Pergola Design Help Please...

[mschmidt]

All,

Homeowner here that has built a few decks in his days but wants to tackle a Pergola over an existing patio.

I understand the basics, but I would like some help deciding if my rafter and beam spans are acceptable.

Largest rafter span is 18.75" with 2x8 Southern Pine PT as the plan. This number will be very hard to shorten with the design as the patio is already there and we want to get the most use out of it without posts in the way. Would butting two 10ft boards together with a third bolted in the middle on one side make it stronger? Sag is my main concern.

Largest beam span is 17.375" of two 2x8 Southern Pine PT beams sandwiched and carriage bolted to a 6x6. Perhaps I can cheat the posts to the middle a bit if needed then add angle supports? The reason I want them stretched far is for a concrete sidewalk we are getting poured to the pool we are building. I don't want a post to have to go in the middle of the sidewalk later. The pool will be done before I start the pergola. Just planning ahead.

Other things that may be hard to see in the pictures are planned 1ft spacing of rafters and 6" spacing of 2x2's on top. The taller half is 12ft tall and the lower half is 10ft tall. No changing this due to the windows.

The rafters will be attached to a ledger board that is bolted to the house with lag bolts via hangers.

Other than the spans feel free to comment on:

- Wood choice - On a budget so PT is the plan right now.
- Dimensions - 2x8 was just my first guess for beams and rafters.
- Any other suggestions.

Thanks in advance,
Mike

[thedeckbarn]

In my humble opinion. and it is only my opinion, Min 2x12 and PT will be ok. I have 2x10 pt on my house. Finished with a solid color stain. Any thing else will sag. I would also use 3 post on each section instead of 2 as there will be lateral stability issues, depending weather you bury the post in the ground, how deep etc, Also a min. of a 6x6 post even if you were to use only 4 post instead of 6 post. Notch the beam into the post don't bolt to the outside.
steve scholl
Here are 3 drawings from the American Wood Council

[mschmidt]

Thank you for the info! Any thoughts to butting two 10ft board together vs a 20ft board?

[Andy Engel]

I would always go with the 20 footer, if that option existed.

[thedeckbarn]

Thank you for the info! Any thoughts to butting two 10ft board together vs a 20ft board?

How would butting 2 boards together work at all? What would hold them together?
I am not sure where you live. If there is a Menards anywhere near you, go there and buy the 2x12"s. I have been buying truckloads quantities of treated for years and sometimes find their retail prices on 2x12's close to or less than my cost.
steve scholl

[mschmidt]

I was thinking of butting them together then add a 3rd board on the side for support. I do have a Menards about an hour away so I will look into that. How is the wood quality?

Each 20ft board will be close to 100lbs if a little wet. Not going to be fun.


How would butting 2 boards together work at all? What would hold them together?
I am not sure where you live. If there is a Menards anywhere near you, go there and buy the 2x12"s. I have been buying truckloads quantities of treated for years and sometimes find their retail prices on 2x12's close to or less than my cost.
steve scholl

[thedeckbarn]

it'll look terrible, the strength will be questionable at best.

to install: you will want to put one edge on the header and walk it down to the joist hanger, Will be manageable, maybe 1/2 the weight than as you will not be deadlifting the whole thing. Once the 1st 2-3 are up you can slide them down the others.
steve scholl

[Andy Engel]

Steve is definitely right here. The effort of raising a 100 lb board will seem negligible in years to come as you look at a clumsy, laminated assembly you could have avoided.

I've lifted a lot of heavy stuff over the years. Get a helper and couple of step ladders, and raise each end incrementally. Or raise one end completely first - that way you're only lifting 50 lbs at a shot. Just be careful and go slow.

Really, this is a piece of cake. It's all ground level. Try setting 20 ft. 2x12 rafters 30 feet in the air!

[piffin]

I can afford to be a bit less tactfull than Andy.

Butt end joints in a structural member would be an unmitigated disaster. Get lumber that is long enough. I presume you meant 18.75 feet and not inches.

[thedeckbarn]

i just weigh a 2' frozen piece of 2x12 treated
8.13 lbs so a 20' piece will weigh 81lbs. the same weight as a bag of concrete. If this makes you feel any better.
steve scholl

[mschmidt]

Thanks for the tips guys! Will for sure get full length boards... I'm sold on the 2x12 based on the chart provided, but is the third post in the middle of the beams nessessary? The two posts in the middle are close enough that I can attach each structure together if needed. Is this asking for trouble or should it help.

[thedeckbarn]

The whole thing here is lateral stability. More post will give more lateral stability. Even with 6 post you still might need to put up those really good lookng x braces you may have seen on other under designed structures. Should it sway or move at all when you are installing the 2x2's perpendicular to the 2x12's you will need to brace this. Somehow. You would be very dissapointed it it fell on you and your guests heads.
I just don't understand where the notion of less is better when it comes to support posts. This thing has to last for years. You yourself already know that each 2x12 weighs 81 lbs. You should really want to consider the footings you use to make this happen, how you install the posts, etc.
steve scholl

[mschmidt]

I don't have the notion that less is better... see attached. I just have a challange with walkways to the pool. I think I have a solution now if I add the 5th and 6th post.

As for footings... two will be in braces on the concrete and the other 4 will be buried 3+ft in the ground with two bags of concrete each. Seem ok?

Thanks again for your help!

The whole thing here is lateral stability. More post will give more lateral stability. Even with 6 post you still might need to put up those really good lookng x braces you may have seen on other under designed structures. Should it sway or move at all when you are installing the 2x2's perpendicular to the 2x12's you will need to brace this. Somehow. You would be very dissapointed it it fell on you and your guests heads.
I just don't understand where the notion of less is better when it comes to support posts. This thing has to last for years. You yourself already know that each 2x12 weighs 81 lbs. You should really want to consider the footings you use to make this happen, how you install the posts, etc.
steve scholl

[mschmidt]

Another close up...

