Removal of roof truss cross-members, to make for easier attic storage access...[??]

[Ken Moiarty]

Background: I'm trying to modify my attic a little bit in order to store some things up there. Currently I'm busy fastening 3/4" plyboard over the ceiling rafters (so objects to be stored won't crash thru the drywall ceiling). However movement/activity up there is hampered by the fact that the roof is supported by factory produced 2x4 trusses, as these are composed of many cross members that switch back-and-forth at oblique angles between the upper and lower rafter sections of each truss. It would certainly make my attic storage efforts easier if some of these cross-members could somehow be removed out of the way (that is... in such a way as to not compromise the structural integrity of my roof, of course).

Therefore I'm requesting feedback on the following idea: 'Sistering' (i.e. reinforcing with) 2x6s to the topmost rafters, followed by removal of some of the supporting cross-members of said trusses. Not being a carpenter or structural engineer of any kind, I'm seeking informed comments/advice from others before going further with this.

Thanks.

Ken

[tbasc]

You need a Structural Engineer. TB

[Olaf]

It seems like you're being careful about this; which is good.

"Never trust a truss" is what many firefighters say. When any of the support members of the truss are compromised the strength of the entire truss is compromised.

Trying to build attic space in a web of truss supports isn't going to be easy. You'd probably have a safer project if you just put your shelving boards in the spaces on the trusses where possible. You don't want to start cutting on any truss. I believe there are issues with doing anything to stop the flexing of the bottom member of the truss (probably the ceiling joists for you). There needs to be some room for movement with temp and humidity fluctuations.

[Joseph Meehan]

What you are suggesting would require re-engineering of each and every truss you are killing. All the parts of a truss work together so if you eliminate one, you have in essence eliminated them all.

[David A]

Ken,

Work with a truss manufacturer and their truss engineers. They have truss design software that can run your project inside out and upside down. A normal engineer can do it, but the truss manufacturers have the software to work up calcs easily.

You might have to pay for this engineering, since you are not buying a truss set.

David A.

[SQLit]

NIX on the sistering.

Truss calculations are figured from the bottom of the truss. Just how much weight are you planning on adding? any significant weight on the top of the truss can spell disaster.

I use my attic to store empty boxes that are broken down. total weight, maybe 50 pounds spread out over a 4x8 sheet of plywood.

[Bob Morrison]

Ken:

STOP!!!!

What you are suggesting is a recipe for disaster.

1) It is quite likely that the trusses were NOT designed for attic storage.

2) Cutting any truss members will void any warranty that the truss manufacturer may have given or may be implied in your state law. In other words if the roof collapses, you will have no legal recourse.

3) Rent a storage locker. It will be cheaper and much much safer.

[tbasc]

David, If I gave a truss manufacturer a careful drawing of an existing truss, a manufacturer could tell me what loads it could support? TB

[Some Guy]

A manufacturer will probably not even give you the time of day.

And I doubt they have a CAD system and/or expert that does finite element analysis on the stuff they do make.

Look. They churn out the same stuff day in and day out. They're not going to have expensive computer software (and an even more expensive engineer) on staff to basically twiddle his fingers. They don't change their designs often enough to warrant that sort of cost. Wood trus engineering was worked out decades ago. It's not rocket science.

For liability issues no-one is going to look at your drawings and give you a thumbs up or down.

[Bob Morrison]

Tom:

Most likely they won't take the time to deal with it. This is a common problem, but the truss manufacturers don't really want to deal with a homeowner on a single project where they are not going to sell anything. The possible profit to them is too small, so they won't consider it. This is especially true if the existing trusses are not ones that they built in the first place.

Most structural engineers (including me) don't really want to be bothered with this type of problem either. The liability is high for a very small return. I'd do it for a regular client, but only then. The cost of engineering services to analyze the truss and its connections, then design a fix is simply not worth the money in my view. You could rent a lot of storage space or buy and build a small storage shed for the same amount of money.

