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Greenhouses & Garden Structures

Stains on Cedar Pergola

14 years ago

I just recently constructed a Pergola on my deck using treated lumber for the main structure, and cedar to wrap the beam and create a planter box around two of the post.

The cedar was treated with Cabot Australian Timber oil series 3400. The inside of the planters are lined with a plastic sheeting to protect the wood from the moisture. To add a little character, I used copper sheeting to wrap around the beams above the planter and I also placed copper plates in the center of the planter walls.

Within one month, the cedar on the planter walls has begun to spot with dark stains. At first I thought it was dirt stains from rain splashing off my deck and figured that I could simply clean it off, but come to find out it is coming off easily. My next thought was with the copper accent peices, could this be causing the discoloration? Some sort of reaction to the untreated copper? I came to this idea based on the fact that the walls with the copper plates that have design worked in to them have more staining then the other sides.

Any feedback on possible causes, as well as possible remedies would be much appreciated. Thank you!

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Comments (2)

elp0901
14 years ago
Hi Caldwde,

Do you have any photos of the planters? I ask for two reasons: One they sound lovely, I love copper details and would really like to see what you did. Two it would be helpful to see the discoloration.

It sounds like it could be mold. I have been reading on allot of the deck forums regarding what to use on my deck to seal it. There are lots of complaints that the ATO is very prone to mold. This has not been my experience but I live in a very different climate.
PRO
Steven Laurin & Company
14 years ago
caldwde,
It is highly possible that your copper-work has stained the cedar. The use of natural copper with cedar lumber - especially the extractives in Western Red Cedar, has long been known to cause wood staining and an eventual deterioration of the copper-work.

For this reason, most architects specify lead-coated copper when planned to be in contact with red cedar - such as with roof shingles. This reaction is amplified when low-grade copper is used - such as the inferior stuff imported from China. It's not pure copper and loaded with impurities.

Don't know how you'd remove the stains. The best remedy would be to initially isolate the two materials - too late and not very feasible with the copper cladding you installed over the cedar pergola beams.

Be cautioned also, that when certain dissimilar metals are in contact with each other - galvanic reaction may cause accelerated corrosion of the less-noble metal. This is why copper nails should be used with copper, aluminum nail with aluminum . . . etc.
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