Prehistoric subflooring strata

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  #1  
Old 07-07-16, 08:06 AM
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Prehistoric subflooring strata

Hello, I have a question about subflooring.

The master bedroom in my house had carpet which was end of life (my way of saying it was disgustingly filthy), so I decided to put in 3/4" pre-finished hardwood floor. I pulled the carpet back and found abandoned 3/4" hardwood floor in about 70% of the floor, vinyl over about 20% (sitting on top of this hardwood), and then the last 10% is just built up scrap wood that was apparently there to repair some water damage? It's very unusual.

Anyway, underneath these decades old layers I found what appears to be 1/2" plywood (the actual measurement is exactly 7/16). What should I do here? I was reading I could put down an underlayment layer of plywood to strengthen the floor (it does seem to flex in some parts, is old, and removing the old hardwood floor is damaging it a bit), though the hardwood flooring people said it wasn't a big deal and to just repair and install as-is.

Since the carpet/padding were sitting on top of the 3/4" hardwood floor (sitting on top of 1/2" plywood subfloor), all the baseboard/trim were cut for this height, so adding a 3/4" layer of plywood on top of the existing subfloor wouldn't really throw off the height of the room much. However, I don't know if this makes structural sense, or if it's worth the effort.

Thanks!
 
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  #2  
Old 07-07-16, 02:23 PM
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When I run into this type problem, it is usually solved by pulling all the odd stuff off the base subflooring and checking its integrity. You should be aware of the size of your joists, their spacing and the longest unsupported run of the joists. This will help to determine if adding plywood will be sufficient without adding support below. If, when you get rid of all the junk, you are left with 1/2" plywood, adding a layer of 3/4" Advantech will quite often be enough. Start with as clean a slate as you can.
 
  #3  
Old 07-08-16, 05:57 AM
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I agree 100%.
It was very common long ago to use 1/2 for sub floor then go over it with something else.
Just made no since to me.
Any time there's been sections cut out and replaced like that is a red flag.
Most often I see this over crawl spaces that are wet causing all kinds of issues to the joist and sub flooring.
 
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Old 07-08-16, 06:55 AM
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Here's an update: the floor is 1/2" all around with joists spaced something like 20" on center. There are a few areas where I'll need to add blocking so I can cut out sections and make repairs. I think the joists are 2x10 but I forgot to measure - this explains the bounciness of the floor, haha. I think I'm forced to add some underlayment here, right?

Is there an advantage to the Advantech over just using 3/4" OSB? It's like a $10 price difference here.

Thanks!

Edit: Meant 2x10, not 2x8. May be 2x12, I am dumb and forgot to measure, will do so later tonight.
 

Last edited by jro29; 07-08-16 at 07:33 AM.
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Old 07-08-16, 07:37 AM
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It's important to know the size, spacing and unsupported span of the joists. If possible, knowing the species is also helpful.
 
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Old 07-08-16, 08:01 AM
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Are there any tricks to figuring out the species?
 
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Old 07-08-16, 04:14 PM
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The spacing, spans, and size of the joist will be enough.
Highly unlikely there spaced 20" apart (though anythings possible)
That would throw off any type of sub flooring.
Advantech has a 50 year warranty, it comes tounged and grooved so it does not flex between the joist.
Still waiting for that picture.
Why are you cutting out and making patches?
Have you been under this floor to check out why there was patches already?
 
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Old 07-08-16, 05:30 PM
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I measured the joists, look to be 2x8's as I initially guessed. The longest unsupported run is 9 feet; they are 18 foot joists but at the midpoint there are 2 2x10s running the length of the crawlspace supported by a concrete pillar. The on center space between joists is 16", what I initially thought was two joists (I measured by poking my head inside the hole in the attached picture) was actually blocking put in because the joists were cut in this area. I am not sure why this was done.

So the plan is to put Advantech (I went ahead and bought the sheets) over the 15/32 plywood (it isn't 1/2, Lowes had 15/32 plywood that matches the thickness exactly) after I patch the two pictured holes, use 1 1/2" deck screws to attach it, and then put my pre-finished 3/4 hardwood over this using the manufacturer recommended 1 3/4" cleats. Does this sound acceptable?

Thanks for the help!

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Old 07-08-16, 06:15 PM
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I would not be wasting my time trying to patch in anything, I'd be cutting out that whole subfloor and replacing.
Just makes no since to me why anyone would cut that floor out on an angle like that.
 
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Old 07-08-16, 06:46 PM
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I agree, I have no idea what they were thinking with that slant cut - they didn't even attach the wood they used to cover the hole. When you stepped on the carpet there, you could feel the board tilt.

I thought about doing the entire subfloor, but I think that project is beyond my experience. All the HVAC (including furnace) and plumbing are under this bedroom and I'd hate to screw something up.
 
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Old 07-08-16, 06:55 PM
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Make your patches and apply the 3/4" Advantech. Lay 15# builder's felt and then your hardwood flooring. I would graduate to 2" cleats, however for a better holding ability. I have less problems with cleats than I do with staples, so good choice.
 
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Old 07-08-16, 07:20 PM
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Thanks for the advice! Should I leave an expansion gap between boards, or, since this is an underlayment, can I butt them all together?

The only reason I'm using 1 3/4" cleats is because the wood I am putting in (acacia) is very hard and the manufacturer specifically recommends 18 gauge 1 3/4" cleats for this wood. I wanted to use my buddy's old 15 gauge stapler but the place that sold me the wood told me it would shatter the tongues.
 
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Old 07-09-16, 05:15 AM
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Absolutely go by the wood manufacturer's suggestion. I didn't know the species. Yes, not only will staples ruin the tongue, some of the staples, if not most, will not sink properly, leaving them proud and you will need to remove them or cut them flush. A total PITA.

Advantech is Tongue and Groove. Leave a slight spacing on the flat edges, but put the tongues and grooves solid.
 
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Old 07-11-16, 06:43 AM
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Larry, do you recommend gluing the advantech in addition to the deck screws? I seem to be getting conflicting information on this point.
 
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Old 07-11-16, 07:03 AM
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Never ever glue down an underlayment!
It needs to be nailed ever 4" on the edges and in every X printed on the panels.
Fastest way is with a framing gun with 8 D ring shank nails.
 
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Old 07-11-16, 01:33 PM
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Thanks - I won't glue. Since I'm putting down the Advantech, is a fancier underlayment like this one a waste of money?

Eco Cork Foam 75 sq. ft. 3 ft. x 25 ft. x 0.126 in. Laminate and Engineered Floor Premium Plus All Inclusive Underlayment-220000503 - The Home Depot
 
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Old 07-11-16, 04:16 PM
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It is for laminate and engineered flooring which usually does not get nailed down. I would just use 15# felt and lay your flooring.
 
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Old 07-17-16, 08:16 PM
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I have another question (I haven't had much of a chance to do this project due to some other stuff coming up): I am about to start laying the Advantech but am not sure about placement. I'm using page 21 of this guide as a reference for placing the sheets, but one of my walls is about 17 feet long. If I place a 92" sheet (so that I'm at the quarter point between joists) followed by a full 96", I'm left with about 4" of space to the wall. Should I cut a sheet to this small length, or should I try to make it so I have three sheets of more or less equal length? Or is there some other method?

Thanks.
 
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Old 07-18-16, 03:53 AM
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I would lay a full sheet centered in the floor, then piece the two sides. Yeah, it takes another sheet of Advantech, but you'll use it somewhere.
 
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Old 07-18-16, 07:02 AM
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Thanks, this makes sense! I bought extra sheets anyway since I wasn't sure how much waste there would be and I am probably the world's worst carpenter.
 
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