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reinforce truss for attic storage

reinforce truss for attic storage

reinforce truss for attic storage

(OP)
I have 2x4 "fink" (or W) trusses spanning 20 foot on 24 inch centers.  I'm not sure the pitch of the roof, but it is pretty steep.  I can include pictures if it would help.

I want to use this attic space for storage, but after doing some searching it appears 2x4's on 24" centers aren't going to support much.

Would I gain anything by sistering a 20' 2x10 or 2x12 next to each bottom chord?  My floor would then go on top of the 2x10 or 2x12.

I don't plan on using this for liveable space, only storage.  I'm an electrical engineer, so go easy on me... :)
 

RE: reinforce truss for attic storage

What are you planning to store in the attic?  Is it heavy or relatively light?

I have 2X4 trusses  with a 24' span on a 4:12 pitch and manage to store relatively light items, with some tax records, clothes, suitcases - that sort of thing.  No problems for me.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: reinforce truss for attic storage

(OP)
snow plow, christmas tree, etc.  i plan on making a lift since its like 11 ft to the ceiling.

i'd say heavier stuff than your average person, but no engine blocks or anything that heavy.

RE: reinforce truss for attic storage

jake65892, If the trusses were designed under the IBC/IRC or the UBC (and maybe the BOCA and SBCCI) codes, then they should of been designed with a 10 psf live load on the bottom chord. If you keep below this loading you should be okay, but you might want to check out the code requirements for your building. I would add a 2x4 to the bottom chord that is taking the full (7 to 10 psf) loading to prevent any additional sagging/creep of the ceiling in that area.

Garth Dreger PE
AZ Phoenix area

RE: reinforce truss for attic storage

(OP)
thanks for the reply woodman.  that is what I was finding in my searches as well.

i guess my main question is:

if i sister a 20' 2x10 or 2x12 next to this, will my psf go up, down, or stay the same?  will i be doing more harm than good?
 

RE: reinforce truss for attic storage

jake65892, 1/2" plywood will add a 1.5 psf load. A 2x10 at 24" o.c. will add 2 psf (but a 2x4 will add .75 psf) to the ceiling. A 2x12 at 24" o.c. will add 2.5 psf. Subtract the plywood and add-on from the 10 psf to get what load you can add.  

Garth Dreger PE
AZ Phoenix area

RE: reinforce truss for attic storage

Hang on, the extra member will add dead load, but it will also add capacity.

RE: reinforce truss for attic storage

hokie66, The add-on will add capacity to the bottom chord of the truss. But, will not add any capacity to the connections of the truss. Usually, the connections of a truss are the weak point of the design. Typically, adding load to the bottom chord of a truss may effect the connections; at the heels first, then to the bottom chord slice and finally to the joints.

Garth Dreger PE
AZ Phoenix area

RE: reinforce truss for attic storage

(OP)
ok, so your saying that I'd actually be losing psf if I sistered a 2x10 or 2x12 on there.  That's exactly what I wanted to know.  Thanks!

Is there anyway to reinforce the connections of the truss if I were to sister a 2x10?

So, another thought popped into my head last night:
what if I just run a 2x10 or 2x12 right next to the truss and the only connection points would be at the load bearing walls.  In other words, the bottom chord of the truss would never be sistered to the extra 2x10 or 2x12 that I would be adding, taking the truss out of the equation.

Would I get better psf since it is just a single board? i guess it would be like a floor joist.

Do I need to be careful not to exceed the strength of the load bearing wall?

RE: reinforce truss for attic storage

That's what I meant.  The joists would span from wall to wall as bending members.  There would be deflection compatibility issues, but I don't see how it could fail to increase the capacity.

RE: reinforce truss for attic storage

(OP)
before i was intending on attaching them to the truss.  now i'm thinking of not attaching them to the truss at all.

RE: reinforce truss for attic storage

How are you going to get the 20' 2x10's up there?
 

RE: reinforce truss for attic storage

(OP)
my roof pitch is pretty steep, (hopefully) allowing for enough space.  and I can always make the opening to the attic larger if I have to.

Can anyone give me the psf numbers on what the "floor joists" will hold?  And will the load bearing walls be able to take the extra load?

RE: reinforce truss for attic storage

"now i'm thinking of not attaching them to the truss at all."

WHOA there nellie!  

If you have sheetrock on the underside of the bottom chord of the trusses, you'd better attach the additional 2X members to the truss or you will likely pop the sheetrock off the bottom of the truss as the 2X12/s are not going to be as strong as the truss and have more deflection under load.

Mike McCann
MMC Engineering
Motto:  KISS
Motivation:  Don't ask

RE: reinforce truss for attic storage

(OP)
thanks for the tables woodman.

hadn't thought about the sheetrock... any idea where i can find the max deflection under load?

if i do attach it to the truss, then we are back to the extra loading issue on the joints of the truss.

so is there any way to reinforce the joints?

right now they have those pressed on metal connectors. maybe if i add some plywood "plates"?

thanks for the replies guys, i'm learning quite a bit!

