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Author: Subject: ? for structural engineers
Santiago
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[*] posted on 1-23-2018 at 08:45 AM
? for structural engineers


I have a 12' X 22' flat roof that we use as a deck. The joists are 2X6 @ 2' centers, covered with 3/4" ply and GacoDeck sealer. The joists sit on top of a double 2X6 rim joist and are secure to this 'beam' with twist ties and solid blocking. Also solid blocking at 4' centers in the field to support the plywood seams. The plywood has been glued and screwed to the joists and the blocking. The rim joist sits on a block wall and never spans more than 3'. Clear span of the joists is a little over 11'.

The problem, of course, is that the 2X6 joists are way over spanned and the deck is too bouncy. We seldom intended to use the deck but find ourselves more and more using it, going so far as to plan on a permanent roof for shade in the summer.

There are two rooms below the deck: a 12' X 9' kitchen and a 12' X 13' bedroom, separated by a block wall. These rooms are 10' tall.

The obvious way to cut down the span is to put a beam below the joists supported at each end and using the block wall between the two rooms as a center support. However, this can't be done as the two end walls have glass block windows right where the beam ends would terminate.

My idea is to add a 2X6 below the existing joists, hang them using a typical Simpson joist hanger as they will butt to the rim joist. Then install an 11"X48" plywood gusset, glued and screwed to the two joists in the center bay between the solid blocking. I am trying to build a 2X12, in other words.

Keep in mind that I am 67 years old and working by myself so any method I must be able to do from ladders and be safe. SWMBO will be there to 'help', but is not allowed on ladders.

Comments and suggestions appreciated but we have already eliminated the obvious "hire someone to do it" so you can leave that one out.


[Edited on 1-23-2018 by Santiago]
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ehall
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[*] posted on 1-23-2018 at 10:07 AM


How about doubling up on all the joist? Not sure how much work it would be with the blocking.
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imlost
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[*] posted on 1-23-2018 at 11:16 AM


Instead of one beam down the center, can you install two beams perpendicular to the joists to break the span up into thirds? Supporting the joists with beams is likely the easiest solution.
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Santiago
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[*] posted on 1-23-2018 at 02:56 PM


Quote: Originally posted by ehall  
How about doubling up on all the joist? Not sure how much work it would be with the blocking.

Cutting the blocking in place is kinda hard but do-able. I've been told by engineers that twice as tall is better than twice as thick.
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ehall
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[*] posted on 1-23-2018 at 04:08 PM


Twice as tall will increase strength 4x. Twice as wide will only double it. The problem with these calcs is they boards have to be bonded together as 1.
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[*] posted on 1-23-2018 at 06:18 PM


If you could remove the blocking and space the joist at 12" on center that would optimul.
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[*] posted on 1-23-2018 at 06:42 PM


If you're trying to make it less flexy, adding more studs of the same dimension isn't going to help much. Increasing the vertical dimension of the joists or reducing the span is way more effective.

Sort of a basic, ballpark rule of sizing joists, is this: For every 1" in height, you can safely span one foot. Ex: a 2x6 will span 6'; a 2x8 will span 8', etc.
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[*] posted on 1-23-2018 at 06:47 PM


Old carpenter told me to do the multiplication, 2x4 will span 8 feet 2x6 12 feet ect. I wonder if they staggered the joints on the plywood, that can make big difference. As an alternative don’t fill the margarita glasses so full and when you hear the first small cracking noise turn the music up.
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[*] posted on 1-23-2018 at 07:28 PM


Quote: Originally posted by Sr.vienes  
Old carpenter told me to do the multiplication, 2x4 will span 8 feet 2x6 12 feet ect. I wonder if they staggered the joints on the plywood, that can make big difference. As an alternative don’t fill the margarita glasses so full and when you hear the first small cracking noise turn the music up.


That is correct with 12" spacing using pine.
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[*] posted on 1-23-2018 at 08:54 PM


Quote: Originally posted by Sr.vienes  
Old carpenter told me to do the multiplication, 2x4 will span 8 feet 2x6 12 feet ect. I wonder if they staggered the joints on the plywood, that can make big difference..


This^^^




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[*] posted on 1-23-2018 at 09:32 PM


Quote: Originally posted by Sr.vienes  
Old carpenter told me to do the multiplication, 2x4 will span 8 feet 2x6 12 feet ect. I wonder if they staggered the joints on the plywood, that can make big difference. As an alternative don’t fill the margarita glasses so full and when you hear the first small cracking noise turn the music up.

There are variables that affect the span. Remember, we're talking about joists; not rafters. Also, the spacing of the joists affects the span. There are lots of calculators available for figuring this out. Here's a pretty good one: http://www.awc.org/codes-standards/calculators-software/span...
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[*] posted on 1-23-2018 at 10:01 PM


In order to span 11' with a 50psf total load (40LL/10DL) you would need 3-2x6 sandwiched or a single 2x12.
Adding a 2x10 alongside each 2x6 would also work.
2 beams spanning the 22' length would require 6x16 beams to do the same job since the span is much longer.

Your plan to add a 2x6 (tightly) below should work with min. 1/2" ply on each side.
With rafters at 2' on center, you will still have a little flex in the plywood roof sheathing.

Hire Someone !! :-)

30 year Civil/Structural PE



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[*] posted on 1-29-2018 at 09:37 AM


Hire someone and do it! Ha Ha. I think your idea is great. Simple and to the point. Glue and screw. And if the beams are currently sagging you can wedge some 2x4s underneath them as you move along so you are starting with straight beams and reinforcing them from there.
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[*] posted on 1-29-2018 at 10:28 AM
I'm not a structural engineer.....


But I built a house for one! It was a split level house at Tahoe, and in order to have a clear span in the garage, he over designed the joists supporting the rooms above it.

The day I finished laying the floor sheeting, he came up to check it out. We were standing in the center of the floor, and felt every step his 40 pound dog took running over to join us. He turned to me and said, "Go ahead and put a beam in the garage!"

I think imlost has the best solution if the beams can be supported by the block wall between the kitchen and bedroom! That would also be a great time to correct any sag that may have already occurred.




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Bob and Susan
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[*] posted on 1-29-2018 at 02:22 PM


you know...you're 67 yrs old

don't do anything...just enjoy your place

NO construction

go fishing

enjoy life and don't worry about the small stuff...and...its ALL small




our website is:
http://www.mulege.org
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[*] posted on 1-29-2018 at 03:55 PM


A quick glance at standard Maximum Span Tables (floor joist, Douglas Fir visually graded) shows that

2 x 6 on 2 foot centers has a maximum span of 9 feet.
2 x12 on 2 foot centers has maximum span of 17 feet ten inch.
So you could put in a load bearing interior wall three feet away from the exterior wall and convert it into a large closet -- :bounce:
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