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juryduty 03-10-2008 12:51 PM

Reinforcing subfloor in 1970s house
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!

guyod 03-10-2008 01:33 PM

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.

juryduty 03-10-2008 01:58 PM

Here's a picture of my crawlspace

guyod 03-10-2008 02:20 PM

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.

inspectorD 03-10-2008 03:50 PM

Someone took all your joists out.:eek:
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.:)

juryduty 06-16-2008 07:03 PM

Is there any way to reinforce from the top?
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?

glennjanie 06-16-2008 07:46 PM

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.

juryduty 06-17-2008 01:35 PM

OK, is there any solution from the top?
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!

Charlie 06-17-2008 02:54 PM


Originally Posted by juryduty
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?

inspectorD 06-17-2008 06:16 PM

It's just hard work
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.:D

Good luck.:)

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