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 DIY Chatroom Home Improvement Forum How do engineers determine weight limit for a roof?
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01-31-2013, 04:29 PM   #1
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## 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

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01-31-2013, 05:21 PM   #2
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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...

 The Following User Says Thank You to Bondo For This Useful Post: noquacks (01-31-2013)
01-31-2013, 05:48 PM   #3
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Quote:
 Originally Posted by Bondo 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.
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 01-31-2013, 05:50 PM #4 Member   Join Date: Jun 2010 Location: se Posts: 1,597 Rewards Points: 1,062 heres the picture of it: (from above/outside, of course) Remodeling deck surface- How? __________________ Alexander Pope: [a little knowledge can be a dangerous thing]
 01-31-2013, 07:25 PM #5 journeyman carpenter     Join Date: Sep 2011 Location: nova scotia canada Posts: 3,315 Rewards Points: 1,248 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
 The Following User Says Thank You to woodworkbykirk For This Useful Post: noquacks (02-02-2013)
 02-01-2013, 12:29 PM #6 Member   Join Date: Aug 2012 Posts: 89 Rewards Points: 93 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.
 The Following 3 Users Say Thank You to dumbengineer For This Useful Post: epson (02-01-2013), noquacks (02-02-2013), woodworkbykirk (02-01-2013)
 02-01-2013, 01:25 PM #7 Member   Join Date: Apr 2012 Location: UK Posts: 714 Rewards Points: 502 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.
 The Following User Says Thank You to tony.g For This Useful Post: noquacks (02-02-2013)
 02-01-2013, 05:27 PM #8 Member   Join Date: Nov 2011 Location: Hartfield VA Posts: 34,440 Rewards Points: 14,010 3 X 6 is massive? Now that's funny. If that's a soild wooden beam it has almost no side load strength.
02-02-2013, 07:35 AM   #9
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Quote:
 Originally Posted by woodworkbykirk 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.
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 02-02-2013, 07:38 AM #10 Member   Join Date: Jun 2010 Location: se Posts: 1,597 Rewards Points: 1,062 I will confirm the size of our infamous MASSIVE beam, people.........prepare yourselves..... __________________ Alexander Pope: [a little knowledge can be a dangerous thing]
 02-02-2013, 07:40 AM #11 Member   Join Date: Jun 2010 Location: se Posts: 1,597 Rewards Points: 1,062 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. __________________ Alexander Pope: [a little knowledge can be a dangerous thing]
 02-02-2013, 08:12 AM #12 Member   Join Date: Jan 2012 Location: ohio Posts: 1,164 Rewards Points: 636 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 __________________ "be careful at listening to just one persons advise" said me "be careful at listening to just one persons advise" said myself p.s. not all i say is a fact but not all i say is my opinion. Last edited by jaydevries; 02-02-2013 at 08:21 AM.
02-02-2013, 03:53 PM   #13
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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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