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tehllama
Apr 30, 2009

Hook, swing.
Hello goons.

My wife and I have been trying to turn our backyard from a horrorshow of rye and chinese privet to a nice place we'd actually like to spend time. To that end, we've cleaned out most of the privet, had some sod and a small patio put in, created a path, tilled some beds and placed a bunch of plantings. I, being dumb and not one to plan too far ahead, decided after the fact that our patio would look real nice with a pergola and that that was something I was capable of designing and building.

Here is our patio:


You might notice that it is somewhat irregularly shaped (its nearly symmetrical but not quite) and doesn't really lend itself to plopping a nice, square, easily measured and designed pergola over top. So, I've been doing my reading for the last month and set out to design a structure that would fit over it in a pleasing way in CAD...and pretty much immediately ran into a problem. I planned on using 4x4's for the posts, 2x6's for the girders, and 2x4's for the rafters. Problem is, the maximum spam across the shorter dimension of the patio is like...16 feet. No matter what direction I lay out the rafters, I can't get even close to having an acceptable span for them, even if I switched to 2x8 girders and 2x6 rafters. Every way I've looked at it appears the structurally sound way to do this is with a center post, which I reallllly don't want to do. I've also though about maybe adding a "suspended" center post with cabling from the actual posts but a) I think I'd want to upgrade from 4x4 to 6x6 if I did that and b) it makes the project a LOT more complicated.

Here's a basic sketch of 10 ft max span girders + long rear end rafters.



I know when a 2x4 is supporting anything heavy its maximum span is like 6 ft. Obviously these would be loaded a lot less but that still seems way too long to not have sag. I've done some smaller carpentry projects and would feel comfortable building a fence but I feel pretty over my head designing something like this. Is there a simpler solution to ensuring that span doesn't sag that I'm just not seeing?

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baromodo
Nov 14, 2012
Well I'm no woodwork PhD-haver, but perhaps you could have two supporting struts forming small but powerful triangles between the two outermost posts and the length of wood you marked in green to give it a bit more support?

Alternatively, you could just put a rectangular pergola structure over the top of the patio and square off the edges of the patio with an outer wooden frame. Then fill the gaps between the frame and the edges of the patio with soil and have a load of plants growing in it, giving it a self-contained "cosy" oasis feel.

Foxfire_
Nov 8, 2010

Metal I-beam instead of wood?

tehllama
Apr 30, 2009

Hook, swing.

baromodo posted:

Well I'm no woodwork PhD-haver, but perhaps you could have two supporting struts forming small but powerful triangles between the two outermost posts and the length of wood you marked in green to give it a bit more support?

Alternatively, you could just put a rectangular pergola structure over the top of the patio and square off the edges of the patio with an outer wooden frame. Then fill the gaps between the frame and the edges of the patio with soil and have a load of plants growing in it, giving it a self-contained "cosy" oasis feel.

This is definitely an option, I'll have to sketch it to see how much it would cut into the overhead space to reach reasonably far out into the span. A stronger central beam (maybe not steel, but like a LVL beam or something) is also a good idea.

cakesmith handyman
Jul 22, 2007

Pip-Pip old chap! Last one in is a rotten egg what what.

I think you're way over thinking this, unless you plan to climb on top with your mates there's almost no load on those rafters. Buy a 2x6 and stand it on a couple of blocks that distance apart to see the deflection unloaded and see how comfortable you feel having it above your head.

Kaiser Schnitzel
Mar 29, 2006

Schnitzel mit uns


cakesmith handyman posted:

I think you're way over thinking this, unless you plan to climb on top with your mates there's almost no load on those rafters. Buy a 2x6 and stand it on a couple of blocks that distance apart to see the deflection unloaded and see how comfortable you feel having it above your head.

I'd agree with this. You don't really need to worry about load. What is the long dimension of the pergola? If you ran a double 2x8 or 10 beam that way (basically along the line that says 'right plane') it would break the span of your rafters a bit. I'm not sure if pressure treated/exterior LVLs are a thing or not, but that would be an option too.

I like your patio-what kind of stone or w/e is it edged with? It looks very clean and nice.

tehllama
Apr 30, 2009

Hook, swing.

Kaiser Schnitzel posted:

I'd agree with this. You don't really need to worry about load. What is the long dimension of the pergola? If you ran a double 2x8 or 10 beam that way (basically along the line that says 'right plane') it would break the span of your rafters a bit. I'm not sure if pressure treated/exterior LVLs are a thing or not, but that would be an option too.

I like your patio-what kind of stone or w/e is it edged with? It looks very clean and nice.

Looking at it on edge I think its granite. The guys that put the patio in told me where they sourced it in case I needed to find more but not exactly what it was. I modified the design with parallel beams to break the span (and also because my wife really wanted double beams...). I also upgraded to 2x8's and 2x6's because she thought they looked better. Max span is now only 10 ft which should be fine even for a loaded 2x6 I think.

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cakesmith handyman
Jul 22, 2007

Pip-Pip old chap! Last one in is a rotten egg what what.

You could park a car on that. Not at speed mind you.

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