Originally Posted By: mpettitt This post was automatically imported from our archived forum.
Florida concrete block home.
Could someone explain (in a technical manor) the purpose for the metal shim that separates the bottom chord of the roof trusses from the top of the concrete block walls. I know its there to prevent a chemical reaction between the wood and concrete. But wood like to know exactly what will happen to the wood w/o them and how long before problems will develop. This home is a under construction and about 15% of the shims are missing. Also, would it be possible to install the missing shims after the trusses are already up and how much of a “big deal” should I make of it. Thanks
Originally Posted By: ccoombs This post was automatically imported from our archived forum.
My understanding of the issue has to do with moisture from the concrete. I know that trusses are lifted to add straps, etc. during construction, so this shouldn’t be a huge issue to add the missing items.
I don't have a good answer to how long it will last with out the shims...but I would classify this as a long term issue vs. a short term life safety issue. My response to the issue/concern would depend on the purpose of my walk/inspection.
Originally Posted By: srowe This post was automatically imported from our archived forum.
Hey Mike,
Although not a technical answer, I have found that where there is an inadequate coverage (poured) at the lintels, the bearing of the trusses will start to crush down if the steel plates are not installed. Just FYI.
Originally Posted By: Steven Brewster This post was automatically imported from our archived forum.
Not seeing alot of concrete block construction, shouldn’t a sill plate (moisture resistent) be installed and bolted to the block prior to the truss installation?
Originally Posted By: pabernathy This post was automatically imported from our archived forum.
IN many places you are correct in that the metal shim is needed not only for the contact issue but to also add to the structure stability but again to fill that void between the truss and the wall member.
They can always be hammered in later...as long as the process does not go on too far to the point they can't be accessed afterwards.
As a HI...note it....and make sure the are aware that YOU are aware of it and if they choose to discount the knowledge..atleast you have noted it as again we are not structual eng.'s....and it is only a observation.
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Originally Posted By: mpettitt This post was automatically imported from our archived forum.
I think the plates are there only to separate the concrete from the wood. They are less than 1/16" thick and have pointed punch outs for attachment, much like you’d see on a joist hangar. They are installed at the truss manufacturer and I suspect during shipment and handling that some are easily knocked off and then not replaced. It is my understanding that their purpose is to prevent a corrosive reaction that would damage the wood over time. Paul, I think you’re right, they can easily be hammered in. Steve, those were my thoughts after first moving to Florida. Here, the trusses are just set in place on top of the block wall and the 1 1/2" metal strap that is imbedded into the concrete is wrapped and nailed to the truss. This is the only method of attachment to the wall. Shawn, good thought, not so in this case but something new I’ll be watching for, thanks. Thanks everyone for your comments and if anyone comes across what will actually happen to the wood and how long before it happens, let us all know. Mike Pettitt
Originally Posted By: mcyr This post was automatically imported from our archived forum.
Mike; I worked in Florida in 1970 and back then wall plates were installed with anchor bolts at the top of the masonry walls and the trusses were installed and nailed in place.
Today, technology has changed and is very hard to keep up with.
I am not familiar with steel shims required to install wood trusses, and would like and appreciate someone to elaborate on this subject.
I would imagine that all that would be required would be to install a bond beam block at the top of the wall with anchor bolts and a pressure treated lumber to set the trusses. Check with the AHJ on this subject and write it accordingly.
Originally Posted By: mcyr This post was automatically imported from our archived forum.
Curtis;
Thanks for the help intended, but I believe I once responded to this photo where termite shields were installed on temporary supports that were intended to reduce bounce on the floor and I commented on the support structure as a whole picture.