[thedeckbarn]

http://www.thedeckbarn.com/howto-buildadeck.aspx here are footing calcs., use this as a guide. Post need to be evenly space and the sad truth about building is it cost money to do it right. Before i would move post for sidewalks I would put the post where the post are supposed to go and move the sidewalks. You are going to affect the whole look of the back of the house for what, A couple of $$$$.
steve scholl

[Andy Engel]

I can afford to be a bit less tactfull than Andy.

Butt end joints in a structural member would be an unmitigated disaster. Get lumber that is long enough. I presume you meant 18.75 feet and not inches.

Ah, Piffin, good to see you here.

[Andy Engel]

Lateral stability is indeed the key here. The straight down dead and live loads are negligible. What matters is how the structure behaves in the wind, or when a guest with a few too many of whatever you or some future owner is serving leans against a post.

I'm not generally a fan of burying posts in concrete because it makes them impossible to inspect. With decks, I prefer other ways to get lateral stability. Others will disagree. However, pergola structures made from wood have no bracing or inherent shear strength, so burying posts in concrete is likely the best way to go.

I'd go so far as to saw cut the existing concrete sidewalk and break it out for a foot or so on each side of post locations. Then I'd dig the footing to frostline, or deeper - say, at least 3 feet - and set the post in 'crete. You need the depth to resist sideloads, even if you don't have frost. Now, I'm on the fence about whether I'd replace the sidewalk at the same time as I poured the footer. It might make sense to do that if your ground is very well drained, or frost is not an issue.

If that's not the case, then I'd probably pour the sidewalk seperately, isolating it from the post with a couple of wraps of sill seal so there was enough freedom of movement to keep the post from cracking the sidewalk later on. After the sidewalk cured, I'd fill the joint between it and the post with urethane or butyl caulk.

In any case, setting the post within the sidewalk will have the sidewalk helping to resist lateral movement. Since a 4 inch slab weighs in at about 2 tons per 80 sq. ft., even a small slab would offer significant lateral reinforcement.

[mschmidt]

Thanks again for the tips. I updated the smaller portion of the design with the 3rd post and 2x12's... Will do the other half tomorrow. Look ok?

[redwood]

I'm not sure that you have as big a problem as is being discussed here.

First off, you can bolt the beams to the outside of the posts. This is a pergola, not a deck, there is no live load. Use a minimum of 1/2" bolts. I prefer machine bolts with malleable (bridge washers), to carriage bolts. On longer spans, I like to put intermediate dummy posts, that bolt the 2 beams together. I also like to bolt a pairs of rafters on each side of the posts. Side trim, on the posts, under the beams, looks good and adds a little safety factor, supporting the beams.

The posts should be 6x6's. If the elevations work out, you can share one post, for both arbors. For lateral stability, I would set those posts in concrete, a minimum of 4' deep. Anything else and you will need knee bracing. Really, the posts should be isolated from the walkway. If not, I can almost guarantee cracks, at the corner of the posts. A isolation membrane is all that is needed.

We don't use your species of wood, but certainly, you can span 18', I don't have span tables in front of me, but I would think that 2 - 2x12's, will probably make it. That rafter span is probably farther then I would like, but with the correct size, you can make it work. You will probably need some blocking in there, to keep the rafters straight, and from rolling over. Whatever the calculations come up with, If you use the minimum size, you can expect some sagging. Always go at least one size larger.

[mschmidt]

Here are the final design pictures based on all the initial feedback... As redwood stated... this is not a deck so without the live load, do the rafter and beam span calculations change? I agree better safe then sorry, but with the 2x12 the ledger board is getting a little close to the windows and I will likely have to shim the back of the board to get it flat on the house as there are some funky patterns with the brick above the windows. Also the 2x12 stacked do reduce the height a bit more. I will take a pic when I get a chance.

[redwood]

You are correct. You don't need to use the span tables for floor joists. The rafter and beam loads will be much smaller. I would still probably use 2x10 rafters, and probably a 6x12 beam sitting directly on the posts or a pair of 3x12 beams, sandwiching the posts.

Can the line of posts go back towards the house any. It will look better with more cantilever and less rafter span.

I would not notch the posts, but then that is just our way.

[mschmidt]

Seems like a bit of guessing from experience... do you have some tables without live load?

I do not want to push the posts back at all, I actually wanted them out further to get the most use out of the existing patio. I was limited by the 20ft rafters for the one end of the taller portion.

Mike


You are correct. You don't need to use the span tables for floor joists. The rafter and beam loads will be much smaller. I would still probably use 2x10 rafters, and probably a 6x12 beam sitting directly on the posts or a pair of 3x12 beams, sandwiching the posts.

Can the line of posts go back towards the house any. It will look better with more cantilever and less rafter span.

I would not notch the posts, but then that is just our way.

[redwood]

Very observant. Yes it is a very educated guess. I tried to fire up my old span calculator and it doesn't behave well in my current operating system. I guess it is time to upgrade. I could calc it all out, I have a old engineering degree in me, but I don't have time right now. We don't use southern pine, but I'm pretty sure that those members would be OK.

Personally, I would sacrafice a little of the patio to get the overhang. They are only posts. It is not like you can't use the room between them. I also don't like that double post that you have in the center. Work it out to have one post there. That would mean going the double beam route, outside of the posts.

[BuiltByMAC]

To cut down on lateral sway, use decorative structural knee bracing...keep 'em high enough though so you don't create skull busters.

Mac