# Help with bouncy floors



## papakevin (Oct 24, 2011)

I've got a house that I'm fixing up over time, and I'm looking for some help. 

Long story short, bought it distressed, fixed foundation issues in back of house and replaced subfloor. Had major plumbing issue and crawl space completely flooded (had 4 feet of water in it).  Pumped water out, dried it out over the next week, and now three months later it seems floors are bouncy in living room where the original floor was and kitchen where I replaced subfloor. 

Looking under house, everything looks ok. House was a kit house from the 40's, and all wood looks good. The 2x8 beams connect to center support, which has concrete blocks supporting it every 7 feet or so in the middle of the house.   Wondering if water flooding crawl space made blocks settle. Have small access to crawl space via the closet and trying to avoid tearing up original hard wood flooring to address issues. Might be able to punch out vent screens along side of house to allow for additional boards to be feed into space so they can be sistered onto the existing 2x8s, but don't know if that's the best course of action. 

Question if adding additional supports to main beam in between the existing blocks would help. Have thought about adding cut 2x4s as cross bracing in between joists at bottom of joist (to help stabilize) and transfer weight to adjacent beams, but don't know if that would work.  Have also thought about adding a middle support to help offset flex in living room, but thinking this may not be best plan either. 

Looking at options, so any help is appreciated.


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## nealtw (Oct 24, 2011)

You didn't say how long the floor joists are. In new construction, we always use 2x10, and when the span is over 16ft the engineers often call for double floor joist or changing to 12" on center. 2x8s would be shorter. Bridging between joists are added when joists are more than 7 ft long, 2 times for 14 ft. Your 2x4 blocking between joists will work as bridging if none is there.
Adding a center support for the joists only works if you have a footing installed below frost level and covered, you wouldn't want to have the frost heave the new supports.


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## papakevin (Oct 24, 2011)

nealtw said:
			
		

> You didn't say how long the floor joists are. In new construction, we always use 2x10, and when the span is over 16ft the engineers often call for double floor joist or changing to 12" on center. 2x8s would be shorter. Bridging between joists are added when joists are more than 7 ft long, 2 times for 14 ft. Your 2x4 blocking between joists will work as bridging if none is there.
> Adding a center support for the joists only works if you have a footing installed below frost level and covered, you wouldn't want to have the frost heave the new supports.



Thanks for the reply.  The 2"x8" joists are 12' in length and spaced approx 18" apart. (Haven't measured, but looks to be about that wide.)  The joists do have the little X boards in the middle of the joists, which are 1" by 4" slats. Thinking I can cut to fit and stagger the 2 by 4 boards in between the joists to help stiffen the floor. Just making sure I'm barking up the right tree before I do it. Thanks.


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## nealtw (Oct 24, 2011)

I don't know but I would think 12 ft is about max for a 2x8. You maybe want to double up 2 or 3 of them just to see the diff. that makes.


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## papakevin (Oct 24, 2011)

I've been reading up on bridging and it seems like it may help. Originally I was thinking I could stagger some 2x4s as bridging, but now thinking I might need to go 2x6's or even metal bracing. 

In one of the articles I saw, it said that staggering the bridging - so you can nail directly into the board - is not recommended and that all boards should be in line.  You direct nail one end and toenail the other. Does this sound right or will staggering work?  Also, is it better to install the thin metal bridging of the wood?  Final question: would it help to liquid nails in the wood bridging as well as nailing, or would that be a waste of time?


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## nealtw (Oct 24, 2011)

The first reason for bridging is to stop the joists from rolling sideways when it warps and maintain spacing. I have seen solid blocking tried and I am not a beleiver. If you think about a stone arch with a keystone, it holds the arch in place, but when your floor bends down your arch is upside down. I would still go for double joists.


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## BridgeMan (Oct 25, 2011)

If you're sure the joists are deflecting from being over-stressed, and not the beam (supported on piers) they tie into, a better solution than bridging would be to add flitch beams to the bottom of each joist in the "bouncing" area.  Doing so is fairly easy, and can do wonders for increasing the section modulus of the members in question, making them stiffer and stronger.


