How much weight can this truss system hold... building home theater room...

[Sonnie]

Hello from Alabama...

I am trying to determine if I will need additional support when adding more weight to our current ceiling.

We are in the beginning stages of converting our garage into a home theater room.

I am a bit concerned about the truss system being able to hold the ceiling weight in the HT room.

I did some measuring and it is already sagging about 3/4" from outside walls to center. Currently the garage is 24' x 25'. We will be reducing the 25' span to 19.5' when we add the HT wall. The 24' span will be reduced to 23'.

I can pick the wall back up some when I add the HT wall at the 19.5', but we are adding to the current 1/2" sheetrock another 5/8" plywood plus another 5/8" sheetrock. That is going to add a LOT of weight. Then we will have the HAC unit up there, of which we can always move it over closer to the side, but still it is more weight on the truss system.

This is nothing more than a 2" x 4" truss system. Here are a few picks of the area above where the HT room will be.

http://www.snapbug.ws/images/attic-1.jpg

http://www.snapbug.ws/images/attic-2.jpg

http://www.snapbug.ws/images/attic-3.jpg

There are two 2" x 4" boards forming an L shape running down the center of the trusses... not sure what that is all about.

What I was thinking is maybe adding a couple of 6" x 6" posts... or maybe a couple of 4" round steel post at the edge of the 3' step down next to the riser in the HT room. This would be at about 2' off the center line of the trusses, but it would narrow the span from 19.5' to 13'. The wall at 19.5' is a 7" wall, so there would be supports at 5.5' and 8.5' on one side of the 25' span... and another support at 3' into the other side of the 25' span, with 13' being the longest unsupported span. While it is not dead center of the truss system... it seems like it should be sufficient.

I don't think the post would be distracting at all... but maybe even add a little decor to the room. See orange marks.

http://www.snapbug.ws/images/ccc2-floor-layout.gif

Not really knowing much about truss systems... I am not sure what I am up against here, but I believe I would feel more comfortable adding the post.

Anyone have any ideas?

Many thanks!

Sonnie

[Richard A Hetzel]

In the first place, when you add walls beneath the trusses, you will not be reducing their span one bit. Trusses are designed for a certain span, and they are built to be supported ONLY at certain points, in your case, the ends. Secondly, if the walls you build do in some way end up carrying load, there would be no footings under them to carry that load to the earth. One cannot simply move support points to anywhere on a truss.

The trusses were probably designed for a limit of deflection of 1/360 of the span. The 3/4-inch deflection is slightly less than that, and therefore normal.

I suggest you not add ANY additional load to the trusses. Build your room with a separate ceiling supported on the walls, and have that ceiling construction designed for the heavy loads.

That L-shaped arrangement in the attic is called a "strongback", and it's function is usually to suspend the center of the ceiling joists from the roof structure above. In your case, it appears that it is a pert of the truss system supporting the bottom chords of the trusses.

Trusses are not things to be messed with. Leave them alone and build an independent structure for your theater room.

[bighammer]

Trusses are not things to be messed with. Leave them alone and build an independent structure for your theater room.

I like that...I would take that with more than a grain of salt...

[Sonnie]

Thank you. I would sure hate to have the roof cave in on us due to my ignorance.

On our last home theater I built a completely independent ceiling joist system that supported tow layers of 5/8" sheetrock.

I will most likely remove the current sheetrock that is there now, then install new 2 x 6 joist supported by the new walls that will be installed on each end of the 23' span. I can run those up and in between the current joist, with the bottom of the 2 x 6 about 2.5" below the current joist. Then add the weight to the new ceiling joist.

The next question is can a 24' 2 x 6 support that same two layers of material, the 5/8" plywood and 5/8" sheetrock?

[Sonnie]

I don't see that happening... that is much too long without some type of center support.

[Richard A Hetzel]

No, 2x6s won't span anything near that distance, but 6-inch heavy gauge C-section cold-rolled steel joists MIGHT, depending on the gauge. Exact spans for wood joists depend greatly on the species and grade of wood used. All 2x6s are not created equal, but even the strongest of softwoods won't span that distance. Another possibility to consider would be 7 1/4-inch laminated veneer lumber used as joists. The steel would probably be cheaper, if you can find a supplier near you. Local lumberyards may be able to get them; if not, try gypsum board supply houses. Either can probably help you with the engineering, if you can be clear about the loads they have to support.

