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fahartle 07-23-2010 01:36 PM

Pergola construction - beam span without sagging
Hey all. I'm building a deck and am going to add a pergola above it for a little shade and possibly some vines. My main concern is sagging in the beams and joists. Many thanks in advance for a little advice. Here's my plan:

Pergola - 24' by 12'

6x6 PT posts on concrete footers (coming through deck)
-- 6 posts total, three along the 24' span (so 12' span between)

Main support beams running along either side of posts (sandwiching posts and lag screwed in)
-- 2x10s cantilevered two feet beyond posts
-- 2x10s cut to 14' lengths and butt joined at the center 6x6 post
-- 4 beams total (2 on either side of posts)

Joists are 2x6s cantilevered two feet over 12' span on each side (so 16' each)
-- secured by a 8" lag screw down the center of the board into the beam
-- joists spaced a foot apart, so 10 boards along the beams on the inside of the posts, and 2 boards outside of the posts along the edge.
-- at 2lbs per foot and 24 boards, that's around 768 lbs of weight (not counting the weight of the beams), with 10 boards or 320 lbs of weight on the beams between each post.

I'd like this structure to last a good long time and would rather overbuild a bit than underbuild. Questions for you guys:

1) With the plan as is, will my beams or my joists sag? If so, what could I do about it? I'd rather not have support beams coming up through the middle of my deck (splitting the 12' small side) but I could put in extra posts along the length of the deck if I had to (maybe a 4x4 post between the 6x6s?).

2) Should I double up the 2x10s along the posts (so have 6 or 8 beams instead of 4)? If so, should I double them so that both boards butt join in the center post, or should I cut them different lengths so that one of the doubled boards has it's longest stretch in the center and the other joins in the center (i.e. for the 28' beams have one board be two 14' sections and the other board be a 6' board, a 16' board in the center, and another 6' board).

3) Would support boards or smaller sections of post running diagonally from the posts to the beams do much to counteract sagging?

4) Lastly, I live in high mountain Utah (semi-arid desert). Can I use untreated lumber for the beams and joists (posts will be PT 6x6s)? Will that alleviate or worsen any sagging?

Thanks again for any help you guys can give me!! All the best,
Brandon in Utah

Ron6519 07-23-2010 04:31 PM

The 2x6's are under sized for the span. Minimum 2x8's, but I would do 2x10's for the over built catagory.

fahartle 07-23-2010 08:37 PM

Thanks Ron. 2x10s all around sound pretty doable. My better half wants to space the top boards further apart so there'll be fewer up there (so probably no increase in cost to use 2x10s).

What type of wood would you use for the pergola? I'm bad at regular maintenance, so I'm hesitant to use standard fir if I'd have to seal it every couple years. But I also hear PT boards can get twisty as they dry. We're in a very dry area, but there'll be snow sitting on the boards for a couple months in winter. Thanks,

Daniel Holzman 07-23-2010 09:23 PM

In dry climates you can use untreated fir for the framing, assuming there is a roof to keep rainwater off the framing. If there is no roof, you can paint the wood. I would stay away from PT for anything but the posts, not necessary if moisture precautions are used, plus PT is toxic and expensive.

I have a shed for my lumber, probably similar to the type of structure you are planning to build, I did not use any PT lumber at all, should last 50 years since the framing stays dry.

forresth 07-23-2010 09:44 PM

PT warps when it dries unevenly, as would any wood. its just that PT is usually wet when you buy it.

I wouldn't sugest using any wood other than rot resistant woods like cedar or PT for an outdoor structure like this. you cant seal it anymore once the vines are up. better make it maintenance free.

one nice thing about the larger lumber is it usually tends to be of a higher grade than the smaller stuff.

for added supoirt if needed, run diagonal braces from the posts. it'll be much stronger and look pretty good too IMO.

Ron6519 07-24-2010 06:51 AM

I don't know of too many woods, set outside, that are maintenance free.

forresth 07-24-2010 01:28 PM


Originally Posted by Ron6519 (Post 474380)
I don't know of too many woods, set outside, that are maintenance free.

Cedar, Redwood and Tamarack come to mind. I'm sure there are others. Ipe and Mahogany maybe

Ron6519 07-24-2010 04:21 PM


Originally Posted by forresth (Post 474554)
Cedar, Redwood and Tamarack come to mind. I'm sure there are others. Ipe and Mahogany maybe

So you're saying these woods will last as long untreated(stained painted, etc..) as treated?

forresth 07-25-2010 10:46 AM

A raw cedar shake or shingle roof will outlast an asphalt shingle roof. a cedar post will last 20 - 30 years or more half burried. and that is the absolute worst case scenario for rotting wood. I've heard Tamarrack is better. I'm not sure how redwood lasts in the ground, but it does exposed quite well. The redwood porch on this house seams to be lasting comperably to the pressure treated decks. Hard to find redwood any more. I've heard it all gets shipped to Japan these days.
one thing about PT is every so often you get a board that hasn't been permiated all the way (most often 4x4s) and they rot out from the inside.

fahartle 07-25-2010 07:54 PM

Thanks again for all the help guys! My job got a tad easier because we found we couldn't really afford the 12x24' deck. So we're going to downsize a bit to a 10x20' which I'm guessing will help out the spanning issue. After the advice I think I'll go with PT wood all around. Maybe next summer I can put on a good sealant as that summer project. Thanks!

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