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## papakevin (Oct 25, 2011)

So, I've looked up the flitch beam. From what I can tell, it appears to be sistering in another joist with a steel plate in between, is this correct?  If yes, where would one locally source these magic steel plates and what size would they need to be?  Do they run the entire length of the beam?


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## papakevin (Oct 25, 2011)

Wait, just read this again and saw you said to the bottom of each joist. Are you talking about underneath each beam?  I'm missing something, but would like to know more please.


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## papakevin (Oct 25, 2011)

Ok, I see where others have used plywood instead of steel for a flitch plate. Assuming I that's what you were talking about, but still interested in learning more please.  Thank you.


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## joecaption (Oct 25, 2011)

We have worked on 100 plus year old houses for the last 15 years.
One sure way to fix this is to add footings 24" X 24" by 6 to 8" thick with rebar in them every 8' running along the middle of the floor joist and add a doubled up 2 X 8 beam the full length of the area your trying to fix sitting on concrete block piers.  We use constrution adhesive when forming the beam and through bolt with 1/2" carrage bolts.
We have been able to use bottle jacks to lift the floor so there's no more sag and stop the bounce with this one beam. 
If the old floor joist are soft, fungus eatten or broken we sister new ones and just tack the two ends, install the beam them them attach the new joist to the old one.


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## BridgeMan (Oct 26, 2011)

papakevin,

Yes, in the sophisticated sense, when massive increases in strength are required, a steel plate is often installed at the bottom of any member in bending.  In your case, however, that would be definite overkill, and not at all necessary (unless your joists are made of bamboo, or balsa wood!).  But seriously, for your situation, simply gluing and screwing a 2 x 4 or 2 x 6 (the flitch plate) to the bottom of the joist (bigger, the better, but often not necessary for shorter span, residential framing) in the center 2/3 or so, where bending stresses are greatest, can often double the strength (and resistance to deflection) of each joist.  With the 2 x in the horizontal direction, and relocated away from the joist's neutral axis, it contributes significantly more to the moment of inertia and subsequently, section modulus, of the built-up joist.  I won't try to confuse you by showing the formula for computing a member's overall strength, but trust me, it's true.

Think of it as acting like a steel girder, whose flanges are always oriented perpendicular to the web, and whose bottom flange is often larger and thicker than the top flange, to contribute to its resistance to bending stresses, and subsequently, deflection.  As a test to convince yourself, you can take a standard 2 x 4, 8' long, and nail it, 3-1/2" dimension vertical, on some raised supports at each end (to prevent it from tipping).  Then step on it in the center, and note how much it deflects.  Then screw (skip the glue, it's just an experiment) a flat 2 x 4, 5' or 6' long, to the underside of the long 2 x 4 in the middle, and repeat the step test--you will notice a significant difference in the amount it deflects with the same amount of downward force--probably only half as much movement with the same downward force.


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## nealtw (Oct 26, 2011)

Bridgeman; I have never heard that before and it makes sense I think. So if I have it right you would just strap the floor joist with a flat 2x4 on the bottom of each. I kinda hope papakevin tries this so he can tell us the results. It would be the quick fix.


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## papakevin (Oct 27, 2011)

Ok, I like the 2x4 idea and will give it a try. Not sure I get the concept exactly, but then again, all I need is for it to work. I may come back and post a drawing of how to install to verify I understand the installation process. 

Prior to reading the last few posts, I had my options narrowed down to bridging with 2x8s twice on each span (there's an existing 1x cross bridging in the middle I didn't want to remove - or - cutting 8" strips of 1/2" plywood, glue and screw into the existing joists, then repeat a second time staggering the seam.  Since there is insulation in place and I have a limited amount of height in the crawl space (it is a crawl space after all) the 2x4 option seems like a much easier and quicker solution. 

Taking some vacation time next week, so planning to give it a shot then.


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## papakevin (Oct 28, 2011)

OK, BridgeMan, I want to make sure I understand your 2x4 suggestion before I try it.

