Plywood floor for an old house attic

[john70t]

Can you educate me on plywood?

The framing I assume is ballon-type.
The floor joists are true 2x6 and true 2x5, one foot on-center.

After about 140 years, the existing tongue-in groove floorboards are about the same weight as balsa.
They are held in place with a few #6 nails and are hacked up over the years.
There has been some repeat cracking in the ceiling above.

I want to sheet with 5/8" or 3/4" plywood, using liquid nails and deck screws to lock it all in place.

Weight is of great concern...

There is CDX and RTD(Resistance Temperature Detectors) type plywood but I'm not sure which to use and whether the 5/8" would conform to code.

[rfuerst911sc]

1.Sounds like you want to go over the existing old tongue and groove boards is that correct ?
2.Are you currently using the space for storage ?
3.If you are using for storage are the cracks in the ceiling caused by weight from stored items ?
4.Any concerns about existing insulation or air infiltration ? If yes now would be the time to tear up existing floor and make corrections .
5.Your concern about weight is valid , any chance getting a structural engineer to look things over ?

[john70t]

The old floor boards would be taken out completely.
So would the 1/2" OSB sheets someone used to cover the current chopped up floor.

Only light storage before. It is empty and clean now.

There is a lathe/plaster ceiling underneath, which seems to be the only thing holding the joists together.

I'm going to replace any missing insulation batts and maybe add some cellulose for any gaps.
It is an unheated space.

[red-beard]

Originally Posted by john70t

The old floor boards would be taken out completely.
So would the 1/2" OSB sheets someone used to cover the current chopped up floor.

Only light storage before. It is empty and clean now.

There is a lathe/plaster ceiling underneath, which seems to be the only thing holding the joists together.

I'm going to replace any missing insulation batts and maybe add some cellulose for any gaps.
It is an unheated space.

Then I don't think weight is going to be an issue. I hate overly thin plywood. What are your plans for the attic? If living space I would say 3/4 plywood for the floor. If not, think about what will be stored, etc. up there. Even 1/2" will be stronger than what is there. I have 3/8" loose sheets in one of my attics. The sheets are actually my hurricane window covers.

[CurtEgerer]

John, installing plywood will make the old attic floor far stronger. In fact, we used to use this as a method of repairing sagged and cracked ceilings. Removing the old floor boards and then gluing & screwing the plywood down will, essentially, create T-beams out of each joist. Screws about 8-inches on center. Use Loctite PL-series glue and make sure the old joists are cleaned and rough-sanded for good bond. Dead weight of the plywood will not be a significant factor.

5/8" CDX is fine - there is no prescriptive code for this retrofit application. It's whatever an engineer specifies.

If it's balloon-framed, you want to check the joist end connections at the wall studs. That's going to be the weak point. Are they just nailed into the sides of the wall studs or is there a ledger board of some sort? If just nails, those connections should be beefed up, ideally with metal connectors.

Obviously, the joists need to be checked for cracks and repaired with splices if needed before the plywood goes down.

*I don't play one on TV, but I'm a structural engineer in real life

[dad911]

Originally Posted by john70t

Can you educate me on plywood?

The framing I assume is ballon-type.
The floor joists are true 2x6 and true 2x5, one foot on-center.

After about 140 years, the existing tongue-in groove floorboards are about the same weight as balsa.
They are held in place with a few #6 nails and are hacked up over the years.
There has been some repeat cracking in the ceiling above.

I want to sheet with 5/8" or 3/4" plywood, using liquid nails and deck screws to lock it all in place.

Weight is of great concern...

There is CDX and RTD(Resistance Temperature Detectors) type plywood but I'm not sure which to use and whether the 5/8" would conform to code.

No floor would conform to code, typically attics are not rated for storage. We are not allowed to put down a 'floor' in an attic space of a new house, without architect upgrading joists. The weight of the plaster may be overloading the 2x6 framing as it is. Hard to tell why it's cracking without more info. We usually find plaster is separating from scratch coat.

Practically, if you need the storage, I would throw a few sheets in areas where it is supported by wall below, and store stuff there.

If you want to strengthen the ceiling system, I suspect a strongback will do more. As in pic below, you would support ceiling joists mid span with a perpendicular beam.




Edit:

Are the existing joists sagging? How long is the span?

[john70t]

There is a 17' x 26' box...which is attached to a 17' x 19' box.

Great ideas. Thank you! I did not think of the joist end connections.

Also on the list:
Some of the joists in the basement were hacked through by plumbers and never repaired.
A few more are toe-nailed with the other half hanging in the air.
I also realized there are no collar ties for the rafters.

