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pylot_gw

Pergola / arbor : how to prevent rot?

pylot
14 years ago

I'm planning to build a wood pergola for growing grapes - size about 8 ft by 12 ft, and 7 ft tall. I need some advice in how to make the posts (more) rot-proof so they last at least 10-15 years. We get about 20 in of rain, Nov through Mar.

First question: Should I place the post directly in the ground or in cement? Should I use some waterproof barrier (plastic bags, roof tar, etc?) Or should I just place some column bases / stirrups in cement and then attach the posts to them above grade?

Second question: Should I use redwood (very expensive), pressure treated pine (could be toxic eating the grapes if they touch the wood?) or douglas fir? I can stain it with something as long as it is non-toxic and maintains wood color.

I think I'm leaning toward using douglas fir, waterproofing the base of the posts (what works best?) and setting them in cement.

Comments (5)

  • gartenmiester
    14 years ago

    Doug fir will not last without annual treatments. Re-staining will be difficult once the vines are estabilished. Your best low maintence choice is pressure treated pine. If toxicity were a problem; every grape grower in America would be in trouble.
    Remember that P.T. pine will warp and twist unless properly designed and constructed. P.T. pine comes in several grades- the grade I would recommend is called "ground-contact". This is getting harder to find since Lowes and Home Depot don't typically carry it. If you purchase the non-ground contact you can coat the post bases with fence post paint (tar in a bucket). I get this at our local Tractor Supply store. If you decide to use concrete for the bases, be sure to slope the concrete away from the posts. Note: Do not try to paint or stain new P.T. pine - it needs to weather a season before it will absorb.

  • blueberrypancakes
    14 years ago

    Hi,

    I built a Kiwi vine Pergola 2 years ago. I used Cedar for the posts, and heavy metal stirrups in 10" diameter x 2' deep concrete forms (Sonotubes) set in the ground. The stirrup bases set about an inch above the concrete forms so that the posts dry faster and don't sit in pooled water.

    I used a water-based semi-transparent stain (from Wolman's) on the Cedar before I put it together so that all the sides of the wood were covered. As the other poster noted, it is difficult to stain a Pergola once the vines are established. I am attaching some photos of my finished Pergola.

    Doug Fir is definitely less expensive than Cedar or PT pine. And it might hold up just as long if kept stained (to protect from rain, sun, wind) and not in direct ground contact (where wood eaters will easily get to it). After 4-5 years though, the stain will be wearing off and you might want to figure out how to recoat the upper grid where the vines are likely to be. The posts should be easy enough to recoat though.

    Or you could use all-PVC plastic lumber...which never rots and never needs staining, but does cost a tiny fortune :).

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  • pylot
    Original Author
    14 years ago

    Wow, that's a nice pergola :) Thanks for the comments, here's my thoughts:

    1. I don't claim to be an expert on toxicity, but all PT pine comes with scary warnings. I don't think it's bad for the plants in the soil, but the grapes (the fruits) are likely to touch / rest on the arbor, and that's what I don't want.

    2. Should I cover the posts in tar and place them in cement, or just attach them to column bases that are placed in cement? In the first case they may rot, in the second case I'm not sure the pergola will be strong enough to withstand wind, not being attached to anything else.

  • kudzu9
    14 years ago

    blueberrypancakes-
    You have a very nice pergola. However, I couldn't tell if it is attached to your house or not. The reason I bring that up is for the original poster's benefit, since using only metal stirrups for a structure like this with no attachment of the top of the structure does not meet code, at least in many areas. If you look into the specs for this type of hardware, you will usually see in the fine print that it is not approved for this use if the top of the structure is free-standing.

  • brian123welder
    14 years ago

    I build trellises pergolas and arbors. I use mild steel. If you use a solid mild steel those structures will be around indefinitely. I have pictures of some arbors and such on my site. www.graydiewelding.com

    Here is a link that might be useful: www.graydiewelding.com

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