My assumption would be you attach the 2X4 to the bottom of the floor joist, with it laying flat, so it makes an upside down T, attaching with lots of Liquid Nails and screws. 
The other option I can see is to sister it to the side of the joist, at the bottom side and not to the bottom of the joist, but this doesn't seem to be what you are saying. 

Please let me know if either option above is what you are recommending or if there's something else. I plan on tyring this next week and I will take plenty of photos to document the process for future posts. 

Again, appreciate any and all information.


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## joecaption (Oct 29, 2011)

A real simple and 100% sure way to correct this problum is to add a row of footings 8' apart running down the middle of the room your trying to stop from bouncing. Add a doubled up beam of 2 X 8's and have it sitting on concrete block piers.
No form of 2 X 4 is going to do anything to stop the sagging or bouncing.
By installing this beam you can also lift the floor in the center and take out the sag that's most likly there now.


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## papakevin (Oct 30, 2011)

joecaption said:
			
		

> A real simple and 100% sure way to correct this problum is to add a row of footings 8' apart running down the middle of the room your trying to stop from bouncing. Add a doubled up beam of 2 X 8's and have it sitting on concrete block piers.
> No form of 2 X 4 is going to do anything to stop the sagging or bouncing.
> By installing this beam you can also lift the floor in the center and take out the sag that's most likly there now.



Well, that was my first plan, but thought there might be a simpler and easier way to correct the problem.  In my primary residence, that's exactly what I did to fix my bounce issue, in my 13 year old home with composite iBeams. There, I had an unfinished basement, so I was able to fix it when I finished it out. 

In this instance, it's a small crawl space, maybe 3 feet of headroom between the floor and bottom of the joist.  With the tight space, I wanted to look at different options. The floor doesn't have any noticeable sag now, but I haven't stuck a 8 foot straight edge across the middle either. The bounce isn't terrible, but it is definitely there. 

Curious, have you tried the 2x4 method and found it to be ineffective or are you simply pointing out the best option which is guaranteed to work? I figured I'd give the 2x4 approach a try first on four or five beams to test it since that solutions seems to be the easiest.  If it didn't work well enough, then I could look at adding cross bracing (bridging) or metal straps.


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## BridgeMan (Oct 31, 2011)

joecaption said:


> No form of 2 X 4 is going to do anything to stop the sagging or bouncing.



Sorry, joecaption, but the following numbers I've crunched clearly prove that a 2 x 4 flitch plate installed flat on the underside of a floor joist can help considerably to reduce unwanted deflections.  I know it's a concept that some (many?) may find hard to believe, but perhaps the following will change a few minds. 

For this purely hypothetical example, I used a modulus of elasticity (E) for both the joist and flat 2 x 4 of 1,800,000 psi, and assumed a simple (not continuous) span length of 10' = 120".  I've used a 2 x 8 joist section having a depth of 7.25", a width of 1.5", an area of 10.880 sq. in., and having a moment of inertia (I) of 47.64" (to the 4th power).  The flat 2 x 4 has a depth of 1.5", a width of 3.5", an area of 5.25 sq. in. and an I of 0.984" (again to the 4th power--it's so small because of the flat orientation).  
The total I of the built-up section is computed by adding the individual I amounts to the value of A x d squared, where d is the distance from the revised neutral axis of the built-up section (computed by moment-area method) to the centroid of the flat 2 x 4.

Applying a concentrated 10,000 lb. load at midspan of the plain 2 x 8 results in a maximum deflection at midspan of 4.198".  Now, if we glue and screw a flat 2 x 4 (only has to be 7' long, centered, to resist the maximum bending moment occurring at the point of load) to the bottom of the 2 x 8, and apply the same 10,000 lb. load, the deflection of the built-up member computes at only 2.112".  The numbers speak for themselves--4.198" vs. 2.112".

