wet service factor: what exactly is "extended period of time?" 1

[SLTA]

I'm designing a covered screened-in porch with a long span, and using an LVL would be ideal for the roof edge beam. It would be covered by 1.5 ft of overhang. I'd really like to stay away from wolmanized PSLs as they mean using extra corrosion-resistant hardware (read: much more expensive) and they're not readily available. The required span makes using PT lumber difficult.

So, background in place, here's the question. Is this beam even considered moist-use? It seems to me that it couldn't get all that wet, especially for an extended period of time like the NDS says. But what exactly is that mysterious extended period of time? And does the problem go away if the beam is protected by something like a miratek trim board?

thanks,
Linnea

 

[Ron]

slta...getting the "wood" people to put time frames on their criteria is futile...they won't do it. Same argument for plywood/OSB exposure conditions. Their description of "extended period of time could be from 90 to 180 days....but they won't say...sorry.

 

[BAretired]

Is this beam even considered moist-use?

It depends on your location. Inland regions may be dry service whereas coastal regions could be considered wet service. You haven't told us where the structure is located. The safest route is to assume wet service.

BA

 

[SLTA]

We're in western NC, USA - definitely not coastal. So why do the wood folks even say extended period of time if they're not willing to quantify it? This goes against everything in my engineering brain...

 

[Qshake]

It seems you all are getting to lose a bit of your witty personalities. If the others won't say it I will.

To wit: An extended period of time is defined as a period of time that is extended.

See, that was simple!

On a more serious note, is this an application for more local and ample use of moisture barriers in order to dissolve the issue?

As an example on another topic, retaining walls. When water table behind the wall (for some applications) indicates a very high EFP, we'll go to extreme measures to reduce the water as that is usually much cheaper than designing a larger concrete wall. The extra drain tile, engineered backfill, etc are usually pretty cheap.

Good Luck.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Ron]

Hey Q! I get accused routinely of being a half wit!

slta...the explanation I got from APA was that they cannot accommodate all of the variables of environment, exposure, protection, etc. in a given locale or on a given project site, so they will not quantify exposures or limits.

This is what the Southern Forest Products Association says about Cm....

Wet Service Factor, CM – A reduction required for lumber used under conditions where the moisture content of the wood in service will exceed 19% for an extended period of time, such as for uncovered outdoor decks regularly exposed directly to rain or other sources of moisture. Members that are protected from the weather by roofs or other means but are occasionally subjected to windblown moisture, such as for covered porches, are generally considered dry applications.

I get from this and other references that Wet Service conditions refer to intermittent, but repetitive wetting in an exterior application.

I would not consider your application to be wet service, but you would do well to protect yourself (and the beam) with some ancillary waterproofing, as Qshake noted.

 

[SLTA]

Cheers, Ron! You found what I couldn't!

 

[msquared48]

I have a little problem with that quote considering the 19% factor and the fact that here in the Northwest, the relative humidity is 50 to 60% on the average. In SC and other subtropical regions, extended periods of 80 to 100% is common.

Personally, I would still treat the wood with some form of a vapor barrier be it preservative, oil, ... whatever, if you want to use the dry condition in order to try to maintain the low moisture content within the wood. I would stay away from paint, but that's just me.

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

 

[hokie66]

Wood needs paint. Two coats of 100% acrylic should do it.

 

[SLTA]

I decided to go with the PT lumber, and carefully checking my spans/loads/etc, found that it will work. Thanks for all the input.

 

[Ron]

Mike...the ambient relative humidity has little to do with the moisture content of wood. I routinely see wood moisture contents in the 12 to 14 percent range. Wood fibers are usually saturated at about 30 percent moisture by weight.

As for treatments, coatings, and preservatives...each can work well in its own right; however, you must select the correct one depending on the exposure, the wood type, and the potential for wetting/drying. In some applications, acrylics work well on wood...in others, they promote decay. Penetrants will generally work in most applications (oils, oil stains, etc.).

 

[hokie66]

This link gives a good description of the difference in the performance of acrylic and alkyd coatings on external wood surfaces when moisture is the consideration. Acrylics also exhibit superior performance against UV deterioration.

http://www.paintquality.com.au/press/newspaper/BOAPT01.html

 

[Ron]

hokie66..thanks. Good info.