looking for guidance in evaluting strength reduction in highly mildewed structural plywood sheathing

[Engineerataltitude]

I've been asked to evaluate the usability of some highly mildewed plywood structural sheathing on the inside of some cripple walls in the lowest level understory area of a condominium complex that was built without understructure vents. I've looked on the APA website, called their technical support team and reviewed the 2010 Wood Handbook by the USDA Forest Products Laboratory. The only information I can find is for solid wood. Nothing for plywood.

Anyone have any technical references or experience to help guide me in this?

 

[rb1957]

i'd worry about mould forming ? more a health thing tahn a strength issue.

Quando Omni Flunkus Moritati

 

[boyze]

test

Have Fun!

James A. Pike

http://www.xl4sim.com

http://www.erieztechnologies.com

 

[Engineerataltitude]

There are companies who clean the mildew/mold. What they don't do is evaluate the extent of the damage (if any) to the shear strength of the paneling. The Wood Handbook make a distinction between fungal growth and decay. They are not the same. One can lead to the other however. Just don't know how to evaluate when decay in plywood has begun.

 

[dik]

The conditions for mildew are also good for brown rot... mildew has almost no effect on the strength of wood products but can be a health hazard...It does not attack the cellulose, hemicellulose or the lignin... rot can have a real significant effect on the strength... could be brown or white or soft...

Mildew can affect finishes and can be a health hazard... and the spores are transportable to other areas.

Dik

 

[dik]

Catch the link:

http://hytechsales.com/mildew.html

Dik

 

[rb1957]

look at it from a different perspective ... how would you feel if a contractor built you some renovation using "highly mildewed" plywood ... would you feel like it was done on the cheap ?

Quando Omni Flunkus Moritati

 

[Engineerataltitude]

Thank you for the input and reference

 

[msquared48]

"...that was built without understructure vents." Not much of a local inspection process either here.

Unless this is a very old structure, this should never have happened.

Mike McCann
MMC Engineering

 

[Engineerataltitude]

re: "not much of a local inspection process here"...No kidding!

The practical matter for this project is that it would be very expensive to remove and replace all the cripple wall sheathing that has bad mildew on it. The buildings are big and the cripple walls are high (average about 6ft in a building with about a 3600 sq ft footprint). If there is no structural degradation, they would rather just clean up the mildew, treat it, add vents and be done.

I just have to give them some kind of assurance that the sheathing hasn't lost its strength.

I think I'm just going to do an inspection and use the old screwdriver penetration test to see if I can detect any softness in the areas with the worst mildew. If the plywood is still pretty solid, I'd say it's fine. If spongy, should be replaced. Not super rigorous, but reasonably practical.

 

[dik]

Have to be careful that your mildew isn't mold... some toxic black molds out there... and very unhealthy. Disturbing it can cause serious health issues... best use a mask, etc. and, maybe have it tested.

Also careful that you don't have brown rot... same kind of environment...

Dik

 

[msquared48]

Hopefully they used plywood with exterior glue. I would suspect that too though.

Mike McCann
MMC Engineering

 

[a2mfk]

I think this has been stated above, but not all fungi are the same, and I am no biologist so I am just relaying info that I have read. The fungus that you may be describing may be harmless mildew or similar surface mold, and just needs to be cleaned and treated for possible health concerns. Wood-decaying fungus that feeds on the cellulose of the wood (rot damage) is completely different.

Good primer on fungi and wood, and actually has some humor in it for a technical article:

http://www.buildingscience.com/documents/published-articles/pa-mold-explosion-why-now

Some in-depth info:

http://www.esf.edu/nekda/documents/WoodDecay-Anagnostsmallversion.pdf

Did you do any moisture content readings? Generally 20% is the lower end of the "danger zone" for being the right environment for wood-decay fungus to grow.

moisture content levels primer

http://www.rlcengineering.com/wmc.htm

In my forensic experience, traditional plywood (not OSB) is very good in getting wet and then dry (if allowed to dry), and then not having any warping, edge swell, or other structural stability or strength issues.

APA are the plywood gurus, and you can contact one of their engineers or tech guys for specific info. But they have a PDF in their library that is free if you register:

http://www.apawood.org

Technical Topics: Wood Moisture Content and the Importance of Drying in Wood Building Systems

Guidance on how to avoid potential moisture problems that could lead to costly and hazardous deterioration as well as health risks when using wood structural products with impermeable materials. Revised March 2011. (Form TT-111 - 3 pages)

very in depth article on moisture content in wood:

http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr190/chapter_13.pdf

 

[Engineerataltitude]

Great references. Thanks.

In normal circumstances I don't deal with moisture problems very often. I practice at high altitude on the east side of the Sierras in CA where it is quite dry. This project is unusual because the source of the moisture was a broken underground landscape pipe that went undiscovered for a long time and the understructure area had been built (inexplicably) without any vents. Pipe has been repaired and vents are being added, so I think once the plywood is cleaned up, and I "poke around" on it a bit to check out if it has gotten soft or not, I think it will be OK for the future.

We are lucky it is plywood and not OSB though.

 

[a2mfk]

Sounds like you got it figured out. If it was related to an event like a plumbing leak and that has been corrected, and the plywood is not soft, flaky, or has any other indications of deterioration, you are probably OK. Knowing if the current moisture content was less than 20% would be good assurance.

I am in the opposite climate down here in Florida, and there are areas where the surficial water table is only a few feet below the surface. I have seen more than one structural failure due to inadequate ventilation of a crawlspace, not to mention all the other types of nasty fungi. Not a fun inspection on your belly under a crawlspace either...

 

[dik]

a2mfk:
Some fungi feed on the lignin... these are considered as 'white-rot' fungi; they leave the lighter coloured cellulose and hemicellulose. Some feed on the cell walls, and these are generally considered the 'soft-rot' variety.

If there is any doubt about what 'critter' has come to roost, then he should take it to a lab for some testing... it shouldn't be too expensive and it should be quick.

Dik