| Author |
Message |
Nathan Woods, CCCA, LEED AP
Senior Member
Username: nwoods
Post Number: 179
Registered: 08-2005
| | Posted on Monday, March 12, 2007 - 12:53 pm: |    |
We are seeing an increased amount of work in hot dry climate areas, particularly the desert areas of Southern California, Arizona, Nevada, etc...
I'm looking for guidelines and recommendations regarding exterior exposed wood, for common decorative elements such as a trellis.
On other projects, we have witnessed significant twisting and warpage and cracking in exposed rough sawn Douglas Fir-Larch Grade 2 lumber, even in sizes as large as 6x8. We don't want to have the same issues those other projects experienced.
Where would you recommend I go to find information about what works best in these hot dry climate areas?
I have searched:
- http://discus.4specs.com/discus/messages/24/2060.html
- www.wwpa.org/
- http://www.awc.org/index.html |
Anne Whitacre, FCSI CCS
Senior Member
Username: awhitacre
Post Number: 478
Registered: 07-2002
| | Posted on Monday, March 12, 2007 - 04:38 pm: | |
go to WI (Woodwork Institute) which used to be the Woodwork Institute of California. www.wicnet.org is the web site. Even before they started expanding, they have always covered the southern part of California, Nevada and Arizona. they can help you with what you need.
Cedar is common to this area, and I've used tropical hardwoods pretty successfully. Doug Fir is definitely out of its element... |
George A. Everding, AIA, CSI, CCS, CCCA
Senior Member
Username: geverding
Post Number: 267
Registered: 11-2004
| | Posted on Monday, March 12, 2007 - 04:48 pm: | |
I can't refer you to a specific paper on this topic, but I love this website for wood: Forest Products (http://www.fpl.fs.fed.us/) Look around and you're likely to find something informative. |
John Regener, AIA, CCS, CCCA, CSI, SCIP
Senior Member
Username: john_regener
Post Number: 264
Registered: 04-2002
| | Posted on Monday, March 12, 2007 - 04:56 pm: | |
Nathan:
You could specify "all wood shall be nice and straight and not have any cracks or visual defects" and make it the Contractor's problem (yeah, sure).
The grade of lumber is going to affect appearance. Specifying "No. 1 Grade selected for appearance" is a step toward better visual quality.
The species of lumber is also a condideration, as Anne pointed out, with Western Red cedar having less cracking and twisting than douglas fir/larch. And, as you know, Alaskan Yellow cedar is exceptionally high quality for visual properties and weathering; it is also exceptionally expensive.
If the lumber has a darker stained finish, then resawn glulams could work, with the stain hiding the laminations. Resawing also mitigates the laminations.
I have been specifying a water repellent stain to keep moisture out of the wood, to reduce future checking. OKON is the brand I've specified. My clients have been pleased with its appearance and performance.
Capping the wood members with sheet metal will help to minimize future cracking and may be required for exposed, cantilevered beams and joists from the building structure. It is a building code compliance matter if naturally decay-resistant lumber is not used.
Finally, I've noticed on some site structures (El Toro Road in Lake Forest, in your area) that square and rectangular structural steel tubing has been used instead of wood. This seems very expensive but it means no checking and twisting.
I've had no experience with wood composites such as used for decking. |
John Bunzick, CCS, CCCA
Senior Member
Username: bunzick
Post Number: 673
Registered: 03-2002
| | Posted on Monday, March 12, 2007 - 05:53 pm: | |
The twisting and warping phenomemon has as much to do with the way the material is used as it does the desert environment. In New England, exposed dimension lumber would not fare any better than you described unless it is well-treated with repellants or preservatives and is attached to underlying construction to keep it restrained against warping. A trellis is a difficult item to keep from self-destructing. Exposed wood such as this here in NE is always either cedar or PT. Even then, it has a limited life, and repellants need to be regularly re-applied. Large size dimension lumber isn't necessarily any less prone to warping than smaller sizes. Often it is the heart of the log and not very warp resistant. |
Nathan Woods, CCCA, LEED AP
Senior Member
Username: nwoods
Post Number: 180
Registered: 08-2005
| | Posted on Monday, March 12, 2007 - 06:21 pm: | |
Regarding the use of metal trellis members, it actually is quite effective cost-wise, but it gets so hot in the desert areas that plant life won't grow on it (it gets scorched).
The wood trellis members we are designing are all painted with a high quality exterior paint coating suitable for the intended use... but so where the ones that failed.
Anne, regarding WIC, I initially considered them, but they seem to be soley focused on interior millwork these days, though their website says otherwise. I initated contact with them just in case, as well as WWPA.
We are looking to hardwoods as the primary ingrediant in our spec that needs to be revised, but picking the right species and cut seems debatable, depending on what resource you look at. |
Mark Gilligan SE, CSI
Senior Member
Username: markgilligan
Post Number: 142
Registered: 05-2005
| | Posted on Tuesday, March 13, 2007 - 04:12 am: | |
The problem has to do with shrinkage due to change in moisture content. Several things aggrevate the problem:
a- The low humidity of the desert can cause even kiln dried lumber to shrink.
b- 6x and 8x members are difficult if not impossible to purchase dry, and because they are large there is more to shrink.
c- Large timbers often have the the heart center, as part of the member. When there is heart center in the piece of wood it is more likely to twist. This is because shrinkage of wood is different in the radial and tangental directions.
d- The ratio of tangental to radial shrinkage varies with the species of wood with larger ratios being associated with more shrinkage. It is interesting that Douglas Fir has a lower T/R ratio than do many hardwoods. The perceived better performance with hardwoods may be due to the fact that hardwoods are more likely to be dry.
My advice in order of importance:
1- specify Free of Heart Center (FOHC). The grading rules use this terminology.
2- Get dry lumber. In the case of desert exposure consider paying more for already used lumber.
3- Consider lumber with low ratios of Tangental to radial shrinkage.
An interesting reference is http://www.fpl.fs.fed.us/documnts/fplgtr/fplgtr113/ch12.pdf |
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