# Cedar vs Fir... Pergola Advice for a 14' span?



## Fingerless (Aug 25, 2015)

Hi team,

New to the forum, but looking around it sounds like there's lots of wisdom around here, so I am hoping to tap into some of it.

I am building a pergola (24' long x 18' wide) to cover my concrete patio.
My problem is that I am using 2x6x18 rafters (on 2' centers) which will have an unsupported span of 14' before landing on my two headers (2x10x24 supported by 3 posts). These 18' rafters will have cross braces (2x4x2) inserted every 3' to prevent them from twisting over time.

The pergola will be covered in suntuf corrugated pvc roofing so I can enjoy the patio in the fall+winter as well.

Question time:
Is 2x6 Cedar strong enough to not sag over time across a 14' span? (taking into account the added weight with the 2x4 bracing + pvc roofing + very minor snowload: west coast rarely gets snow).
or should I completely ignore cedar and go with Fir which is exponentially stronger.

My 6x6 posts are cedar for esthetic purposes and braced laterally + longitudinally. Everything else will be the same timber.

Here is a small sketch to put things into perspective. 
I welcome all your advice/opinions.


----------



## NickB (Sep 24, 2013)

According to This chart, depending on the grade, 24" OC would be between 6' 8 " and 10' 8" for cedar (read "western woods").
According to Fine Homebuilding it's 7' for cedar for a deck.
The problem is that the charts aren't completely applicable because you don't really have measurable live or dead loads (well, maybe with your situation with the PVC, you have a very small measurable load). Also everything changes under wet conditions. 

I opted for 2 x 8's (#2 Fir) for my pergola. Partially because of sag over the years, but also for aesthetics and sun block. The longest span is about 14' and I may have gotten away with 2 x 6's but I like the "heft" of the 2 x 8 and it gives me a bit of that invaluable_ peace of mind..._









I don't think I would have been able to convince myself to use 2 x 6 cedar.


----------



## Fingerless (Aug 25, 2015)

Thanks Nick!
I had another friend recommended I use 2x8 as well, due to the span.

I definitely do not want to be doing this again in the next 5 years...

Have you had any issues, or foresee any, with the 2x8's twisting/warping?
I was told to use cross braces at 3' intervals to prevent the wood from twisting as it ages... is this a real concern?


----------



## Chamfer (Sep 6, 2014)

With that span anything will sag over time. The bigger you go the less it will sag, but it will still sag.

Since you're using 6x6's I'd at LEAST go with 2x8's, if not 10's. While Nick's is nice looking, the 4x4's look kind of wimpy to me with the wider boards on top.

I've seen far smaller Pergola's built with 6x6's and 10's so it's not only a sagging issue but a preference on looks.


----------



## Ghidrah (Mar 2, 2010)

Agree with Chamfer, with nothing on top of it, it'll slowly sag under its own weight. A 10' span isn't too far and with the 2X4 trusses it stiffens up, but you can slow it down more with solid blocking or cross bridging. Even with all that 2X6 is asking for sorrow, 2X8 better 2X10 better still, but may look over loaded with mass, maybe an attractive recurve and sloping pattern to the butt ends.


----------



## Steve Neul (Sep 2, 2011)

With a 18' span it would certainly sag even if you used 2x6 yellow pine. That is just too long of a span for wood. What you might do is run a 1/4" dado in the bottom edge 3" deep and glue a piece of flat steel in the wood and then put some kind of trim over it to hide the dado. Another option would be to make the boards easily removable and periodically turn them over. Of course you would need to make the design on the ends reversible too.


----------



## Develin (Oct 1, 2012)

It really needs a joist in the middle. If it isn't fixed to anything above it substantial in weight it will definitely sag in the middle. I've had to repair ceilings because of this problem. I imagine it would be worse for an external project.

Then again I don't know your climate so I could be talking out of my...!

Hope you get on ok with it


----------



## Toolman50 (Mar 22, 2015)

Fingerless said:


> Thanks Nick!
> I had another friend recommended I use 2x8 as well, due to the span.
> 
> I definitely do not want to be doing this again in the next 5 years...
> ...


+1: 2X8". I think the span is too long for 2X6 even without a load. 
I think the 2X6 would sag. 
If you go with fir, you will need treated lumber. 
I would go with the cedar and stain it.


----------



## k9scooter (Mar 5, 2014)

I just built these two out of Douglas fir. It is much smaller, but the mill cut the 2x10's a full 2" thick.

I had a covered one in Minnesota that was closer to your size and was made from 2x 10's in reclaimed barn Cedar that was a full 2" thick. It had a metal roof and never had any sagging issues.


----------



## k9scooter (Mar 5, 2014)

Here's a framing photo of the MN project..


----------



## WesTex (Jan 5, 2014)

SWMBO & I put this one up from a Home Depot kit. It's cedar, 16 x 20. 8 x 8 posts. 2 x 8 joists. The kit came with 2 x 2's to be spaced on 4" centers. Turned out that it let too much sunshine through, so we bought more from our local Ace & added them. Much better! It's only about 3 yrs old now, so I don't know about sagging issues.


----------



## k9scooter (Mar 5, 2014)

Nice! Now you need to add a fire pit.....


----------



## WesTex (Jan 5, 2014)

K9scooter, I like yours. Can't do the fire pit, though. It and the sidewalk are the only concrete around, so we move the furniture back and the smaller grandkids turn it into a tricycle Grand Prix. 

We do have a portable metal pit.


----------



## Fingerless (Aug 25, 2015)

Team, thanks for all the input, especially the photo's. 

I am convinced Fir is the way to go with 2x8 for the rafters. I will have 2x4's inserted between the rafters at 3' intervals to primarily reduce twisting, but also add some lateral rigidity. 

If it starts sagging, I can always throw up 3 pieces of painted channel steel parallel with the rafters to connect the 6x6's posts. This would essentially remove the majority of the load from the remaining rafters.

Now to decide if I should use 2x8's for the headers, or 2x10's to add contrast to the rafters.

Thanks again Team. Great input!


----------



## ecr (Jan 4, 2011)

This is one I built about 5 years ago. It is 2 x 10's the span is 12'-6" beam to beam. I was over at their house last year and everything looked great no sags.


----------



## Chamfer (Sep 6, 2014)

Very nice ECR. Did you build the deck too?

I like the big 45* braces on both. Were they purposely arched or some kind of live wood? Hard to tell from the pics.


----------



## ecr (Jan 4, 2011)

Yeah I did the deck two. braces were arches cut from 2x12's. Deck was pretty interesting the previous deck had one 4x4 in the left corner that looked like a banana. That was the only support post. The room the deck was attached two was cantilevered out 8 foot. The problem was who ever built it ran the floor joist the wrong direction. So they were not going back into the structure. Basically all that was holding the room and the deck was 1 4x4 and some masonary anchors. It had sagged about 3 inches. I drew some plans and took them to a engineer. He reviewed them and made a few revisions and stamped them. Then I built as per plans. The engineer was shocked it was still standing. The new deck structure also supports the room. It was fun I did it solo so that was fun.


----------

