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dcunited122 07-29-2011 05:45 PM

Pergola Notched Cross Beams Are Bending and Warping

I have a pergola with 20ft long girders and 14ft long cross-beams or rafters. I have notched only the cross-beams and installed them onto the girders. Unfortunately, I couldn't get to installing brackets on the beams until a week later, and the beams became warped and bent. All the wood is pressure treated and I assume the reason why they bent is because the wood is so moist and saturated from the treatment process.

I removed the beams and am going to let them sit under weight for a couple of weeks to flatten out again while I'm on vacation. Now, could someone suggest a method so that the beams won't bend when I put them back up? The notches were very snug and in some cases I had to hammer the beams on. Should I widen the notches so that the beams easily go onto the girders? Should I notch the girders as well?

Any help would be greatly appreciated.

Thank you in advance.

Leah Frances 07-29-2011 07:53 PM

You may or may not be able to 'flatten' some or few of the boards. Pressure treated really needs to be allowed to dry before trying to do something with it. Some of it is going to twist and bend - no matter what you do.

My friend's deck was fairly well built by a licensed contractor but done in a hurry to flip the house (before he bought it) so they used fairly wet PT. Now even well secured boards are starting to cup and twist.

So, you may be out of luck. Let's see what the pros say.

Daniel Holzman 07-29-2011 09:03 PM

PT lumber is well known for twisting, bending, and cupping. There are a few reasons for this. First, the wood tends to be low quality pine, and is not dried before treating. Second, the PT wood is typically sold very green, moisture content well above 30 percent not uncommon. As soon as it starts to dry, it begins to twist.

Since PT is so problematical, it is best to use a design that functions structurally even if the wood bends. Simpson connector brackets are good for this, even if the lumber warps, the brackets maintain strength. If you are looking for furniture grade appearance, forget PT lumber, not gonna happen. Trying to force PT to maintain a straight, unwarped shape is like herding cats.

dcunited122 07-29-2011 09:15 PM

Thank you for your responses. With that in mind I will need to buy new lumber for the beams. What type of wood would you suggest? I know cedar is more expensive but its a nicer looking and won't bend. I will be staining the wood white so the finish doesn't have to be the best.

Thank you.

Leah Frances 07-29-2011 09:32 PM


Originally Posted by dcunited122 (Post 696517)
Thank you for your responses. With that in mind I will need to buy new lumber for the beams. What type of wood would you suggest? I know cedar is more expensive but its a nicer looking and won't bend. I will be staining the wood white so the finish doesn't have to be the best.

Thank you.

Knotty Cedar isn't that bad. You want STK - sound tight Knot.

I had a project a year ago that I really wanted to do in PT. I bought the PT six months before I planned to use it and stacked it carefully in my garage. When I was ready to work I had to discard (use on another project - not throw away) 20-35% of it. Then after I assembled the railings I left them in the garage for another few months until they were bone dry so I could paint them.

My current project is a gift and one I can't babysit that long so we splurged on Cedar.

Maintenance 6 08-01-2011 06:30 AM

Pressure treated lumber is kiln dried before it is treated. Otherwise it would not absorb enough treating solution. PT lumber can be gotten in number 1 or 2 grade. Number 1 will tend to be somewhat more stable. What makes it wet is the remaining solution fom the treatment process. What really creates the problem can be the quality of the wood, but more important is the species. Southern yellow pine is the preferred wood for treatment. It is by nature, very unstable, but the grain structure adapts well to the pressurization process during treatment.

Daniel Holzman 08-01-2011 07:24 AM

Not sure where you got your information Maintenance. I checked six manufacturers of PT lumber. Several of them advertise that their lumber is "Kiln Dried After Treatment" , the industry acronym is KDAT. For example, Prowood, CoxWood, and YellaWood advertise that they are KDAT, and of course they charge more for this type of wood, since they first treat the wood, then have to kiln dry the PT lumber later.

"Marine Treated Pilings", on the other hand, advertise that they kiln dry before treatment. They seem to be one of the few that actually does this, possibly because their application requires deep penetration of preservative, and the standard PT perhaps is thought not to require total penetration.

Madison Lumber seems to be typical, they offer either undried (standard) PT, or #1 KDAT lumber at additional cost. The PT lumber I have purchased at big box stores must have been standard, not KDAT, because it was quite dense. In retrospect, maybe I should have sprung for KDAT, probably costs at least 50% more, but the greater stability might have been worth it. The claim is that KDAT lumber is stable and ready to paint, warps, cups and twists far less than standard PT lumber.

Maintenance 6 08-01-2011 02:18 PM

Well Dan,
I will avoid an argument, but I have two pieces here with grade stamps. Both say "KD" nothing about AT. One is treated with CCA. The other with ACQ. One is number 1. The other is number 2. Both are treated to .40. The info that I got came from the local rep for one of the suppliers (Can't think of which one at the moment). His statement was that the pine used is dried to a certain percentage of moisture prior to treating and that is for consistant quality control of the finished product and proper absorption of the treating material. Perhaps the better quality materials are also dried after treatment. Regardless, Southern Yellow pine is the lumber of choice for PT and is still an unstable species. Also, much of the yellow pine used is milled from very young trees which in my opinion makes the situation even worse.

kwikfishron 08-01-2011 03:04 PM

199 Attachment(s)

gridirongal 06-24-2012 09:29 PM

On this same topic, we are in the middle of a pergola build and also used PT lumber. Two of the 4 girders are warped, one is very warped at the very end. Would this warping cause the post on the same side, opposite end ,to lean? We have tried multiple fixes, including reseting the post, which corrected a great deal but it seems to keep falling back after being level for a short period. We just cant figure out what is causing the lean and wondering if it has something to do with the warped board. Thank you, and be kind, I'm a wife asking because my stubborn husband won't ask for help! :)

oh'mike 06-25-2012 06:20 AM

Post a couple of pictures---there is a good chance the lean is caused by a lack of proper bracing--

the post itself might be warping---

the girders may be shrinking as they dry--

pictures would get you some good answers---(don't be to hard on your man--real men don't ask for help very often)---Mike----

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