RE: reinforce truss for attic storage

I would scab to the existing truss...
If you only plan on loading the the middle panel of the fink (which should be about 6'6" on a 20' span), then a single 2x8 x 12' scab centered on the truss will be sufficient.  The oversized scab will allow you to get about (3) nails in each intersecting web to cover the increase in force in the web just in case the tooth holding of the plate is not sufficient.  If you are worried about eccentricity on the joint, apply a 24" scab centered on the back face of the joint and use (2) nails per face per web.  For the heel joints-1/2" plywood x 16" wide x height to match the pitch on each face.  The peak joint - 16" x 16" x 1/2" plywood cut to fit.
Alpine makes the nail-on Trulox plate (I'm sure Mitek has an equivalent), so instead of using the oversized bottom chord scab, you can apply the trulox plate to each face at the bottom chord panel and then use a single 2x4 scab on the bottom chord.  Same goes for the peak instead of plywood.  i would still use plywood on the heel joints unless you want to cut a wedge to fit into the heel that the trulox can nail in to.  The wedge is to prevent the trulox plate from buckling where it is not in contact with wood, which is common on steep pitches.

RE: reinforce truss for attic storage

I bet you don't need to reinforce the trusses.  For a 20ft span and a steep roof pitch the trusses can probably take much more load than what is needed.  This is due to the height of the truss (5ft tall for a 6:12 pitch, sounds like you may have a taller truss) and the minimum chords the truss needs.  
Yes, plates are the week spot.  But before you start adding a bunch of wood "joists" or some sort of system to provide additional strength to the plates, talk to a truss engineer or structural engineer in your area.  They can calculate what your truss can support.  I'm not sure on the price the truss engineer would ask but shouldn't take them more than 1/4 hour at about $80per hour for a technician to run.  Maybe they can just run it while you're on the phone.  Just a matter of finding a truss guy willing to help you since this is not the type of work they normally do.   
But if you would rather beef up the existing trusses, OSB gusset plates work well when glued and nailed.    

Jim Houlette PE
Web: www.evstudio.us
Online Magazine: www.evstudio.info
 

RE: reinforce truss for attic storage

(OP)
Thanks for the replies KRW7 and jhoulette.

Since it sounds relatively easy to beef up the joints as KRW7 proposed, I think I will go ahead and do that.

If someone is willing to, I can post a picture with the measurements and if you could calculate the loading, that would be awesome.

Again, thanks for all the replies.  I appreciate all the help!

RE: reinforce truss for attic storage

Why 'sister' the joists and complicate the situation?  Why not install new joists such that the new joists completely support your new attic loads independent of the roof trusses entirely?

RE: reinforce truss for attic storage

If you were to ignore the trusses and design for joists spanning 20ft at 24"o.c. (you have to at least work around the trusses).  You would need 2x12 @ 24"o.c. of DougFir #1 or Southern Pine #1 or 11 7/8" TJI 210s at 24"o.c. (or equal) just to support 20psf live and 10psf dead.  
That's a lot of material for just some storage area.
I'm curious jake65892, have you made your choice and started work?    

Jim Houlette PE
Web: www.evstudio.us
Online Magazine: www.evstudio.info
 

RE: reinforce truss for attic storage

(OP)
efsinc:
I thought about that, essentially making floor joists.  Someone stated earlier on that they would bend under my load and cause problems with the sheetrock ceiling in the garage.

jhoulette:
I haven't started the work yet (have to finish running electric to the shed), but I think I am going to follow KRW7's advice, as it seems the easiest (and cheapest) with the best reinforcement.

RE: reinforce truss for attic storage

regarding the bending under load interfering with the sheetrock, the deflection is easily calculated and a shim could be placed under the 'floor joists' at each end so that the floor joist fully deflected still would not touch the sheetrock beneath.
 

RE: reinforce truss for attic storage

Yes, and that deflection is just under 1".  Adding 1" shims does not sound like fun.  