It's a sad old house but this year has been full of improvements. Tenants are happy. I'm happy.

[URY914]

5/8" plywood is more than enough. Could even go to 1/2" with joists on 12" centers.

[tcar]

Originally Posted by john70t

There is a 17' x 26' box...which is attached to a 17' x 19' box.....

What does that mean???? Span????

How Far are those Joists Spanning???? That's critical.

[john70t]

tcar, It's a 17' span on both runs. Interior walls are mostly parallel.

I cancelled the friday delivery of heavy 3/4" and am going to get a structural engineer. (aka call the guy)
I'm personally sort of 'leaning' towards using 1/2" and beefing up every other joist per advice here.

The stairs are narrow so there would be little storage use up there. I'm sure it was a summer sublet for some kids back in the day.

At this point there are probably a few other factors involved in the house which need beefing up.
It's a cobbled web, everywhere, the more it's thought about.
What I could only do is probably fix part of it, but that would affect somewhere else. Then there would be shifting and popping and repainting for years.

Can 'o worms situation.
There is a balance between doing what is right, and doing what is possible, and doing what is affordable.


Thank you all again for the advice.

[island911]

You have some good info above.

A few comments:

Most floor construction is not concerned about strength, it's all about stiffness. For an attic floor, who cares? 1/2" plywood as PW suggests.. Plenty, on 1' centered joists. I don't like OSB so much. YMMV.

Wood moves (expands/contracts) most with moisture, not thermal. So, if it's a vented attic moisture content will change with the seasons.

Personally, I would not glue. It's nice to have access, should you want to run wiring in the future.

I take it that you want this for extra space for storing seasonal decorations and such. (?)

[john70t]

Originally Posted by island911

Personally, I would not glue. It's nice to have access, should you want to run wiring in the future.

That is apt.
I was planning on boxing in any of the runs coming up from below, and putting a painted marked separate access panel above them
It would be obvious.

Also will leave a quasi-blueprint of the runs (with photos) in an envelope on the wall for the next poor sucker..I mean...fellow dealing with it.

[john70t]

I didn't actually measure on-center.....so it might actually be closer to eighteen instead of twelve from my own pics.
I'm an area-code carpenter.
Shame on me.

There is a full week scheduled ahead to get rid of all the remaining nob-and-tube.
(That Greenlee outlet tester goes off half a f'kin foot away....)

There is also some braided two-wire which is less of a concern to the electricians, but will also get taken care of if they have the time.

Pics of the project:

[island911]

Another thought; Decking screws are typically pretty big.
I would go with a much lighter screw.

[island911]

Originally Posted by john70t

I didn't actually measure on-center, so it might actually be eighteen instead of twelve from my own pics. ..

maybe 16" OC (typical). 3x16=48 which makes easy work of butting up 4x8 plywood panels. (and having a common edge on joist)

Cool that you have an actual stairway.

[island911]

And Another thought; You could also just replace any rotten boards with more of the same.

It looks like shiplap. Smaller boards certainly are easier to get up and into the space than big ol' slabs of plywood.

The only caveat there is much of the newer shiplap is thinner than the old stuff. (and still called 1x6, 1x8 ...)

[john70t]

That is an entirely new idea. Thanks Island.

(I hope I got this right)
Replace a section of critical-area boards with long heavy duty ones, which provide a tad bit more vertical strength but lock together the joists?

Even as a temporary measure...that would add significantly more to what is already existing.

[island911]

I'm happy to help ya.

Understand that the joists primarily span to hold the lath & plaster, and in your case, give some structure for flooring. They also hold tension, w/o which the roof would push the walls out..

Again, stiffness is the big concern and what is needed so that the plaster ceiling doesn't crack. -that would be a sign that there is not enough structure to provide adequate stiffness for the weight. IOW, the wood will flex quite a lot before breaking, whereas the plaster is much less elastic. So the wood portions need to be over-built to be extra stiff. And that is what you have... as long as you don't start storing bags of concrete up there.

Think of the long boards as really there only to provide a surface to walk on. Locking the joists together isn't a need. The lap, or tongue & groove, helps transfer a small amount of load to the adjacent board, when walking around. It also does away with straight cracks between boards.

[island911]

for refnc "Shiplap Nickle Gap"

[john70t]

The minor ceiling cracks are perpendicular to the joists...which sounds like joist flex and/or lathe separation.

I like that flooring idea though.
Those are easier to carry and might blend with the existing a bit easier.
Replace same with same.

There is a lot to think about and that part of the tired brain is burned out at the moment.
I appreciate the idea. thx.