To summarize, the difference in deflections is very close to a factor of 2 if the flat 2 x 4 is not attached to the bottom of the 2 x 8.  Or in other words, the joist will deflect only half as much with the 2 x 4 attached as it would without it.  If anyone would like to check my arithmetic, the formula for maximum midspan deflection is /_\ (delta, total deflection) = P (load) x L (span length, in inches) cubed, divided by the product of 48 x E x I.  The formula for moment of inertia of any rectangular section is I = b x d cubed, divided by 12.  The foregoing formulas have been used and  accepted in the engineering world for close to 100 years, should anyone think or suggest I'm making things up.


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## joecaption (Oct 31, 2011)

No form of cross bracing will do anything to stop the bounce. Cross bracing is only to stop the joist from twisting.
A 2 X4 is not good for any type of support because there only rated for a very short span and will do 0 good if just attached to the bottom.
 Take even a 4' long 2 X 4 and set it between two cement blocks and stand on it and see what happens, it bends, if stood on end it's stronger but even an 8' one would stap if stood on.
A 3' High crawl space is more then enough room to work in, not fun but can be done, we've done dozens of them.
We use a metal concrete mixing pan with holes drilled in two ends and attach a rope to each end. Ones for pulling the material in and ones for pulling it back out.
Do not try and preform the pads then try to pull them in place, it takes over 3, bags at 80 lb. each to form a proper pad, 240 lb.


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## joecaption (Oct 31, 2011)

Maximum Span Calculator for Joists & Rafters


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## BridgeMan (Oct 31, 2011)

Thanks, joecaption, for furnishing the link to the span tables and mentioning how ineffective cross-bracing is for reducing joist deflections. But you haven't presented anything showing the example numbers I presented in Post 18 are incorrect.  

To summarize those numbers--for a typical floor joist, the amount of vertical deflection caused by applied loads can be cut in half by simply adding a flat 2 x 4 to the bottom of that joist.  For the example I used (a 10', 2 x 8 joist with a 10,000 lb. concentrated load in the center), the actual deflections are 4.198" (without the 2 x 4) and 2.112" (with a 2 x 4).  In summary, the difference is a factor of 4.198 divided by 2.112, or 1.99.  Pretty close to 2 when rounded (or twice as much deflection without the 2 x 4 flitch plate on the bottom).

Let me try to explain the concept in a different way--the next time you are out and about, driving your vehicle on public highways, take a look at the shape of each girder on steel bridges you drive under.  You'll notice the girders are almost always shaped like an "I".  This particular shape has been used for more than 90 years because of its ability to economically resist applied loads, while minimizing vertical deflections caused by those loads.  And the lower flange of each girder (the flat, horizontal part on the bottom) is what enables each girder to resist heavy loads applied to the span.  By itself, the flange would be useless in resisting loads (it would bend and deflect, just like the flat 2 x 4 you mentioned).  But because it's attached to the rest of the girder, it becomes integral with the overall member, enabling the member to withstand loads.  And if we take a closer look at the lower half of a typical girder, you'll notice how it resembles an inverted "T".

Which is the same shape as a floor joist with a flat 2 x 4 on its bottom.  Acting just like the bottom half of a highway bridge girder, carrying its applied loads safely and without abnormal deflections.


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## papakevin (Nov 2, 2011)

Ok, started installing the 2x4's on the bottom of the joists today. Only got 3 installed, so won't know fully it is works until the rest are installed. 

Ended up installing a new vapor barrier first because I had to pull up the old one after the crawl space flooded (bad plumbing leak) few months back.  Made it easier and more pleasant to move around in the cramped space. 

Predrilled the 2x4s before taking them under the house so the center screw holes were there and aligned. Used about a 1/2 tube of Heavy Duty liquid nails on board to secure it to bottom of joist. Cheated and got 10' 2x4s which allowed me to put one end on top of the block foundation, which helped with my one man install. 

I will post results and photos once this project is complete. 

Disappointed to hear that cross bridging will not help bounce. My plan B was to install solid block 2x8 bridging, but it sounds like that isn't a good option, right?


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## nealtw (Nov 2, 2011)

Solid blocking only helps a little when one person is on the floor, when six people are on the floor, it bends down, open up the bottom of the blocking and starts squeeking. If Bridgeman's 2x4 don't work, you are either doubling the joists or installing a beam as Joe suggested.