[Sonnie]

I finally spoke with the company that built my trusses. The first gentleman has been with the company forever, not an engineer, but knows his stuff. He stated I will have no problems adding the extra 5/8" plywood and 5/8" sheetrock to the current ceiling. However, he gave me the phone number to the engineer and stated I could call him to verify if it would make me feel more comfortable. So I did... I was able to speak directly with the engineer. This truss system is what they consider a very minor span of only 24'. He stated it would take about 800 pounds per square foot to cause that truss system to give any. He also stated it could easily handle the extra weight.

I asked him about adding an LVL in line with the "strong tie" (as he called it) from one support wall to the other and tying in to the current truss system just to keep if from deflating any further. He stated it would not hurt anything. He stated it was not necessary, but if I wanted to do it that it would indeed give more strength to the truss system. He also stated that adding a few 2 x 4's from the center peak to the top of the LVL would add additional strength, although again it is not necessary.

I had another guy come over who stick frames houses... he agreed, no problems, but he did think the LVL would be a good idea just to be on the safe side, especially since we are going to have 4 x 18" subwoofers and 2 x 15" subwoofers in the room... which will be like a real earthquake during some like War of the Worlds.

[Richard A Hetzel]

Methinks the engineer exaggerates somewhat.

[Sonnie]

He may very well have exaggerated it to simply put an exclamation mark on the fact we are no where close to straining it. Let's say it is only 400 pounds per square foot... or even only 200 pounds... we will only be adding about 3.5 pounds per square foot... about 1700 pounds to 480 square feet.

[concretemasonry]

Sonnie -

You might be absolutely correct, but all you talk about is psf, which is dangerous when you are also talking about system consisting of numerous trusses to uniformly support a load over a large area.

If you have any concentrated loads, they must be distributed over several trusses. It is surprising the "engineer" you talked to on the phone did not have all the information so he could look into the concentrated loads and the normal "psf" loads. All it takes is one truss to fail, and the entire "system" can collapse or fail (severe cracking or deflection). - An AC unit is not that much, but it can cause server deflection and cracking if it is loaded on 1 or 2 trusses, especially at the wrong locations.

Did the engineer ask where your AC and any othe heavy loads would be located and where in relationship to to the trusses they would be. there is a difference between deflection and strength, but both are necessary structural criteria. - A ceiling can hold the load, but it may deflect too much.

As a registered engineer and a previous owner of a truss plant, I feel we are hearing only part of the story/plan and not the loading diagrams that would be needed for a real answer.

Too often people get lost in "psf" if the loads are not distributed to the area considered.

Since this is change of use for the area, you will need a permit if you want to include the area in the livable square footage of the house for future mortgage purposes. Realtors, home inspectors and financial agencies are getting very good at spotting the traditional loop-holes. If you do not have a permit, you may not be grandfathered in when you get around to sell and will have to face different codes (structural. use, HVAC and electrical). This puts you in avery poor position after you have accepted a future purchasers offer. If you were asking for a signature from the engineer over the phone, he may have asked more questions before putting his professional life on the line.

P.S. - those trusses do not seem to be all 2x4 but seem to have some 2x6 SYP members unless the photos distorted things.

[Sonnie]

We discussed quite a bit, which included everything I intended on doing. We also talked about quite a bit more than just PSF. I also intend on adding the LVL, so I can hardly see any worries.

And yes... the photos are deceiving... it is a 2 x 4 truss system.

And FWIW... the only way our home will be sold is if it is sold by the bank in foreclosure. We are here to stay. My wife would not even sell a half acre to her best friend. We own 63 acres in the country... nice fish pond... nice farm... mom and dad live next door on our property... absolutely no reason to go anywhere.



I appreciate everyone's input.

[Richard A Hetzel]

A ceiling load is normally about 10 PSF if there is no accessible attic space above. No one in their right mind will design a component with a safety factor of 25x...not if they want to sell their product competitively. Concretemasonry makes some good points, and one would be wise to heed them. It is quite possible that an LVL added to the strongback area will have the opposite of the desired effect...it could easily add more load on to the ceiling framing.

Without the specific design drawings, no sound opinion can be given, and there are several flaws in what the engineer said. Obviously, the trusses cannot support anything close to 800 PSF. Less obviously, a truss is designed differently for a 25-foot span than for a 40-foot span. They don't just use the same truss, because again, they would never sell their product competitively.

I repeat my previous warning: trusses are not things to be messed with if you do not know what you are doing! The trusses have already deflected near their maximum deflection with virtually no load on the bottom chords. Now you're planning to add heavy wallboard, plus plywood, plus equipment, without having a clue whether they can support those loads or not. A chain-reaction collapse is not out of the question, as concretemasonry suggests. All that is needed is for one single connection to fail, and they may all come down in a heap.

You are doing all this on a pure guess. That's simply asking for it. I hope you don't get what you're asking for.