# Reinforcing subfloor in 1970s house



## juryduty (Mar 10, 2008)

I have a 1970s house with 1.125" plywood subflooring over 4x6 joists 48" on center. It's pier and beam over a crawlspace.

How sturdy is this subfloor?  Is it common?  Can it handle most types of flooring? Frankly it doesn't "feel" all that sturdy, but it has lasted 30+ years.

I'd like to strengthen it in some of the larger rooms and am wondering if I can just add another layer of 3/8" plywood and then some nail-down hardwood flooring. Is this sufficient?

Thanks in advance!


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## guyod (Mar 10, 2008)

I would add some extra floor joists. 2x4's or 2x6's will work fine. what ever fits better.  If you could post some pictures  of the joists and piers we could help you out some more. I dont know how much flex you have but I dont see how 3/8 plywood will help and be a waist of time and money.If you still want to add subflooring i would make it at least 5/8 or 3/4. And that plus your 3/4" hardwood floors will cause all kinds of problems. I would Install the hard wood floors perpendicular to the joists. and if they are still flexing then add floor joists.


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## juryduty (Mar 10, 2008)




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## guyod (Mar 10, 2008)

never seen a floor framed out like that. I would add a 2x4 every 2'. if you have a nuematic nailer it will go real quick.


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## inspectorD (Mar 10, 2008)

Someone took all your joists out. 
This was common in Ca in the 70s. They had a boom, and energy crunch. 
I would update it a bit with some 2x6 joists. Adding more plywood will not do the trick. It would have to be 2x4 tongue and grooved boards, but maybe you are into that kind of floor.


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## juryduty (Jun 16, 2008)

Can't I lay some strong plywood over the top and then nail-down hardwood flooring?  I'm thinking 1/4" ply, making the seams perpendicular to the joists, screwed and glued down.  That should make it quite a bit stronger, no?


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## glennjanie (Jun 16, 2008)

Sorry, no. InspectorD is correct in saying it needs support from underneath. Even with the glue and screws, the plywood layers would act independently; if what you have is moving, the 1/4" would move also.
Guyod's suggestion is good also. Extra framing would stiffen the floor and the hardwood would be a nice finish.
Glenn


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## juryduty (Jun 17, 2008)

Thanks for the replies.  I'm wondering if there is *any* solution that will work from the top side.  Can I lay 2x4 tongue-in-groove across the floor?

The problem I have is that the crawlspace clearance is very tight, about 24" all around (code minimum) and even a little less, like 18", near the corners.  It will be tough for anyone to put up joists from underneath.

Is there any structural solution from the top that will give me more strength in the floor?  Even if just a little more strength?

Thanks in advance!


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## Charlie (Jun 17, 2008)

juryduty said:
			
		

> It will be tough for anyone to put up joists from underneath.



I don't think it will be too hard as long as you have some sort of nail gun and take your time. This will probably be the easiest and most cost effective method of fixing the floor.

The wood looks to be in very good shape do you know when it was put in?


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## inspectorD (Jun 17, 2008)

The issue's with adding to your existing floor are many.
The exterior doors and thresholds will need to be lifted up to clear any wood you put down, The doors will not open in. Any stairs will need to be raised to meet code. The doors on the interior will need to be all cut down and baseboard all redone. If you have baseboard floor heat it will need to be adjusted up also.
And the floor will still have bounce when you are finished.
All this is a lot more work and money than what is needed in the crawlspace.
Buy a nail gun and compressor and take your time. Get a pair of overalls and a good dust respirator.
This is hard work and I know it is not easy, but you will be much more satisfied when you are finished for doing it the right way. 

Good luck.


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## ChrWright (Jun 19, 2008)

I agree. It won't be fun, but adding joists from underneath is the only thing that will prevent deflection in this floor.

2x6, 16" o.c.


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## juryduty (Dec 9, 2009)

Hi folks,
Sorry to resurrect this old thread, but I am finally going to have a new floor put into the bathroom of this house.  My contractor says he can put in 2x6 joists every 16" to reinforce the bathroom floor, and is installing marble.  Again the floor is framed with 4x8s on 48" centers with 1.125" plywood.  Will this be strong enough for marble over concrete backerboard once the additional 2x6's are installed?

Thanks for any clues!


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## GBR (Dec 11, 2009)

http://www.trioforest.com/pdf/Load-Span_Tables.pdf
Be safe, Gary   And get a 6mil. vapor barrier on that, dirt? And insulate the floor or the concrete walls if closed crawl: http://www.eere.energy.gov/buildings/building_america/pdfs/db/35379.pdf


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## btw (May 3, 2010)

Hello,
    I have the same subfloor as the person above except that I have 2x4 or 2x6 tongue and groove.  I need to go down and measure it again.  Anyway, I want to install a 90 gallon aquarium.  The spot for the aqaurium actually falls in between the two floor joists.  Should I install another floor joist?  If so, do I have to pour concrete columns; how deep do you dig or can I buy post jacks from home depot?


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## btw (May 5, 2010)

I have the same subfloor.  I need to reinforce for a 90 gallon aquarium; which is about 1,000 lbs.  What is the best way to reinforce this.  I've had a couple guys take a look at it and they suggested reinforcing the footprint of the aqurium underneath with 4x6 beams and pillars.  Do I need to tie this into the existing beams or do I just reinforce the aquarium footprint underneath which is 4'x1.5'.  Could somebody point me to a good book or some detailed instructions for how to accomplish this?


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## longboard (Oct 23, 2010)

I too have the same type floor and have been under it many times to put in 2x6 joists between the 4 foot beams. I also added some 2x6 blocking in between the added 2x6 for additional support. I used joist hangers and no compressor. I have only 18-24 inches under most of my subfloor and I am almost 60 and it is a pain. If you really don't want to get under your house and you don't have a finished floor yet, you can cut your existing subfloor from the top where it is nailed to your existing beams. With the subfloor removed and the crawspace open, you can easily add your new 2x6 joists from above and then replace your subfloor.

Hope this helps.


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## nealtw (Nov 8, 2010)

crawl under there and add one 2x6 between the joists with hangers. go upstairs and jump on it. then you will feel the difference.   From top down would be a waist of time and money.


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