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Discussion Starter · #1 ·
My garage roof is 2x6 T&G decking on 4x6 beams that span 14 ft. They rest on the top plate of 2x4 stud walls. The beams are about 5 ft on center. The flat roof material is torch down rubber. The roof sags about 1" in the middle causing leaks and ponds. I need to jack the roof center up and strengthen the beams to eliminate the sag so that rain will not puddle and leak. We are going to put a membrane roof on to finally eliminate leaks. Any suggestions as how to reinforce the roof beams? I was thinking about sandwiching the beams between 2x10 or 2x12's. Not sure how best to attach? screws, bolts ?
 

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Discussion Starter · #3 ·
Thanks for that. The 4x6 beams (rafters) rest on the top plates of the garage walls. The 2x6 decking is nailed to the beams. And yes, I jacked up one of the beams (in the center of the span) and was able to raise it about an inch which eliminated the pond when I sprayed with water. Need to jack up at least three, if not four of the beams and then figure out a way to hold them so they don't sag again. The garage is 25 feet long. Hope that answers your question.
 

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Thanks for that. The 4x6 beams (rafters) rest on the top plates of the garage walls. The 2x6 decking is nailed to the beams. And yes, I jacked up one of the beams (in the center of the span) and was able to raise it about an inch which eliminated the pond when I sprayed with water. Need to jack up at least three, if not four of the beams and then figure out a way to hold them so they don't sag again. The garage is 25 feet long. Hope that answers your question.

Have you checked it with a level to see just how bad the sag is?
If you lift just one and check it with water, it will be higher than the rest but may not be high enough.

You could jack one up like that and place a 2x4 to hold it and move to the next one so you have them all jacked up.
Then if you wait a day or two you could likely lift it a little more. But there is a fear you could stress it to much and lift it off the wall.
You might want to add hurricane ties to make sure they stay attached.



You would like to get them high enough that you have some hump in the middle.

A 2x12 on each side should help it. With nails you would want 3 every 16". Screws don't have the same sheer strength as nails so you might want 2x as many screws.
 

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Discussion Starter · #5 ·
Thanks very much for the comments. Glad to hear about the nails vs screws. I jacked up the center beam but will also do the adjoining beams I think that a very good plan. I'll be happy to have this problem behind me. thanks again
 

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Discussion Starter · #7 ·
I don't think so. Seems like sandwiching the existing beams with more appropriately sized material will provide what should have been there in the first place. At least that's what I think. I guess we will see. Hopefully for sure it will eliminate the drain in the center of the garage put there by the previous owner. Flat roofs shouldn't be flat!!:smile:
 

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I don't think so. Seems like sandwiching the existing beams with more appropriately sized material will provide what should have been there in the first place. At least that's what I think. I guess we will see. Hopefully for sure it will eliminate the drain in the center of the garage put there by the previous owner. Flat roofs shouldn't be flat!!:smile:
Yes agree, even a small slope can help.
 

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The most bang for your buck would be to glue and nail a 2x6 to the bottom of your beams. Get a basement jack, glue the beam then put 2x6 under and jack up beam until just past level (or build a beam and do several at once, if you have helpers). Then nail to ensure good glue contact, glue is what's actually doing the work. You'll gain more support for less money than sistering.

Reason for this is that all the support is done by compression and tension at the top and bottom of a joist, when you sister you're only adding 1.5" of max tension/comp material, and as you move towards the center of the joist it does exponentially less. When you glue to bottom chord you utilize almost all the material at max leverage to resist tension forces.

See chart here - https://www.decks.com/how-to/40/beam-span-chart-table Going from a double 2x6 to a 2x8 gains you 26" of span with only 2x3" of wood added, tripling up that 2x6 only gains you 15" of span.

So, adding mass below rather than to the side give you 43% more span and uses 28% less wood. Just butt up to walls of course, no need to set on plate. Just another option, either way works, I've done both.

Edit, also that roof should have a slope built into it with iso foam when it's reroofed, either towards a drain or to the outside of roof.
 

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retired framer
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The most bang for your buck would be to glue and nail a 2x6 to the bottom of your beams. Get a basement jack, glue the beam then put 2x6 under and jack up beam until just past level (or build a beam and do several at once, if you have helpers). Then nail to ensure good glue contact, glue is what's actually doing the work. You'll gain more support for less money than sistering.

Reason for this is that all the support is done by compression and tension at the top and bottom of a joist, when you sister you're only adding 1.5" of max tension/comp material, and as you move towards the center of the joist it does exponentially less. When you glue to bottom chord you utilize almost all the material at max leverage to resist tension forces.

See chart here - https://www.decks.com/how-to/40/beam-span-chart-table Going from a double 2x6 to a 2x8 gains you 26" of span with only 2x3" of wood added, tripling up that 2x6 only gains you 15" of span.

So, adding mass below rather than to the side give you 43% more span and uses 28% less wood. Just butt up to walls of course, no need to set on plate. Just another option, either way works, I've done both.

Edit, also that roof should have a slope built into it with iso foam when it's reroofed, either towards a drain or to the outside of roof.
I have heard of putting a board under the joist and thought it to be a good idea. Have you done it.
 
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