pergola design? stregnth

shadegoddess15(z5 WI)December 26, 2005

My family and I have agreed on building a pergola in our backyard. We have found the location for it and I have an idea in my mind on what i want it to look like, but we recently purchased a bench swing and I need the pergoloa to be able to hold up the weight of the bench and two full grown people...all 3 combined, less then 500 pounds. Is that possible? It will of course have more support than the 4 posts in corners, and the posts that I will purchase will most likely be 4x4 or bigger... What do you think?

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virgo91967(z6 KY)

When in doubt about strength, go up une lumber size. But for just two folks, 4x4 should be fine.. with 2x6 cross beams and 2x2 or 2x4 rafters. I recomend galvanized carriage bolts to hold the major structural compnents together and deck screws for Pressure treated lumber for the minor fastenings.

I know Screws take a little longer to instal becasue of having to pre-drill, but they will hold better for longer

Anthony B.

    Bookmark   December 26, 2005 at 10:46PM
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stressbaby(z6 MO)

I agree with Anthony, go up a size. Not only do you want this structure to be structurally sound, you want to give it visual weight. If made with 4x4 and 2x6, it is likely to look skimpy, even if it is structurally sound. Go with 6x6 and 2x8 or more.

Here is a photo of an arbor I built. 4x4 construction would have probably been structurally ok but the arbor would have just looked skimpy with 4x4 posts.

Note that ordinary galvanized fasteners will corrode with the new ACQ pressure treated lumber. You must use hot-dipped or stainless fasteners with ACQ. Use 3/8" bolts and screws as Anthony said. Also, if your swing sways back and forth, your post anchoring system will have to be carefully considered. I've generally used the metal post anchors that bolt onto a concrete footing and elevate the base of the post off the concrete. Rot-wise, this type of anchor is preferred. But if you use these anchors and your swing sways back and forth, you may need additional bracing up top. An anchoring system with the posts in the ground might give you a structure with less sway from the swing.

    Bookmark   December 27, 2005 at 9:12AM
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shadegoddess15(z5 WI)

thank you thank you both!

    Bookmark   December 27, 2005 at 2:31PM
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draftsman28(5 IN)

You should find out how deep the frost line is in your area. Your post's if set in concrete will need to be below the frost line. It would probably also help with the swaying if they were set in concrete. Check out some library books on outdoor projects or thumb thru the books at a home store and see what they use to support the bench swing. The chain that attaches to the pergola will also need to be sized and secured accordingly.

    Bookmark   December 28, 2005 at 3:48PM
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garyfla_gw(10 Florida)

Since there seem to be some professionals here Maybe I could ask a question.?? How would you secure posts in the ground in hurricane zones?? i used 4x4 set in concrete and they were snapped off at ground line or lifted out of the ground. Any suggestions??
Thanks gary

    Bookmark   December 29, 2005 at 7:38AM
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draftsman28(5 IN)

Gary I'm not a structural engineer so I can't tell ya the sheer value of a 4x4 post is. How far down were the posts set and how deep do foundations need to be in your area for hurricane resistance? how much concrete and did you use? did you use any sort of anchor on the bottom of the post for the concrete to anchor into?

    Bookmark   December 29, 2005 at 11:27AM
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cottagefarmer(z4b WI)

You should consider steel posts attached to cast concrete blocks that have embedded high tensile bolts. Cover the posts with polyurethane decorative cladding (which can easily be replaced).

    Bookmark   December 29, 2005 at 3:46PM
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nathanhurst(VIC Aust)

If you set timber in concrete it will rot quite quickly, and the concrete will crack due to the expansion and contraction of wood with changing moisture. Use .

    Bookmark   December 29, 2005 at 6:18PM
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garyfla_gw(10 Florida)

The structure was 4x16 with 9 4x4 posts set in two rows tied together with 2x4's and covered with vinyl lattice.Posts were set in 18 inches of concrete. Survived Frances and Jeanne with minimal damage but Wilma either snapped the posts at ground level or lifted the posts from the ground..
Maybe it would be better to not tie it into the ground??
My 8x15 aviary had only slight damage and is just sitting on the ground though attached to the house on one side. My guess is that it didn't resist the wind as the shadehouse did??
Would like to rebuild this structure but no use making it too expensive as insurence won't cover it.
thanks gary

    Bookmark   December 31, 2005 at 5:33AM
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We chose pressure treated wood for our pergola primarly because of the high cost of ceder which my wife and I prefer. Are there any concerns with painting pressure treated wood (My wife would like the pergola white)? If so, which stains would you recommend that would lighten it up and keep it attractive yet durable?

    Bookmark   February 7, 2006 at 11:16AM
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I've been using opaque, acrylic latex stain instead of paint for years with excellent results. Unless you get up real close, you woudn't notice my front porch railings and stair risers (all treated lumber) are stained, not painted. There's never any peeling or checking of the finish and the wood appears to be still in excellent condition. Caveat: These were all constructed before the new type of treated lumber came out.

It did require more than one coat the first time to get a good, opaque finish, but was well worth it. Now it just needs an occasional single coat to keep it fresh looking. I don't use paint outside any more - scraping and sanding in preparation for a new coat of exterior enamel is too much ... I'd rather be gardening!

Good going! I'm still trying to get my menfolk to build a pergola over our patio.


    Bookmark   February 7, 2006 at 12:22PM
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One thing to remember...all the lateral strength(wind load) in a pergola is obtained in the footing. Pergolas by origin do not have any diagonal bracing and/or brackets. Using metal stamped brackets and other fastening devices, unless 1/4" hot dipped steel, is a waste of time. We build our pergolas/structures by utilizing ss40 galv pipe inside fiberglass columns or handmade wood columns. Yes it adds to the price, but I can assure you that it will last for decades and never blow over. Also, stainless steel fasteners are a must for 2 reasons. If you use west red cedar you will not get the staining associated with galv. fasteners. Also the new AQC treated wood is not consistant in the chemical compound quanities, therefore hot dipped fasteners and brackets (ZZ) rated for this wood all not failproof. Thats if you wanted to know......

    Bookmark   February 14, 2006 at 10:26PM
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Hello, my husband is building a pergola over our patio. it is 16' by 25'. He has it attched to the house.He is using 16' boards coming from the house to the end of the patio.They are being supported by #4, 4 by 4's. It doesn't look substantial enough. Is that enough to support all 12 of the 16' boards? Is there anyone with experience that can give us a good idea on the best way to do this? We live in the Dallas/Fort Worth area and get some strong winds.

    Bookmark   May 15, 2006 at 7:51PM
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nathanhurst(VIC Aust)

patina: you might add some bracing strength by running some steel wire on two diagonals on two sides. You could then grow a vine up this (perhaps add more wires for looks).

    Bookmark   May 16, 2006 at 3:52AM
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