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## Dionysia (Nov 3, 2011)

I am fascinated and eager to know how this experiment turns out. Please keep us posted papakevin! And BridgeMan, thanks for the mathematical rundown. I've never had the time to try to puzzle out the engineering stuff on my own.


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## papakevin (Nov 3, 2011)

OK, I'm trying to post some photos, so I hope it works. Here's an update on where I am as of tonight:

(1) - I'm sore as hell and feel like I've been beat with a bat.  Prior to putting down the new vapor barrier there was one down when the crawl space flooded.  When I crawled around under the house, I created a ton of small imprints in the mud below.  It dried hard when I removed the old vapor barrier, so it's like crawling / laying on large rocks and it sucks.  Plus it's a tight crawl space, so I used some muscles I haven't used in a while.  

(2) - Smart thing I did do was buy a couple clamp lights to use to light up the crawl space.  Prior to doing this I was working with a flashlight which sucked.  Being able to see what you are doing without holding a light is a big help.

(3) - A +1 vote for NASA.  As my coveralls, I wore an old Challenger flightsuit / jumpsuit that I purchased at Kennedy Space center years ago.  It's not a real one, but the cheap ones they sell to visitors.  Anyway, after the Challenger accident, they removed the shuttle patch and sold them at discount.  The placement of the zippered pockets on the arms, chest, made really great places to hide a small flashlight, camera, screws, etc.  

(4) - Used longer 2x4's than necessary so I could rest them on the block wall.  Because the house is old construction (40's) the sill plate is just a bit thicker than the 2x4, which allowed me to prop it up while I attached the other end.  Installation was easy once it was in place, which was good because I was working solo.

(5) - Strong drill with a fully charged battery is key.  With the number of screws you install per board, you need to do a lot of screwing (with the drill that is), and you really need to push up hard to verify you are getting a good bite.  Used a Craftsman 19.2 volt drill and it worked well.  

Will check it out tomorrow to see how successful the process is thus far.  I didn't want to bounce on the floor or walk on it until the liquid nails has dried.  Hopefully more updates will follow this weekend.  

Trying to post photos... we'll see if this works.


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## nealtw (Nov 4, 2011)

Just had a look at your plumbing thread. Can you have a look at the beam running the center of the house that we see in those pictures. Is that one or two plys 2x8 on top of the flat 2 x ?. 
Bridgeman will be pleased with you work so far, that old lumber can be a bear to screw into. 
Looking forward to the results and hopeful that it works, we all have things to learn.


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## papakevin (Nov 4, 2011)

OK, here's a couple updates.  First, the ruling on the 2x4's, then an attempt to answer the questions on the beams.

So, the verdict is....... it worked!  BRIDGEMAN for President!  Yes, there still is some bounce in the floor, however the improvements are noticeable and it should be good enough that no additional floor support is needed.  My only regret is not completing the entire job all at once, because now I need to go back under and do the 3-4 joists that I didn't do in the living room.  And yes, I'm still sore and walking with a slight limp from my last trip in the crawl space....

On a related note, I may go ahead and install underneath the beams that support the floor that supports a couple of the interior walls.  (Let me explain...).  Apparently the house was built so that the entire hardwood floor was installed first, then the walls were built on top of the wood floor.  The walls themselves are not necessarily aligned with the beams, so in some instances the walls are actually resting on the wood floor in between two beams.  Seems a little odd to me, but maybe that's the way they did it back in the 40's.

Now the beam question Nealtw asked about.  Sad to say I didn't give the center beam a close look, but I did take some random photos while I was under the house.  From what I can see, I believe the center beam is actually two 2x10's nailed together, with some 2x 2 block strips installed along the bottom to hold the 2x8's in place.  Guessing they did this to rest the 2x8's on the rails, then toe nailed the 2x8's into the 2x10's.  Maybe they didn't have joist hangers back then, or maybe this was just the cheap way of doing it.  I'm adding some photos for additional review / comments.  Don't know if adding anything to the bottom of these joists (like with the 2x4's) would help add structural support or hurt, but open to feedback and discussions.  

