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Sonnie 02-05-2009 11:00 PM

Adding more weight to truss system... what can it hold?
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.

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.

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!


Maintenance 6 02-06-2009 06:36 AM

If you have a copy of the engineering data sheet that was supplied with the trusses, it will give you the allowable loads on the top and bottom cords. Around here, that data sheet is on file with the building permit at the building codes office. You might check there for a copy. If not, then I would consult either an engineer or a truss supplier to see if they can run the calculations for you. Installing walls under a truss system with the intent of having them become bearing walls, is not a really good idea. Trusses are engineered with tensile and compression points at different places in the structure. What you or I think is a good place to gain support may be just the opposite.

Willie T 02-06-2009 08:15 AM

I vehemently second that warning about adding extra support walls beneath trusses. DON'T DO IT!!!


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.
One of those is designed to be in tension, the other has a specified, engineered job of remaining in compression. If you mess around doing anything other than making either of them stronger by adding required and called-for side strengthening, you stand a very good chance of ******ing the pooch, as they say.

Trusses aren't made to be messed with too much.
Way too risky.

Michael Thomas 02-06-2009 09:35 AM

First question: are the trusses designed to carry this new load?

Some are, some are not - a truss is designed to carry loads applied in specified directions at specified locations and can be subject to failure if (relatively) small loads are applied in some other manner.

For this reason, per code, modifications to existing truss systems MUST be evaluated and approved by a qualified licensed design professional:

2006 IRC - R802.10.4 Alterations to trusses. ... alterations resulting in the addition of load ... that exceeds the design load for the truss shall not be permitted without verification that the truss is capable of supporting such additional loading.

Most places that will mean review of the manufacture's specifications, load calculations for the revised structural design (otherwise you don't know if the new load "exceeds the design load for the truss" ) resulting in a design stamped by a SE or architect and approved by the local building department.

If you do such work without an approved design, you can find yourself responsible for expensive corrections when the problem is discovered - and anyone who buys the property may still be responsible for performing the corrections even if they did not do or casue the work to be done, which is how sellers of such property can find themselves in "failure to disclose" lawsuit territory.

My recommendation to inspection clients when I observe evidence of such repairs or modifications is that they request the seller provide verification of proper permitting and municipal inspection, and that if the seller cannot provide it the buyer perform a permit check to determine if the work was properly permitted and passed all required inspections.

I also recommend that they retain all documentation related to truss repairs or modification and file it with the other paperwork (such as tile insurance) related to the property as the question may come up again at subsequent sale of the property.

concretemasonry 02-06-2009 09:49 AM

Even adding a wall below that cuts the span can be a disaster.

The wall completely changes the the loads go and can result in a failure after you have reduced the deflection (which always is present with longer spans). You do not want to apply loads or supports between joints because that changes everything.

That is on top of applying more load than was designed for.

Since you are going to spend a lot of time and money, do it right and get it permitted with the proper paper work and justification from a the truss supplier or an engineer. When you go to sell, then you will have no trouble getting paid for all the finished and livable spaces the buyer is financing.

Not an unusual problem and there are many classic ways of handling it and someone with experience may be able to give you some options or things to avoid.


Maintenance 6 02-06-2009 12:32 PM

I should have added, that it's a fair bet that the L shaped 2x4 assembly is probably a bracing requirement by the truss manufacturer.

Sonnie 02-06-2009 05:37 PM

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.

Michael Thomas 02-06-2009 08:45 PM

IMO, at this point it's really a question of what the AHJ will accept.

Based on your report of a verbal assurance form the truss manufacturer, they might agree "that the load carrying capacity of this truss system is obviously far in excess of any load reasonably likely to applied to it".

Or, they might have someone in the building department review the manufacturers design guide, and decide the same.

Or, they might require (most around here would) that a qualified design professional actually calculate the loads which will be applied, compare them to the maximums in the design guide, determine that the strength of the system is adequate, and stamp the drawings to be submitted to the city for review.

It comes down to what the AHJ will accept a an adequate demonstration that there has been " verification that the truss is capable of supporting such additional loading."

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