Weathering of OSB floor sheathing during construction

[S3Engineers]

I have a multifamily housing project that was stalled for more than a year due to financing while in the wood framing stage. The OSB floor systems show signs of weathering due to rain intrusion. TJI joists are spaced at max 24" o.c and 23/32 OSB SturdiFloor (24" floor span rating) sheathing has been used. The floors will receive 1 1/2" lightweight concrete overlay.

The weathering of the OSB surface may extend 1/8" deep or more which means the OSB span rating may be compromised. Of course the lightweight concrete will add significant strength to the floor which is usually neglected in span rating specifications. Does anyone have any experience with testing of weathered OSB? or composite design of lightweight concrete/OSB composite floors? It would be very difficult to replace the floor sheathing and adding sheathing to the top will compromise all door openings in the project. Please advise.

 

[Ron]

yes..I have experience with this. The APA will not give a clear statement on time periods for exposure, but it is generally accepted that anything exceeding 6 months is too long.

Inspect and measure the edges of the OSB. If the edges are "feathered" or show evidence of particle separation, or if the thickness of the edges has expanded to a thickness greater than the manufacturing thickness tolerance, reject it.

 

[S3Engineers]

Replacing the floor sheathing on this project will require the structure to be torn down. The OSB sheathing is glued and nailed.

What is an alternate repair solution? Are there test methods established by APA? Using a six month time period for exposure may not be applicable in all locations. Winters in Minnesota are certainly much wetter and harder than winters in the Southern California, which is where this project is located.

Allowing for no expansion due to weathering seems very restrictive. Is there an industry standard for expansion due to weathering that applies?

What about composite design of the floor system using the lightweight concrete plus the sheathing similar to composite steel deck system design? Without a doubt the light weight topping will stiffen the floor system, but can that be accounted for in design?

I can certainly determine composite section properties and justify a 24" span rating based on the composite section but is there any precedent for this approach?

Once the structure is protected, are there any long term materials consequences due to light surface deterioration on OSB sheathing?

 

[Ron]

You hit on exactly why the APA won't give a guideline on weathering limitations....different exposure conditions.

Considering that you are in southern California, your exposure conditions would be conducive to a longer exposure time. The probability of significant detriment is fairly low.

I didn't state that you couldn't have any expansion...just limited to the manufacturing tolerances. Yes, that's tight, but defensible if you needed to do so. Outside that, there are no allowable criteria to determine the detriment from moisture or exposure expansion.

There are no tests to determine the effects on OSB by weathering, other than to run the material through the same criteria as new board would have to meet. That would be PS-2, a Public Standard used for the testing and rating of wood based sheathing materials.

One test that I've found to be helpful in evaluating wood products is fastener pullout resistance. I would purchase a new board of the same thickness, run a series of fastener pullout tests in the new board, then do the same with the existing board. If the retention in the existing material is at least 90% of the new board, I'd be inclined to accept it, given a detailed visual inspection of other conditions of the board.

Assuming you accept the board, you might want to consider applying a sealer, particularly at the board edges, before you place the gypsum decking.

As for the residual strength, I doubt that it has been affected greatly, but the pullout testing should indicate so. You could do a load test on the assembly to further assure yourself of the strength. I'm a bit surprised that the building official has not required a load test before resuming construction. Keep in mind that deterioration of the OSB is not the only effect of exposure. Truss plates, fasteners and other metal items should be evaluated for corrosion and wood shrinkage at the specific location (e.g. shrinkage separation of truss/joist plates).

Good luck.

 

[msquared48]

In lieu of removing the OSB, what about overlaying it with 7/16" OSB and glue-nail, then reduce the concrete overlay to 1.25".

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BridgeSmith]

Another possible option would be to reinforce the concrete overlay, either with welded wire mesh or maybe fiberglass reinforced concrete (fiberglass strands in the concrete mix). The mix with the fiberglass is available here in WY from our local ready-mix plants for a nominal cost increase (about $20/CY) and it provides some tensile strength to the concrete.

 

[Ron]

HotRod10...check a little closer. The fibers offered by your ready-mix supplier are probably polypropylene, not glass. Fibers do not "reinforce" the concrete in the same manner as rebar. They enhance the properties of the concrete (flexural strength, first crack resistance, etc.) so in your calculations, consider the effect to be on the concrete only, not on the reinforced system.

If the OSB and structural components with OSB are deficient, reinforcing the concrete won't help.

 

[S3Engineers]

I'm the original author of the thread. I contacted APA and they do have a document available that offers guidelines on the issue. It is titled "Assessing Water Damage after a Flood" X-501 on

http://www.apawood.org.

This document discusses various assessment and repair methods for sheathing that has been exposed to severe moisture conditions, such as occurred during Hurricane Katrina.

The building in question has been exposed to intermittent moisture conditions during rain events for approximately one year. While greying of the surface is evident, there is no evidence of swelling or buckling. The contractor will be replacing the roof and exterior sheathing, then soda blast the structure inside to eliminate any issues with mold. The floor sheathing is solid in almost every location. There are very few areas where floor squeaks are evident. The contractor will repair or replace these sections.

If we felt there was a structural issue, we would overlay with 7/16" sheathing and reduce the concrete thickness to 1". However, this is the minimum thickness allowed for a 1 hour fire rating (UL593)and if there are any issues in the field where the floor is built high, the fire rating could be jeopardized.

 

[BridgeSmith]

Ron, you are correct - they are polypropylene fibers, and the tensile strength gain is nominal. I wasn't clear on the reasons for my suggestion. (I was rushed by a planned power outage in the building when I wrote it) The advantage I saw was just what you suggested - to minimize cracking. The OSB should retain adequate strength, but the weathering may have reduced its stiffness somewhat, allowing it to deflect and produce unsightly cracking in regular concrete at the joists.

 

[S3Engineers]

Self leveling gypsum floor underlayments are generally pumped into the building thru a 1" hose. Fibers could not be used in this product as they would clog the hose.

However, the manufacturers do provide an optional metal lath product in sheets that can be installed on the sheathing prior to concrete placement that will reinforce and provide crack control, similar to the product used in exterior stucco.