Bridgeman, thanks again for the suggestion.  I really didn't know if it would work, but I did have some faith and it worked for me well enough to become a believer.  Thanks again!


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## BridgeMan (Nov 5, 2011)

You're totally welcome, papakevin, and just glad I could help.  I suspect there still may be a few "unbelievers" out there, but all I can say is that what I've suggested is based on proven engineering principles.  And in reference to my earlier posting explaining the action of a flat member, acting integrally in the tension zone of a bending member, more than 2.5 million* bridge girders can't be wrong.

*  (My estimate, probably very conservative, for the total number of steel girders in bridges in this country--double or triple that for the rest of the world)


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## Dionysia (Nov 7, 2011)

So BridgeMan, would the same theory apply when trying to strengthen ceiling joists in an old house? And since the load is on the bottom, would the 2xs need to go on top? 

I am asking because my 1920's bungalow had some storm damage and we had to take down the lathe and plaster ceiling in one room. The 2x4 ceiling joists span about 10 feet and all but 3 of them are spliced. Without the lathe, I don't think they will hold up drywall, and there is no room under the bottom of the roof slope to put up 2x6s.


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## BridgeMan (Nov 7, 2011)

Ceiling joists experience what's known as positive bending moment (just like floor joists), a downward force due to the weight of the members themselves, ceiling surfacing/covering, attic insulation, etc.  Floor joists often don't have drywall and insulation loads, but have heavier live loads in the form of people and furniture.  

Think of positive bending moment as bowing downward like a bowl, and holding water.  Such bending moments place the bottom fibers of the loaded member in tension, while the tops are in compression.  Any additional stiffening (flat) members would still best be added to the bottoms, primarily because most wood species are typically 50% stronger in tension than compression, parallel-to-grain.  In the case of ceiling joists,  you'd want to install the flat 2 x 4s full-length on the joist bottoms in a given room to make the drywall application surfaces uniform, even though the additional members are only needed at the center 2/3 of any span for structural reasons.  An added advantage of flat 2 x 4s on the bottom of each joist is gaining the additional nailing/screwing area (3-1/2" vs. 1-1/2" wide) they provide.

An exception to the foregoing is, if instead of being oriented in simple-span configuration, load-carrying joists are continuous over one or more interior supports.  Not normally seen in residential construction, as the members would be abnormally long and not economically practical.  In such a scenario, however, there will be negative bending moments (tendency to bow upward) at the interior supports, and in such instances the 2 x 4s could be installed on the tops of the joists at support locations, and at the bottoms in mid-span locations, to do the most good.


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## BridgeMan (Nov 7, 2011)

Dionysia said:


> . . . . and there is no room under the bottom of the roof slope to put up 2x6s.



Actually, 2 x 6s could be sistered onto each 2 x 4 ceiling joist by taper-cutting the ends where the roof slope interferes.  But a stiffer ceiling will result if each 2 x 4 joist would have a 2 x 4 flat member glued/screwed to its bottom.  Crunching the (approximate) numbers for the section modulus of each configuration, it works out to 13.62 cu. in. vs. 10.63 cu. in., or a difference of approximately 28% stiffer with the flat 2 x 4s on the bottom.


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## Dionysia (Nov 8, 2011)

Thanks BridgeMan - I will pass this info onto the Mr. As always, I appreciate your number-crunching!


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## joecaption (Nov 9, 2011)

In your case I'd just use at least 2 X 6's.  That's the way it's always done with a new home with a low slope roof.
Those 2X 4's are not even close to being wide enough to support the load or plaster. I'd remove the plaster and use light weight 5/8 sheetrock.


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## papakevin (Nov 14, 2011)

This will likely be my last post on this thread unless something else comes up.  

Ended up getting a few more 2x4's to finish up the floor under the living room.  It might have been a mental thing, but it seemed like I could tell where I stopped putting the 2x4's in the middle, so I went ahead and finished off the room.  Execpt for the insulation hanging down, I think it turned out alright.  

Thanks again Bridgeman for the idea.  Love it when a plan comes together.


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