Pergola questions

[MikeFromVA]

Hi everyone.

I'm considering putting a pergola over my patio and I'm having a little trouble locating information on beam and joist spans. Most likely I will be using pressure treated pine but I might go with cedar. I want to find out how much span I can get so i can figure out my post placement. The minimum size for the pergola will be in the 18 x 18 range but I need more information to finalize my plans. I do plan to grow wisteria over the finished product but I have no idea how much weight that adds. If its better to go with lattice work over it for lighter weight, I could lean towards that plastic lattice that should reduce maintenance.

Thanks for looking.

 

[EddieWalker]

Is your span from post to post? At 18 feet, you should be ok with 2x10's, but I'd use 2x12's myself. It's not so much the load, but they tendancy to sag over a large span like that. Buy 24 footers and have them cantelever out past your posts 3 feet on either side. This will give you a little more strength and greatly improve the look of it.

I'm guilty of using latice, and probably will again sometime soon, but it's really not what I'd use if I had a choice. 1x2 strips look really nice, or for longer distances, you can use 2x4's on edge or even 2x6's on edge for 12 foot spans.

How far apart will your posts be?

Cedar posts are nice, but pricey. How are you going to finish the posts? If painting, then PT is your most affordable material. I really like PT posts that are smoothed out with bondo to make them look perfect, then build up the base of them with brick or rock. You can also wrap them in cedar, but you won't save any money that way unless you get a vineer or find some type of plywood that's reasonable.

Is this something that you want to do up fancy, or just to create some quick shade?

Eddie

 

[MikeFromVA]

Eddie,

Thank you for your reply. Although I'd rather have the openness of only 2 posts per beam, I would use 3 if it is needed for durability. If the beam posts can span 18' (post to post total) that would be ideal. I'm sure it depends on the number and type of joists that I place on top of the beams. I like the idea of the 3' cantilever on the beams. I do plan on (2) 2x10 beams on each side with lap joints on the 6x6 posts. If I need 2x12s that would be alright also.

For the joists themselves, I'm guessing that a 2x6 on edge will not be able to support it's own weight over a 16' span even - hopefully a 2x8 can. As for how to space them, that may depend on the beam span and what load I can expect there. If I truly need a beam in the middle, then I will have to see how to fit it around some stairs that lead from the deck to the patio.

I'm not sure if I'm making sense so I'll restate it. I would like to have (2) 6x6 posts that are 18' apart. I would bolt a 2x10 to each side of the post which I would notch to hold the beams. I would place another post/beam set 16' (minimum - prefer 18') away. Across the beams I would run a 2x8 on its edge every 1.5 to 2'. On top of that I would run 1x2 strips in the same direction as the beam every 1.5 to 2'. I would like to cantilever the joists and beams by 2 to 3'.

I was not sure if cedar was a better construction material vs. PT. If PT gives me greater spans and durability, I will go with it hands down. However, if cedar gives me more spans and greater beauty and durability, I will be faced with a decision as to whether the additional cost is worth it.

In the end, I want it to look nice and last a long time so I'm really not looking for quick shade. I'm not a huge fan of PT as it seems to require more care over time. I will stain or paint it if that will help me with long term maintenance.

 

[JohnSinVA]

SYP is considerably stronger than cedar, so you'll get longer spans from it than from cedar of the same dimensions. In this area, it's likely much cheaper too.

What you're proposing is an awfully big pergola. You could use the Loudoun County deck guidelines as a starting point for spanning information, but your spans can be considerably longer than those listed since there are no live load considerations. For something that large you'll need to address racking, since a structure that large and lightweight will be very suspect to it. Lattice would provide greater rack resistance than strips. Is there a structure you could anchor one end to? That will help some with stability also.

As far as looks go, cedar is subject to the same problems as PT - exposure to UV will eventually grey it out, humidity swings will eventually cause checking, etc.

 

[Timber]

I would not use PT in a pergola. 24 on center will be fine.

 

[EddieWalker]

Mike,

I think your a little ahead of me here. Lets start with the posts. You want four posts in a square, with the side to side distances to be 18ft?

How do you plan to attach these to the ground?

How tall will they be? or how high will your beams be?

Racking is gonna be a real issue. Especially at those distances.

I'm not a big fan of metal, but if this is what you want, then you might want to consider steel posts set in concrete. You can get square tubing that's 6x6 inches for the same look as painted wood posts. Then you can weld on some brackets to attach the wood beams to it pretty easily, but I don't see why you couldn't drill holes in it and bolt the beams right to the metal.

Eddie

 

[MikeFromVA]

Thank you for all of the responses. It's starting to look like I may need a middle beam and perhaps center posts on all of the beams. That takes me from 4 posts to 9. I may actually go with 8 giving the center beam no center post but I'm not sure that make sense. I was really hoping to avoid the center beam altogether but only if it could be done safely. There just isn't too much guidance for pergolas to be found.

John, I have the Loudoun County information but it really isn't too helpful since it is for a deck that carries a much higher load. According to it, if I use (2) 2x12 SYP to make the beam, I can place posts 12' apart if the joist span in no more than 9'. If the joist span is 18', the posts should be 6'6" apart. Of course I will be using fewer joists and carrying a lighter load so who knows how much more I can go - I was hoping someone might have that information. The guide further states that if I use 2x10 joists, I can span 17'3" and 2x8 joists can span 13'6". That's why I was hoping to get 16' out of the 2x8 joists with no load.

Racking is twisting - right? Do you mean when the wind hits it? I was considering bracing the corners facing both ways to help with that. As for anchoring the 6x6 posts, the reason for the size is that I am building it over a flagstone patio. I was hoping to place 3 of the posts just off of the patio in 24" deep footings (24" min depth then put 8" of concrete in for pad - when dried place post in hole and fill with concrete). I would drill anchors into the patio for the last post. The planned height is 9' or 10' from ground to top of beam. If the lattice could really help with racking, I can do that pretty easily.

 

[EddieWalker]

Racking is when the building twists.

Take four pencils, pens or anything of equal length. Place all four of them in a square that's wide enough to cover with the palm of your hand. Put your hand over them like a roof. Now move your hand around and see what the pencils do. They rack.

They move at the bottom and the top. Racking is this movement and a big reason that some structures fall down.

Tieing the posts to the top of the cement footings that you plan to pour will anchor the post, but it wont stop the post from moving around. Setting the post four feet in the ground will.

Two ways to stop a post from moving is to brace it at the top and the bottom. In your case, you can iliminate the bottom bracing, but will have to compensate with top bracing. This can get sort of complicated and it's not something I'm comfortable, nor qualified to design here.

Most pergola's are attached to a house or building. This give them the stability to stand on there own. A rigid structure is already braced to stop it from racking.

I don't know my definitions enough to say one way or another, but I think a free standing structure like you're planning on building falls into the gazebo area of design.

Do some searches on gazebos and pay attention to the railins and fancy detail work that's alont the top of the posts, below the actual roof. This isn't just decoration. It's bracing done in a fancy method to stiffen up the posts and stop the building from racking.

18 feet is a long way to span, but not impossible. Do you have to have a flat roof? Is it something you really want to have, or just an idea you're floating around? Why are you spanning 18 feet by 18 feet?

If you did the 18 foot span on one side, you could easily solf allot of your problems by going with three posts on one side. You could easily make it larger by spanning 18 feet on one side and 24 feet on the other with posts every 12 feet. Six posts total and nothing in the middle.

Eddie