Hello all. What a wonderful forum.

I have a simple newbie question that (I hope) requires a simple answer. My wife has asked me to build a flat-roofed 22'x22' pergola. Assuming the structure ends up at 20' on each side, even if I put posts around the perimeter that are 10' apart, I will still need at least one beam that spans 20', to avoid a centre post. For aesthetics, I'm looking to use three equal beams even if the front and back beams have a post centered under them and only the middle beam spans the 20'. On top of the 3 beams will sit 11 22' 2x6s that will be 24" on center. Nothing too fancy.

The local mill can offer me any size in fir or cedar. I've searched the internet and this forum (which seems to be my best bet to get an answer) end to end but can't find out what size beam I need. Can someone please tell me the minimum size to use? My miller "guesses" a rough sawn cedar 3x10 will do the job - but I'd like to know for sure. I know fir is stronger, but is the difference in strength enough to result in suitable beams being a 4x10 cedar vs. 3x10 fir beam?

Thanks in advance for any assistance anyone can offer.

Cheers, Bob

Bob:

We can help with the calculation, but we'll need to know the load. Will there be any load other than the weight oif the timber itself? Heavy planters, etc?

Yes, load is needed to calculate. Is there a certain thickness you have in mind for proportions sake? How big of posts are you looking to do?

Thanks for the quick replies.

To help you understand load and size the beam - there will be no load other than the rafters that sit on the beams (11 of 22' 2x6s). I'm not planning to use any purlins at this point, only because I think it will look odd.

Looking from the front of the pergola, the three beams will run left to right in front, middle and rear positions. The rafters will then run front to back. Although small (2x3?) purlins might help strengthen or tie the structure together, I think they will look odd running left to right over the rafters. Since I don't want a center post under the front beam either, I don't see any way to have beams run front to back, with rafters side to side and purlins going front to back.

Also, I'm planning 6x6 posts, inside 10" decorative columns.

I hope that helps.

Cheers, Bob

For help visualizing this frame design I have created this:



The posts are 6x6, the beams are 3x10, and the rafters are 2x6 24" on center, no fancy end cuts on anything....with 1' overhang on all sides...

In order to figure the weigh of the 2x6's we would need to know what type of cedar or fir you would want these made out of....

Jim Rogers

Wow! Jim, That's over and above the call or duty!

The structure will be clear western red cedar. Since the center beam needs no centre post, can I assume I can eliminate the centre post on the front beam?

Thanks to all, Bob

Don't jump to any hasty assumptions until it is truly figured out.....

Now we need the weight per board foot of material, so we can calculate the load. CB.

It's not in my chart of weights.....
Someone will have to post a webpage location of a chart or the actual weight.

Sorry, Jim. I misunderstood. I thought the image you created was of the final structure AFTER doing the beam sizing calculations.

To help with calculations, my internet search has come up with the following:

Avg weight of red cedar - 26 lb/cu. ft green 23.7 lb/cu ft. dry (12%)
Avg weight of douglas fir - 35 green 33 dry.
(Of course divide by 12 for board feet.)

Obviously fir is heavier, but also stronger.

Given that I live north of Seattle, can we assume it will spend much of its time closer to green than dry?

To complicate things further, because my wife wants a "Roman" gazebo with round decorative columns covering each post, knee braces are difficult (impossible). To prevent racking, can I use a large dowel (which is easiest for her non-timber framer husband) glued in place and 2 commercially available 7 guage angle brackets per post?

Earlier I had said I was planning to use 6x6 posts. That was because I assumed I needed that to allow for a sizable dowel. Using 4x4 posts would be lighter and allow more room inside the decorative post end caps for longler galvanized brackets.

While I'm asking for help - do I need the purlins/battens to tie everything together? Will they also help with the racking problem?

What makes this all confusing is I've found sites, such as www.baldwinpergolas.com, which sell commercially made pergolas, some with 18 foot spans - simply using pairs of KNOTCHED 2x8 cedar members as the "entablature" (beam). Am I missing something?

I appreciate everyone's help. Thanks again, again,

Bob