Plantation Grown Mahogany Decking - Separating Fact from Fiction

We have recently witnessed a strong marketing campaign urging consumers to make the choice of utilizing plantation-grown Mahogany (i.e. Swietenia Macrophylla) decking when planning their new deck or outdoor living areas.

Homeowners are told that they will be purchasing a beautiful, durable product with superior stability and have the assurance that the deck boards are sourced from 100% sustainable plantation timber. Today we examine the subject and try to separate fact from fiction.

Environmentally Friendly

The plantation Mahogany does not come from rainforests but from tree farms planted by local Fijians after World War 2. Today the Fijian government manages these Swietenia Macrophylla hardwood forests through regulations crafted to prevent over-harvesting and establish the framework for growth cycle with minimal biological impact.

While it is certainly verifiable that the Mahogany sold is grown in plantations, the question of sustainability is still up in the air. Unlike plantation-grown Teak, Pine and Eucalyptus forests found throughout Asia, Africa and South America, the plantation grown Mahogany is neither FSC nor PEFC Certified.

While no third-party certification is yet available, plantation-grown Mahogany from Fiji has already taken some important first steps towards sustainability, recently receiving CITES clearance for export and beginning the certification process with Smartwood/FSC (Forest Stewardship Council).

Low Movement in Service

Mahogany from Fiji shows extremely low overall shrinkage with regards to volume and dimensions during the drying, but what truly makes it the best for keeping shape is the close to 1:1 ratio of radial (measured in the direction between the center of the tree and the bark) to tangential (measured in the direction of the circumference) shrinkage. This scientific ratio is the determining factor in woods' resistance to warping and cupping.

According to information provided by distributors of Fijian Mahogany, homeowners should allow a 3/16" gap between board widths on 4" (3-1/2" net) wide decking, and a 1/4" gap between board widths on 6" (5-1/2" net) wide decking for drainage, airflow and expansion.

Natural Durability and Termite Resistance

According to the USDA Forest Products Laboratory plantation-grown Mahogany heartwood rates as durable in resistance to a brown-rot and a white-rot fungus. The heartwood of the species is also moderately resistant to dry-wood termites but offers little resistance to attack by marine borers.

While it is true that the heartwood of Mahogany may be durable, the sapwood is not suitable for any level of outdoor exposure. It has been shown that Mahogany sapwood is susceptible to wood-boring insects like termites and readily decays when left exposed to the outside elements.

Preservative Treatment for Extra Protection

Due to the less than ideal natural durability of the species, the plantation grown Mahogany decking that is imported from Fiji is pre-treated with MCA to resist decay or termite attacks and generally to prolong the life span of the sapwood. While clever marketers have claimed that any sapwood that exists will absorb the MCA or MCQ treatment to ensure longer life span and protect against degradation of the Mahogany deck boards, the USDA Forest Products Laboratory has clearly stated that "Both heartwood and sapwood of Genuine Mahogany are resistant to impregnation with preservatives".

Furthermore, information from the National Pesticide Information Center advises that Micronized copper wood treatments are merely new versions of the previously banned ACQ and CA formulations. The main difference in these new versions is the use of very small particles of solid copper, usually copper carbonate, rather than soluble copper in the solution being injected into the wood. The small size of the copper particles in these products allows them to stay suspended in the solution being injected into the wood. However, their small size may also affect their potential to leach from the wood. The potential health and environmental risks of these formulations may be affected by the size of the copper particles being used in the product. For more information on MCA treatment please click here.

Not everyone thinks that MCQ is effective. In an interview with Professional Deck Builder Magazine, MCA treatment manufacturer MicroPro came under attack from Viance (800-421-8661, treatedwood.com). Viance makes Ecolife, a nonmetallic, carbon-based preservative, as well as ACQ, but it does not manufacture MCQ. Based on findings from a field test done by Viance (and verified by a third party), that company has claimed the MCQ formula does not provide adequate protection against premature decay, particularly in ground-contact wood.

"The decay we found was due to brown and white rot fungi, two common decay-type microbes. It's our theory that the solid, essentially insoluble copper in MCQ is chemically bound and not available in an ionic form, as the soluble copper in ACQ is. Because of this, we don't think that MCQ is as effective at preventing these organisms. We're also concerned that the copper in MCQ doesn't enter the cell walls during treatment, and so won't be as effective at controlling what's called soft rot. However, this rot takes two to three years to develop and our test only ran for about 10 months."

The crux for the consumer is whether there's substance to Viance's findings of premature decay in MCQ-treated wood.

