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I'd like a pergola with natural wood color and grain appearance. I live near the west side of the Sierra Nevada, where drought and beetles have decimated the Ponderosa pines. An acquaintance has access to logs that were cut and given away by the Forest Service; he has a small mill and will sell me lumber cut to my specs for half what the lumber yard charges. This wood has not been treated in any way. So, is this a good route to go? What would I need to do to keep this wood from warping?

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    What kind of service life do you want? Or to put it another way, how long would you like it to last and in what condition? Untreated wood (of species not naturally resistant) is acceptable outdoors in the right context, you can of course treat the wood yourself to extend the service life but with nothing done to it you can still expect years without much issue although of course the wood will weather (go grey and expect at least some cracking).. – Graphus Aug 4 '17 at 7:17
  • Well if you lock it into a structure that will help prevent warping to some degree. A pergola is usually a lot of interlocking "lincoln log" type pieces that form a grid. The quicker you put it together the less it will be able to warp due to the grid structure locking it in place – jbord39 Aug 5 '17 at 20:35
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It seems like you have 3 questions:

  1. Is Ponderosa Pine a good choice?

It's not a great choice as the heartwood is rated as moderate to low in decay resistance. It depends on your local climate and how long you reasonably expect it to last without refinishing/coating.

However, economics might make it into a better choice for you. It is certainly going to be among your cheapest wood options (even without your friend's deal) and depending on your experience and expertise, it is going to be a pretty easy wood to work with.

Before you choose too quickly, consider Choosing Wood for Outdoor Projects that are also fairly economical (which one depends on what's locally sourced & available).

  1. Does it need to be treated?

Wood, when left outside, will begin to decay -- especially porous softwoods like (most) pines. Generally speaking though, you only "need" treated wood for ground contact -- which is something you can avoid by using a hardware standoff or a spacer of treated wood. The rest can be finished using an outdoor coating to reduce UV and moisture.

This is a project for your own enjoyment. It's not structural and can be made with and how you like.

  1. How do I keep it from warping?

Warping can happen, but as long as things are fastened (by hardware or joinery) on both ends, it's unlikely to do much. Also, you tend to work with larger dimensions (ie. 4x4 or 6x6 posts) that are less prone to excessive warping.

If you're in a rainy climate, limit the amount of truly flat surfaces. Usually you can put a slight pitch on horizontal surfaces that doesn't change the appearance and will last significantly longer.

Good luck with your project.

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