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juryduty 03-10-2008 01:30 PM

Houses with 48" on-center 4x6 joists?
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? 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?

Bondo 03-10-2008 01:42 PM


over 4x6 joists 48" on center.
I think you'll get more bang for the buck by closing up those 4' Centers.......

How about a run of 2X6s to bring it to 16" or 24" centers,..??

juryduty 03-10-2008 03:19 PM

Here's a picture

Daryl 03-10-2008 04:19 PM

i like Bondos idea filling in the "16 OC's" with 2x6.

prior to installing the 2x6 i would mark my layout and install hangers on one side of the bay. next i would suggest a bead of subfloor adhesive on the top of each 2x6 to be installed. Install hanger on remaining end of each joist as you install them.


Daryl 03-10-2008 04:22 PM

or... if you so choose to run your 2x6 joist perpendicular to the subfloor (as you should)

hang a dbl 2x6 at each post that the 4x6 is sitting on, and then hang your 2x6's on the dbls

and DONT FORGET to insulate vapor barrier up if there is living space above.

juryduty 03-10-2008 04:23 PM

So I should run 2x6's perpendicular to the existing joists, without any additional footings (can't easily pour new concrete footings anyway). How many 2x6's should I run? Every 2' or so?

Daryl 03-10-2008 04:51 PM

at each post that post down the beam to the footing... install a dbl 2x6 joist on the beam. Then install dbl hangers on each end. then measure from the beam 16" and make a mark, then 32", then 48" then make an X on the side of your 16" mark that is closest to the beam. square this mark on the beam with a speed square or combo square. then install the joist on the X so there will yield a spacing of 14.5" in between the beam and the joist.


Ron6519 03-10-2008 05:26 PM

Where do you live that this construction would be to code?

juryduty 03-10-2008 05:38 PM

In the San Francisco Bay Area (East bay). The house is part of an 1000+ home development built in the 1970s. Is it considered "cheap"?

Ron6519 03-10-2008 05:46 PM


Originally Posted by juryduty (Post 106319)
In the San Francisco Bay Area (East bay). The house is part of an 1000+ home development built in the 1970s. Is it considered "cheap"?

California probably currently has the strictest structural requirements in the U.S. Maybe the '70's were a little more mellow.

LawnGuyLandSparky 03-10-2008 05:52 PM

Is the entire house like this, or is this an addition. That work doesn't look like it's 30 years old.

juryduty 03-10-2008 06:02 PM

The entire house is like this. The house was built in 1972. You're right, it doesn't look 36 years old and has held up well. The problem is that if you're standing in a room and someone walks by you at close range, you can feel the plywood span give a little bit. This is what I'm trying to fix -- I want to make it feel sturdier and hold up under hardwood or laminate, maybe tile, etc.

joasis 03-10-2008 06:21 PM

I just thought I have seen everything.

As suggested above, add joists.....and if it wasn't clear from above, the easiest way to tackle this would be to run them 90 degrees to the main joists...that makes the added joists like 44 inches long, and if you do 16 or even 24 on center, it will add a lot of rigidity to the floor. Looks like you have plenty of room to work in.

Wanttodoitright 03-24-2008 01:06 PM

I have the same problem in the addition that was built on my house. The addition was put on about the same time that your house was built. The rest of my house looks exactly like yours underneath, but my subfloor is 2"x6" T&G, and is VERY strong, with no flex.

My addition, however is the same specs as your house, including the plywood (called plyscord back then), but they DIDN'T put piers in to support the span. :eek: It is 11'6" by 23'. My 4x6 beams span the 11'6", with no support in between!

MY question to everyone is can I jack up the now-sagging beams and put posts and pier footings underneath?

They only sag about 1/4" - 1/2", which has caused a slope in my floor. Whille under there, I want to add the 2x6 joists mentioned earlier to eliminate all of the flex in the subfloor, as I am going to do 16"x16" porcelain tile in one area of the addition.

Also, underneath the addition is the original concrete patio that was the backyard. Will I be able to set the pier footings right on top of this concrete?

juryduty 03-25-2008 01:59 AM

I am considering simply adding another layer of 1/2" plywood, screwed and glued to the floor, then a layer of nailed hardwood. Can you folks really tell me that this would not strengthen the floor considerably? It seems like an easy, straightforward fix. The problem is the span, right? And this strengthens the span a lot.

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