Rafter storage.

[Priest]

I've got high ceilings in the new shop so I just got back from home depot where I picked up 8 sheets of 1/2" plywood for the rafters. I'm tossing them up there so I can get the camping and x-mas stuff up off of the floor and out of my way. How much weight can I safely put on a sheet of plywood in my rafters? I realize that it is going to vary greatly depending upon the construction but give me a ballpark number?

[EMG7895]

Depends too much on the construction need the spacing and how far do they span etc. to give you a good number

[androbus]

Depends too much on the construction need the spacing and how far do they span etc. to give you a good number

as an ex roof truss designer, I have to say for new structures? with pre-fab rafters you really can't put up much at all without compromising the structure. it's set up for certain loads, and with tossing things up there, pretty quick youi can accidently end up with a lot more weight than you thought you tossed there...if it falls down your insurance will NOT replace things!

Paul

[maddog]

I cannot tell you specifically how much you can load on the bottom cord of your trusses. If you in fact have a 2x4 truss system, but there is a guess tamation you can do providing you have a comp roof. The Uniform Building Code allows you to install up to 3 layers of fiberglass composition shingle roofing layers on your home before tear off. Truss manufactures realize that. And a typical truss is designed at 32 to 37 lbs per sq ft of uplift. One layer of comp roofing depending on grade can weigh up to 150lbs + per square (10 ft x 10 ft = 100sq ft). Three layers @ 150 x 3 = 450 lbs per 100 sq ft = 4.5 lbs per sq ft. One 4ft x 8ft sheet of plywood = 32 sq ft. Keeping it safe at 300 lbs per 100 sq ft or 3 lbs per 1 sq ft (2 layers) would be 32 sq ft x 3 = 96 lbs per sheet of plywood. Believe me when I say Truss Manufacturers over engineer the trusses for the live and dead loads. I hang my FJ-40 tub from my trusses with additional weight (stored items) inside the webbing with no ill affects. You would be surprised in regards to the test results of the gang plates attaching the truss parts together. Then if you’re really concerned you could always add additional 2 x 4 vertical braces from the rafter to the joist. I would place those at or near the diagonal webs especially if you have a Tile roof.

[Priest]

I have a coregated tin roof so that isn't much weight at all... It raining right now so I don't feel like walking through the back yard to see what the spacing is. I'll get back with that info later.

[maddog]

This Fink truss is obviously designed for a snow load at 57lbs. Per square foot.

[PAToyota]

Yeah, but the snow load is on TOP of the truss and not sitting on the bottom chord. And yes, it DOES make a difference...

[Peter_C]

Our tract home uses 2X4's that are held together with these cheese ball pound in flat thingys, so in the rafters we only store cardboard boxes for stuff that might need to be warrantied. What we did for storage was to build 7 4'X8'X24" cabinets to hold all of our junk...I mean prized possessions.

Edit: 1/2" ply is pretty thin...

[maddog]

Yeah, but the snow load is on TOP of the truss and not sitting on the bottom chord. And yes, it DOES make a difference...

Your right about that. That particular Fink was designed for 10 lbs’ per square foot dead load on the bottom cord. It works as a complete system top cord at 40 lbs live (snow) load with an additional 7 lbs. for dead load. With this Truss I would have no fear of storing my extras (within reason) inside the webbing especially with 17 lbs per sq ft total factored in for dead load. I don't know about your state but here in California Structural Engineers will tell you they design their buildings (structures) by a additional 30% calculations purely for liability reasons.

[PAToyota]

Yep, worked for structural engineers for about ten years. Currently finishing up my architectural registration. Always design with a safety margin to CYA! :)

The other thing to consider is the question of point load vs. distributed load. A snow load on the roof sheathing is different from an axle spanning across a couple trusses at midspan.

Unless a truss was specifically designed for a storage bearing condition, I'd be careful about what was thrown up in the rafters.

[maddog]

Yep, worked for structural engineers for about ten years. Currently finishing up my architectural registration. Always design with a safety margin to CYA! :)

The other thing to consider is the question of point load vs. distributed load. A snow load on the roof sheathing is different from an axle spanning across a couple trusses at midspan.

Unless a truss was specifically designed for a storage bearing condition, I'd be careful about what was thrown up in the rafters.