Jim Houlette PE
Web: www.evstudio.us
Online Magazine: www.evstudio.info
 

RE: reinforce truss for attic storage

I work in the truss industry and I analyzed my houses trusses and added screwed 2x10 with the top at about 12" above the ceiling, basically three sds type screws at each intersecting web and then floored it to brace the top of the 2x10. In my state residential housing with attic access is typically designed with an additional 20psf in 24 x 42" tall or taller areas of the bottom chord. The 12" height leaves room for insulation. I had remodeled my kitchen and it involved ripping the ceiling out- needless to say I needed a sealed detail for the inspector for the changes to the attic (light attic storage) even though it had nothing to do with the kitchen.
The analysis showed that the front half worked just fine, but the back half would require plywood or osb gussets at the joints. It would be worth your while to get paperwork from a PE even with a simple fink (mine were modified queen post trusses-low pitch).
On a side note, I've seen repairs to attic frame trusses to allow them to support steel beam winches, so its almost all do-able, just a question of what to do.
fyi
Mike

RE: reinforce truss for attic storage

If you've got a 24' fink, the open area in the center is probably around 8'. You aren't going to cut the webs to make the room larger are you?. Many truss manufacturers oversize the plates, so you may be able to scab a 2x8 in the center of the truss (approx 8' wide which is approximately the width of your room). If you check with the truss mfgr, you may be ok, otherwise you may need to add a gusset to a few of the joints. If you plan on cutting out the webs to make the room larger, that's a different story. At that point you may need to slide some lvl's next to the trusses.

Larry  

RE: reinforce truss for attic storage

(OP)
oh no, definitely not cutting anything that's already in place.

RE: reinforce truss for attic storage

First, it is true that MOST trusses are designed with a bottom chord live load of 10psf(Nonconcurrent). BUT, that only applies if the open area of the webs will allow a "box" 42"x42". Secondly, IF this applies Consult w/truss MFG)the trusses will handle light storage.(not a snow plow)
Third, Never install a "hoist" to the trusses unless they have been designed for this. A hoist will result in Impact loads which may cause the joints to fail. Consult your truss Manufacture or an independant P.E.

RE: reinforce truss for attic storage

ComponentsDesigner - I disagree with the first part of you post.  Current trusses are checked for the 10 psf Non-Concurrent Live Load (no roof live load) along the entire bottom chord.  Then there is a another check for a 20 psf bottom chord live load where a 42"H x 24"W clearance exists concurrent with the roof live load.
How hold the trusses are and under what code they were designed, depends on if these checks were done.

RE: reinforce truss for attic storage

Hello,

This is the perfect thread for my similar questions about strengthening a truss for attic floor use. My situation has a bit of a twist, though. I have moved into a home where the attic space above a garage has been altered for storage. Here are the raw details:

- The floor is 8'X14' square foot OSB flooring, screwed into adjoining framing members
- The floor sits above six 2X4's laying perpendicular above the bottom cord of five trusses
- A load bearing wall holds one end of these perpendicular members
- Trusses are a standard 20'X65" design, with three web members meeting together at the bottom cord, one vertically in the center and two laterally
- The bottom cord is comprised of two 2X4's that is joined with the web members via one large gusset plate
- A six foot 2X4 has been sistered in the middle with the bottom cord on all trusses and bolted in six places with 1/4" bolts/nuts/oversized washers
- Standard framing hardware was used at each joint and intersection of the framing pieces
- Based upon a receipt found under the flooring, this project was completed in 2001

Now, I have no idea if this was professionally done, but doubt it. The previous owner was pretty handy and I think that he just did what he thought would work. I don't even know the kind of storage load that he put up there, but it all seems to have worked for several years. Nonetheless, I can see a tiny crack in the seams of two large ceiling sheetrock pieces. It runs the length of the garage, but shows no bowing or other deformity. Here are my questions:

1. Does this sound safe?
2. Could the 1/4" bolts drilled through the 2X4's mentioned actually lessen the strength of the bottom cord?
3. Does such a design alteration sound like it could hold minimal storage weight (like mentioned previously)?
4. How might I properly reinforce the trusses for the existing flooring? I am willing to tear down the ceiling and retrofit full length members if needed.

Nope, there is no legal liability with anyone's answers and any response is just hearsay. I would be interested in getting an engineer to look at this, but can't seem to find anyone who will take on such a small job.

Like the guy who started this thread stated, "I'm an electrical engineer, so go easy on me." Well, I am a firefighter, so I only really know how to dodge when a truss fails...


Peace,
Dr. Z.
 

RE: reinforce truss for attic storage

Zeigh,
like most of what I've seen, I doubt it would fall down, but the long term effects  (like cracking or bowing sheetrock) you are probably already seeing. There are typically some safety margins built in (particularly with visually graded lumber, and metal connector plates). The actual trusses may or may not have been designed for the attic storage load (and you would only see minor differences in the design anyway- for typical garage spans.

What is scary to me is even with the code changes that mandate running the LAS loads for residential, the software (like Mitek) will now run trusses that depend heavily on the moment carrying capacity of the plates and non-engineer types are encouraged to design trusses. I have been seeing more than a few non triangulated designs for garage trusses in the past couple of years and its the triangulation that gives a truss its strength and stiffness.
anyway- my .02$
Mike

Murphys law 28th corollary- If there are five ways for something to go wrong, and you circumvent all five, a sixth will promptly develop.
Kwazai's addition- if you circumvent the sixth a seventh will develop, etc. etc.

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