Potential Corrosive Nature of Preservative Treatment on Deck Hardware

According to an article found in ProfessionalDeck Builder Magazine the greatest interest to deck builders, perhaps, is that these deck boards treated with MCA and MCQ micronized formulas are said to be less corrosive. The manufacturers claim that aluminum and standard G-90 galvanized hardware can be used in direct contact with micronized copper–treated lumber. The reason is that the copper carbonate used in MCQ and MCA produces relatively few copper ions. This is not the case with ACQ and CA, with which aluminum contact is forbidden, and hardware has to be either the thicker, more expensive G-185 galvanized or stainless steel.

That said, while Simpson Strong-Tie (800/999-5099, strongtie.com), a major manufacturer of framing hardware, acknowledges that while MCQ is less corrosive than ACQ or CA, it still continues to recommend the use of G-185 or stainless steel hardware with MCQ.

Framing Considerations

While Fijian Mahogany specie is the same as South American Mahogany, some people note that the material is slightly softer and less strong. The difference is due to the quick plantation growth. The fiber is of a lower quality with large growth rings, pin knots and bleached color and is much easier to dent and prone to bend/break when force is applied.

According to information provided on the websites of distributors of plantation-grown Fijian Mahogany the Span requirements are either 12" or 16" on center for 1x4, 16" for 1x6 and 18" on center for 5/4x4 and 5/4x6 decking. When compared to other hardwood decking products such as Batu, Cumaru or Ipe (which all require spans of 16? on center for 1x4 and 1x6 decking and 24" on center for 5/4x4 and 5/4x6 decking) this adds up to a considerable amount of extra time and money spent on framing to prepare for a plantation Mahogany deck.

Conclusion

While plantation grown Mahogany is indeed the same specie as Genuine Mahogany and offers beauty and good stability, the jury is still out on product claims of its suitability for decking use due to its relative softness, questionable durability and long-term sustainability.

We at Nova are ourselves still studying the species further to better determine the long term prospects for plantation grown Mahogany Decking before providing a definitive statement on the specie's suitability for decking.

In the end, when analyzing the blur of decking products and brands that are out there, it is up to the individual consumer to review all the relevant facts and decide on an established product which has been time-tested and which suits the home owner's particular needs.

Educate yourself and use wisely your power of choice....

If you have any questions about which specie may be best for your particular deck project please contact the experts at Nova www.novausawood.com

By Bill Christou, 08/09/17

"So you are saying that cutting wood products out of Virgin Rain forests is a better alternative than cutting wood products out of a plantation that was planted specifically for the purpose it is stated? Hmmmmm....."

By Anonymous on 09/19/17

"Bill, I'd be careful about false claims on your blogspot. I just looked over the ONLY installation guide on Plantation Genuine Mahogany decking as I was surprised about you claiming 1x4 could only span 12". This is exactly what it states. Spanning – For minimal waste, we recommend that you build your deck on 12? spans. Otherwise, follow these standard spans for installation for minimal deflection of the deck surface: Fijian Genuine Mahogany Decking Nominal Size 40 psf Span 1×4 16? on center 1×6 16? on center 5/4×4 18? on center 5/4×6 18? on center; Your statement that Genuine Mahogany can only span 12" is completely false and is considered Libel. Now I know you aren't going to allow this to be posted on your blog. But, you should in fact tell only facts to the one follower you have currently."

By Anonymous on 09/19/17

"We appreciate your comments and have made the corrections regarding the span issue. 12" was just their baseline recommendation. As to plantation vs. rainforest / natural re-generation, I think we have to consider why do we have a plantation in the first place? Well, somebody probably chopped down the forest in that area. We are always better off to try and maintain the original natural forest to whatever extent is possible. Don't misunderstand, a plantation growing Mahogany is absolutely preferred to agriculture such as soybeans or cattle ranching. Thank you again for the comments."

By SteveG on 09/19/17

"Note also that standard decking installation should be for 100 pounds per SF rating not 40 pounds per SF as stated in the manufacturer's installation guide regarding spans. Our research has shown that for a 100 PSF rating, 12" spans are required for both 1x4 and 1x6. See http://www.greenworldlumber.com/blog/fijian-genuine-mahogany-decking-installation-guide"

By SteveG on 09/19/17

"Dear Anonymous, Thanks for your comments. No straw men please! See https://en.wikipedia.org/wiki/Straw_man Please note that nowhere in the article was it stated or implied that harvesting wood products from rainforests is a better alternative than out of a plantation. Please do not misrepresent what was said or try to create a counterfeit statement or position to argue against. Sincerely, Bill"

By BillC on 09/19/17

"This blog was written 3.5 years ago. Have there been any changes in perspective on Fiji Mahogany"

By J. McFadden on 04/10/21

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