Finishing garage with truss roof system

[Jim]

Hello!

I recently purchased a house in Upstate, NY with a 2x4 roof truss system in a detached garage. The interior dimensions of the garage are roughly 24' x 24', and the trusses are constructed using plates to connect the 2x4s. There is no bottom center support -- it looks like two 12' 2x4s are connected at the middle via a plate on each truss. Finally, the trusses are spaced 24" on center.

The garage has been in place for around 15 years, so it's plenty sturdy. I've begun finishing the garage, but I just gave some thought to whether the truss system will be able to handle the additional weight of insulation and sheetrock that will be used? The attic space won't be used for anything other than light storage. Any information or suggestions would be greatly appreciated. Thanks!

--Jim.

[HerHusband]

There's no way to know for certain without the specs from the company that made the truss. I had to include the truss specifications with my building permit, so you might be able to get that info from your local building department if you do not have a copy of the plans (for a research fee, I'm sure). The truss may also be stamped with the company name, in which case they might have records of that truss.

In any case, every roof truss I have seen has always been designed for at least 10 pounds dead load on the bottom chord of the truss. A 4x8 sheet of

5/8" sheetrock probably weighs less than 80 pounds. That should work out to about 2.5 pounds per square foot. With insulation I'm guessing you'd still be under 3 psf (insulation is rather light). That still leaves plenty of capacity for light fixtures, garage door tracks and openers, etc.

Personally, I wouldn't store anything on the bottom chord of a truss unless the truss was specifically designed to carry that load. But that's me.

You would probably be fine putting christmas decorations, spare pvc pipe, or fishing poles up there. But, don't try storing your collection of old magazines, books, that old boat motor, cans of paint, or other heavy objects. It probably wouldn't be a problem 999 times out of 1000, but it only takes that 1 time for things to fail. :)

Anthony

[Bob Morrison]

Anthony has given a most excellent answer!

[Dave]

The builder failed to showup the day they dropped off the trusses for my house in building progress. I happened to be at the building site, told the guy where to drop off the trusses. The paperwork the driver provided included the layout of each truss type (hip roof) and so forth, and dwgs of each. On the last page was a disclaimer regarding using the resulting attic space for storage. Dave

[Jim]

Thanks to all for the replies! I'll try to find out the truss specs from the bldg. dept. Out of curiosity (and not wanting to get killed)

-- if there *is* a problem with the trusses, will there generally be some sign before there's a "real" problem. In other words, will I see a crack in the ceiling before there's a catastrophic failure and the roof falls down on my head?

Many thanks again,

--Jim.

[Bob Morrison]

Generally, the collapse mechanism is failure of the press plate into the wood. This is a catastrophic failure and can occur without warning. Failure of one truss can cause a "zipper" effect, also known as a progressive collapse.

[Jim]

Hmm...okay, thanks Bob. I think... Here's another question: Assuming that I've got nothing stored up there that wasn't up there before (aside from the sheetrock and insulation), and given the fact that the structure has been in place for over 15 years, is it fairly safe to assume that it'll be okay? In other words, is history a good indicator, or are trusses like stocks: past performance does not guarantee future performance?

Also, if I wanted to have someone take a look at the situation, who would that be? A structural engineer?

Many thanks again.

--Jim.

[Bob Morrison]

Let's say that the fact that the trusses have been in place for 15 years with sheetrock and insulation in place and they have not failed is a good thing. However, it may also be that the roof has never been fully loaded to capacity by snow load or roof live load, both of which cause tension in the bottom chord.

It will be a lot easier if you can find the original design from the truss manufacturer. Evaluating an existing press plate truss can be time consuming and expense. I've done it - once. It is not the truss members that are the issue, but the press plates. These are optimized by the truss manufacturer's design program.

One option might be to have the trusses completely redesigned with all connection loads taken up by plywood gussets. This will not be inexpensive, not will it be easy to implement any changes.

I just now went back and read your original post. It seems the question has more to do with using the trusses for attic storage than supporting sheetrock and insulation. Most modern roof trusses that I have seen are designed for a superimposed load of about 10 psf on the bottom chord. This is enough to take care of sheetrock, insulation, lights, etc. but not much else. So, bottom line. Don't use them for storage of any kind.

[Jim]

Yeah, that's definitely part of it -- some storage up there would be nice, but it's not necessary. My biggest concern is getting flattened. If I post a photo or two of the press plates, would that tell you anything?

FWIW, I found out that the truss manufacturer is Stock Building Supply. The previous owner didn't have any other info, and a call to them turned up that they don't keep records that long.

[Bob Morrison]

A photo wouldn't tell much. Is there some reason you are concerned about these trusses if you aren't going to store anything in the attic? Are they showing signs of distress in some way?

Again, from your original post I assume the trusses were designed for a pretty heavy snow load in upstate New York. The weight of a layer of sheetrock (even 5/8") and some insulation is most likely only a small portion of the truss design load.

For your own piece of mind you could ask an engineer (structural or civil with structural emphasis) for a brief inspection. If it were me I would probably want to look at 50% or so of the press plate connections just to be safe. If they look okay then I think you could proceed safely.

[Chuck]

Guys. Please remember that if the bottom chord was designed for 10 psf and you store heavy material/books/paper etc that runs around 100 psf, 10 times the design load you might be in trouble not just failure of the plates but also the bottom chord. Not to mention deflection. As Bob says do get an local engineer to do some inspection. For a little expense a good engineer could analyze the truss for load capacity and have some ideas for your storage, such as half of the space where additional structure could be added specific to storage. Just a thought.

CID...