** · 07-30-2010, 08:01 AM

Originally Posted by Dom D'Agostino

Rafter spans don't translate to truss spans. I've seen plenty of 30 ft top chords (or longer) with no issues.

See example here

Sorry, maybe I wasn't very clear.

I was not referring to the overall span of the trusses, but to the span of a single component of the truss... the unsupported span of the 2x4 top chord from one web to another web.

I do realize load calculations for a single simple span are different than those for a series of adjacent spans covered by what is assumed a continuous member.

If you've got trees down in Florida growing 2x4's that can span 30 feet...please ship me some.

**Robert Foster** · 07-30-2010, 08:19 AM

Maybe I should amend my question above and possible report comment to be more general.

Do site built trusses, which have no engineering documentation, warrant a comment ?

The roof of this house appears to be constructed with site built trusses. Trusses are an engineered component and typically have associated documentation. Recommend inquiring with and obtaining from current owner engineering documentation for these trusses.

**H.G. Watson, Sr.** · 07-30-2010, 10:02 AM

I don't see any web bracing (t-bracing, diagonal, CLR/CLB, end) to prevent buckling of the truss webs.

This might explain some of the bounce or "give" you felt when walking the roof. Also appears to be signs of water/condensation damage, the plywood sheathing may be damaged, not installed correctly, and/or originally not of sufficient strength.

Perhaps you might find something in the following clickable links helpful:

Inspecting Wood Trusses | ASHI Reporter

http://www.ufpi.com/literature/siteprodtech-104.pdf

http://www.sbcindustry.com/images/pu...ges/pbrace.pdf

HTH.

**Michael Chambers** · 07-30-2010, 01:10 PM

Nice info on trusses H.G. Thanks for posting!

chris mcintyre · 07-30-2010, 06:34 PM

Rafters, especially in the second pic, look like 2x6's to me. Regardless of the rafter size anytime we run across site built trusses (typically from the 60's and early 70's) there are always major issues with sagging of the roof and ceilings.

**John Kogel** · 07-30-2010, 07:34 PM

Originally Posted by chris mcintyre

Rafters, especially in the second pic, look like 2x6's to me. Regardless of the rafter size anytime we run across site built trusses (typically from the 60's and early 70's) there are always major issues with sagging of the roof and ceilings.

I think they are 2X4.
I would check carefully for sagging or other problems, but don't see problems in these pics. They may be just fine. The gussets are good size, and there are no obvious butt splices.

I have seen gussets popping off because the nails were too short, so, yeah, they need to be checked carefully.

In the 2nd pic, the moisture stain occurred before the gusset was nailed on. Ignore stuff like that.

No harm in calling for an expert opinion, but they need to get someone that will actually crawl past the hatch, eh?

**Matt Fellman** · 07-30-2010, 09:17 PM

I'd focus more on the performance than the design... meaning you noted excessive deflection when walked upon, etc. Re-engineering a truss in the field is a slippery slope IMO.

**Robert Foster** · 07-31-2010, 05:31 AM

As suggested I didn't comment specifically about spans or design of the trusses just what I observed and the deflection experienced.

No 2x6 all 2x4's

Yes, moisture issues around too...minimal attic ventilation

H.G. Thank you for the links...I've read the ASHI article and cruised the other two references and downloaded them into my growing reference library.

Thank you to everyone for the continued education, much appreciated.

**Door Guy** · 07-31-2010, 06:45 AM

Originally Posted by Robert Foster

Sorry, maybe I wasn't very clear.

I was not referring to the overall span of the trusses, but to the span of a single component of the truss... the unsupported span of the 2x4 top chord from one web to another web.

I do realize load calculations for a single simple span are different than those for a series of adjacent spans covered by what is assumed a continuous member.

If you've got trees down in Florida growing 2x4's that can span 30 feet...please ship me some.

You can't look at trusses in the same way as rafters, or try to calculate or estimate the spans between supports or webs. Trusses are designed with compression and tension not single spans. This is how you can span greater distance. So using rafter span tables for the distance between webs or supports is useless. Recommend SE to look at. Also check the thickness of the plywood, it could be some of the deflection problem?