Yes your right but the 10 years is now more so than ever. Especially after the Northridge and Landers Quakes. Now I'm only talking light to moderate storage. I really didn't think this would get into a in depth thread. Your axle spanning across a couple of bottom cords is correct for sure. You would not want to concentrate a 500 lb load on 2 trusses. If you read earlier 96 lbs for a 32 square foot area would not have a significant affect on the 5 trusses. Especially with the Fink truss showed. The concentrated loads ,L, at 240 and of course structural lumber at 24” O.C. Now go to Chapter 23 of the Uniform Building code for the modulus of elasticity which will show allowable bend stress ,Fb, of the truss lumber throughout. That’s why truss webbing is accordingly designed and spaced with the size of gang plates installed. If you have a 20 ft piece of train rail supported on both ends and a fly lands in the middle of it, will it change its values? Of course it will, but will it have a ill affect on it? Of course not. Light to moderate storage spread out on the bottom cords will not have such an ill affect on the overall structure. In fact I talked with a Structural Engineer today and his thoughts were pretty much along the line of my thinking. Hell, I’ve had storage in my Garage rafters (25 ft span Fink design) the day we moved in 20 years ago and it survived the Landers 6-28-92, 7.3 quake which is only 60 miles from Victorville. That did a nice rock and roll but everything survived. I was totally surprised nothing fell out of the rafters. I guess mainly because it was all on ¼” sheets of plywood and spread out for a uniform load.

[PAToyota]

Yep. This has gotten a bit more involved than it looked at first... :)

Modulus of elasticity is dependent upon the species of timber and influences deflection where delta is equal to 5 x uniform load per foot x span^4 / 384 x Modulus of Elasticity x Moment of Inertia for a uniformly loaded beam (showing equations is a bit difficult in this format... :) Fb is the design value for extreme fiber in bending based on grading and width. But anyway...

I'm not sure that 1/4" plywood would provide enough of a diaphragm to adequately spread out the load. Typically you'd start with 3/4" T&G plywood. It seems to have worked well enough for you, though.

I guess the point we are both making is that a uniform load of a reasonable amount is not a problem, but be careful about concentrated or otherwise extreme loads.

[maddog]

I guess the point we are both making is that a uniform load of a reasonable amount is not a problem, but be careful about concentrated or otherwise extreme loads.


Amen!

[Sundowner]

Yep, worked for structural engineers for about ten years. Currently finishing up my architectural registration.

TRAITOR!

[PAToyota]

TRAITOR!

Ahh, but see, I got my architectural degree years ago. Realized that they didn't teach you squat beyond "making things pretty." So I've worked construction, got my welding certifications, worked for structural engineers, and did industrial design. So now I'm going back for the architectural registration. The engineers love working with me because I can talk their language! :cool2:

[maddog]

Ahh, but see, I got my architectural degree years ago. Realized that they didn't teach you squat beyond "making things pretty." So I've worked construction, got my welding certifications, worked for structural engineers, and did industrial design. So now I'm going back for the architectural registration. The engineers love working with me because I can talk their language! :cool2:

Now that should be the inspiration for all Architectural Engineers. Or have the up and coming Architectural Engineers spend some real time into structural theory, calculations and design. We almost on a daily basis encounter Architectural Engineers designing structures outside the conventional light frame construction realm. Structural calculations in many instances are so far off you have to wonder if they ever reviewed Chapter 16 of the UBS > Structural Design Requirements < My hat off to you for wanting to not just make something pretty but to build a structure with sound integrity.

[Bmf24]

Ahh, but see, I got my architectural degree years ago. Realized that they didn't teach you squat beyond "making things pretty." So I've worked construction, got my welding certifications, worked for structural engineers, and did industrial design. So now I'm going back for the architectural registration. The engineers love working with me because I can talk their language! :cool2:

but to a contractor you are still evil.

i started in architectural school, but after a semester i realized architects are only about the looks.

[PAToyota]

but to a contractor you are still evil.

And I've seen more than enough evil things that contractors have done... :rolleyes:

When I was working for the one structural engineer, we spent about six months after a unusually heavy snowstorm going around inspecting buildings for people and insurance companies that were "design/build" by people that really did not understand the first thing about the "design" part...