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-   -   How do engineers determine weight limit for a roof? (http://www.diychatroom.com/f19/how-do-engineers-determine-weight-limit-roof-170679/)

 noquacks 01-31-2013 04:29 PM

How do engineers determine weight limit for a roof?

People,

Now, Im trying to figure out if my flat roof can hold a certain weight which hopefully, will be a new exterior floor on top of it (many posts on my recent threads on various issues involved with this multi faceted project).

Yes, garage roof is made up of 2X6, 16" apart as usual, spanning a 2 car concrete poured foundation garage. 2X6's cross the garage from left to right, and down the middle of the beams if a massive 3X6 supporting the center of all the 2x6's. Of course, on each end the 2x6's are resting on the foundation. Roof has 1/2" ply currently with old asphalt membrane.

How do you determine the max weight of such a roof structure? Can one? Do we need a book?

Thanks

 Bondo 01-31-2013 05:21 PM

Quote:
 Now, Im trying to figure out if my flat roof can hold a certain weight which hopefully, will be a new exterior floor on top of it
Ayuh,.... Is this Floor, the same size as the roof,..??

If so, it'll be bridgin' the roof, 'n the floor weight will be transferred to the Walls, not the roof...

 noquacks 01-31-2013 05:48 PM

Quote:
 Originally Posted by Bondo (Post 1106660) Ayuh,.... Is this Floor, the same size as the roof,..?? If so, it'll be bridgin' the roof, 'n the floor weight will be transferred to the Walls, not the roof...
Thnaks, Bondo. Yes, floor is same size, matter of fact, roof IS the floor from above. But, right now, it IS bridged, and I want to get away from that if able to, so how does one calculate the max weight? Thers gotta be a way. Even if I go with a bridge, still would like to know.

 noquacks 01-31-2013 05:50 PM

heres the picture of it: (from above/outside, of course)
http://www.diychatroom.com/f19/remod...ce-how-169601/

 woodworkbykirk 01-31-2013 07:25 PM

whats the width of the garage..and where are you located. if you get any amount of snow and its more than 8' i would never use 2x6.. if your relating it to a floor system canadian code wont accept anything less than 2x8 for a floor. 2x6's are allowed only in the case of a small landing for a stairwell

 dumbengineer 02-01-2013 12:29 PM

i lol'd at this question. i'm a structural engineer by profession though i do industrial and not residential.

i dont think you will be able to do this the "engineer" way. it involves a lot of math. you need to know the maximum dead and live load then compute for maximum moment and shear and deflection and compare it to the capacity of the beams per buidling code.

better ask a contractor or superintendent, they can give you intelligent advice based on gut feel or experience. which maybe dangerous if you ask a structural engineer.

 tony.g 02-01-2013 01:25 PM

From quick back-of-envelope calcs.,assuming a live load of 40 psf and dead load 10 psf, the bending stress in your joists will be roughly 1000 psi, which is near the limit for lower-grade timber. Your deflection will probably be within the limit (just).
But you don't say what the "massive" 3x6 is (timber? steel? polystyrene?). Whatever it is,it will be supporting half the weight and it, and the supports, are the critical things here.

 joecaption 02-01-2013 05:27 PM

3 X 6 is massive? Now that's funny.
If that's a soild wooden beam it has almost no side load strength.

 noquacks 02-02-2013 07:35 AM

Quote:
 Originally Posted by woodworkbykirk (Post 1106737) whats the width of the garage..and where are you located. if you get any amount of snow and its more than 8' i would never use 2x6.. if your relating it to a floor system canadian code wont accept anything less than 2x8 for a floor. 2x6's are allowed only in the case of a small landing for a stairwell
garage is just enough to fit 2 cars plus 2' on each side (one left, one right), plust the space between cars, about 3". A 2 car garage. Do we need to the inch? Im in S Cal. No snow. just pricey gasoline and high rent/income taxes.

 noquacks 02-02-2013 07:38 AM

I will confirm the size of our infamous MASSIVE beam, people.........prepare yourselves.....

 noquacks 02-02-2013 07:40 AM

Meanwhile, thinking ahead of myself (a dengerous passtime), my intent is, if roof structure is strong enough, to lay on top of membrane, hardyboard, then seal, then MAYBE thin setted ceramic tile. I know its not revcomended, but just want to be able to at least throw out the idea permanently IF the structure wont hold it.

 jaydevries 02-02-2013 08:12 AM

try this for your membrane under tile will do drainage and crack isolation
http://www.schluter.com/142.aspx
or you could do this also
http://www.decksdirect.com/hardware-...-deck-supports

 woodworkbykirk 02-02-2013 03:53 PM

no matter what size beam your using i wouldnt be using 2x6` they will still sag unless you plan on having 3 beams for the ceiling joists to sit on keeping the joist spans under 7 `

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