[Some Guy]

If you're a half decent carpenter and you throw enough 2x4's at it, you'll probably end up with something stronger than you started with.

If you use 1/4 bolts with fender washers and (or) #12 or #14 wood screws and pre-drill the holes (and not nail anything together) you'll have something stronger than the existing framework.

I bet your existing wood is full of splits because of the hack job that is usually done when cutting rafters and pounding over-sized nails in.

Do yourself a favor and make sure each and every rafter space is ventilated out to your soffit overhang. Don't stuff the insulation in there - let it breath.

[Colbyt]

You are joking aren't you???????????

Most residential trusses are designed to hold up the roof and the drywall.

Attic junk not included. Floor load not included.

Colbyt

[Harry K]

Now I have seen some bad advice in this forum before but this ranks right up there with the worst of them.

Harry K

[Edwin Pawlowski]

Crazy idea. Sure, there may be some method, but do you really trust a bunch of us crazies on a newsgroup to tell you how to re-do your roof support? Only way to know is to have a qualified engineer look at the situation. Not knowing the spans, load, new floor load, etc, you can be in real serious trouble with this.

[RicodJour]

I wouldn't bet on that. The odds of him messing up the structure are very high.

Stiffness attracts load. You stiffen up one section of the truss and another area, maybe on the opposite side of the house, will have its loads drastically affected. You're intentions are good, but you're not helping this guy.

The house is built with trusses. No one was cutting any rafters. And the trusses are, dollars to donuts, held together with the gangnail plates.

R

[Robert Allison]

I have to add my professional opinion that this is very bad advice. There is no way that I would give any advice on modifying trusses where the work is going to be done by someone I don't know, much less without even seeing the situation in person.

Trusses are built with enough strength to hold up what they are designed to carry and no more. Even storing stuff on them is considered to be forbidden by the engineers that I use. One of my PEs will not sign off on a job if there is decking down on the trusses because he knows that something is going to eventually be stored there and he will NOT let you leave it. For him to sign off on the job, decking must be removed except where it is necessary to access equipment.

The thought of modifying trusses is not a scary thought to me, because I have done a lot of work that involved modification and I have worked with a lot of engineers on what to do, and I have 30 years of experience in construction. The thought of advising someone else on doing it sends chills up and down my spine.

Like the time I went to look at a job where the garage trusses were failing and looked in the attic to see ENGINE BLOCKS stored up there! I got the heck out of that place as fast as I could.

[Some Guy]

I guess that explains how I can load my roof with 120 bundles of shingles and 5 guys (and that's in addition to the weight of the existing shingles).

[Bob Morrison]

Cut a few truss members and try it. That's what the OP was proposing.

Most roofers I've seen at least make an attempt to spread the load out some. If you pile all 120 bundles in one place then a roof failure would not be unexpected.

As for the 5 guys, I assume they are moving around and not sitting in a bunch having a smoke. That makes them "short-term" load and wood is pretty forgiving for that type of loading.

[RicodJour]

The truss is designed to be toploaded. If you put that same load on the bottom chord of those same trusses they would fail.

Trusses are the most economical use of wood possible. Why do you feel that they would build in all sorts of reserve strength (and give it away)?

R

[Lil' Dave]

Even if the house plans provided an attic access, this does not mean the trusses were designed to take an additional load above the ceiling. Its implied by the attic access provided by the builder, but that doesn't mean its true.

The chords tie the joist to the rafters in a typically angular fashion. These are mutually supportive of both the joist and the rafter.

Trusses are designed to provide a minimum ceiling load as is. Especially the exclusive 2X4 type. I wouldn't trust these without the chords in place. A long run over a bedroom, living area etc is inviting disaster for storage purposes in the attic. You cut the chords, and its an even worse situation.

Conventionally framed roofs with ceiling joists of adequate width meant for storage is probably the only type one can safely store your stuff in the attic. A few truss designers will design these if the ceiling load specs are provided by the builder, but its not seen very often.