I'm new to the site and I hope I am posting in the right area as this is still framing (sort of ) Basically I'm building a pergola over my exisiting deck. The deck runs 20' wide by 22 out to the yard. This will be an attached structure as the house runs 20' wide on one wall and 14' down the other side. So this is basically built off of 2 exterior walls that run perpindicular to each other with an 8 foot overhang for stairs. My problem is that I don't want to put a post in the middle of the deck to support a beam running the entire 20' width. There will be 2 Ledgers ran 9 feet off the deck into the 2nd story and posts around the edges. What size beam can I use from the short wall to the middle post opposite it (20' away) that can support about 10 joists per side of the beam. I will only be using 1X3 slats on top of the joists for shading of the pergola so there won't be any walking weight, it just needs to support itself and anything that may hang from it, such as plants, lights, etc... oh and not fall over on me The lumber yard recommended wrapping a glu lam in cedar to hide the ugliness of it but I was hoping to just run a big heavy beam up there and call it good. Thanks for any help.

You should really consult with an engineer on this one, or at least with your local building inspector. That being said, I've used what is commonly called a "fletch" beam, which consisted of six 2x12x10 with 1/4" steel sandwiched in between. Basically, it's 2 2x12x10s to get the 20 ft., then a layer of steel, then the middle run is a 2x12x5 then a 2x12x10 then another 2x12x5 to give you another 20 ft. then another layer of steel and finally the last run is the same as the first, 2 2x12x10s. All of these layers are bolted together with a minimum 3/8" galvanized bolts, however 9/16" bolts are more substantial. The steel plates will have to be galvanized, or if your inspector permits painted with rust inhibiting paint. Either way this method tends to be a bit expensive for the steel, and some building inspectors will still require an engineer or architect seal before letting it pass. Hope this helps some.

What you've just read is bad advice. If all the wood members are only 10 feet long, and the span is 20 feet, they all have a joint in the middle, and therefore are structurally worthless. And a 1/4-inch steel plate is probably not nearly enough for a 20-foot span.

I do not believe we should be performing engineering on this site. There is too much that can be undescribed. Anyone who would design a structural member or system without actually exanining the conditions is not performing a service.

To span 20 feet with a "flitch beam" probably would require a steel plate so thich that a steel I-beam would be lighter and stronger for the same span, and would not require nearly as much labor-intensive fabrication.

The good advice in the previous post was to consult a good architect or structural engineer locally.

I agree with "build4you"... you really should consult with an engineer... but.. what "we" really should do is set up a system for easily posting plans and pictures.. so that engineers and/or architects can (if they want to) come look at them and answer questions such as this one. Good Day all..
Luke

Thanks for the responses guys. Maybe it's me being new, but I'm really not sure why I need an engineer on this one. It's just a simple overhead structure that has very little weight. You don't think a 2X10 or 2X12 could support itself plus the ends of 10 joists? It's not like they are floor joists, I could even get away with 2X4's for the joists.

Depending on where you are located, the required design load for a roof can be greater than for a floor! Don't assume something is simple, when it may not be simple at all, Twenty feet is LONG span for sawn lumber, probably TOO long for all of it. Have the beam designed by a professional after he examines the complete picture, in person, at the site.

Why is there never time to do it right, but always time to do it over again?