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Here is the design idea I have in mind and I want to avoid putting a post at the end. I'm planning to lay down a lightweight flat rain roof on top of the boards - roughly 20 lbs.

I thought of going with a knee braces may be? But how do I go about the math here? Do I do 45 degree cuts?

My main concern is the middle board as there is nowhere to do a knee brace. Potentially re-enforce it with some kind of a metal part?

Extra details:

  • everything is wood-based, posts are pressured treated redwood/douglas, horizontal boards are likely a cedar (something very light)

  • the main structure is attached to a wall (house)

  • vertical posts are 4x4

  • horizontal boards are 2x4 or likely something smaller - it simply needs to support a lightweight flat rain roof

  • I don't have a joinery details as that's something I'm trying to figure out here

Thanks.

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    What type of material are you using - solid wood (what kind?), plywood, engineered laminated veneer lumber... and what are the dimensions of each piece? What are the joinery details? Is this a free-standing structure, or is it anchored to a wall or into the ground somehow? All these details will be very helpful.
    – MattDMo
    Jul 8, 2022 at 18:22
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    Put a slight angle on the roof to control run-off rather than leaving it to chance. That might be an opportunity to put a "rafter" kind of arrangement to introduce any required strength.
    – gnicko
    Jul 9, 2022 at 3:16
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    Welcome to StackExchange. Since it's not actually the case that this only needs to support a lightweight rain roof (you need to think about wind loading) knee braces or corbels would be a good idea for the outlying two horizontals, but as you say you can't do one for the centre. So I think it will be important to tie this into the two that are, since they are supported; this way you turn it into a single mutually supporting structure. Re. how to do the bracing, don't reinvent the wheel here — find a design and simply copy it.
    – Graphus
    Jul 9, 2022 at 6:21
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    Tell us about the house behind. Do you have wall surface a couple of feet above, or is the top of your new thing pretty much at the top of the wall? (I'm thinking of wire tying back to the house above, instead of brackets below.) Jul 9, 2022 at 18:20
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    55" is a long cantilever. I'd say you need a knee brace, and a to tie the center cantilever to the two ends with a front rail. You also don't say how you intend to secure the posts, but unless they are attached firmly to something solid, they're going to move from the leverage of the cantilever. That is, if they are just posts in the ground, they won't remain vertical for long. Also, you don't give your location. Do you have to worry about snow load, or leaves accumulating on the roof? Jul 9, 2022 at 22:37

2 Answers 2

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I thought of going with a knee braces may be?

Since it's not actually the case that this only needs to support a lightweight rain roof (you also need to think about wind loading at least1) knee braces or corbels would be a good idea for the outlying two horizontals2.

But as you say you can't do one for the centre, so I think it will be important to tie this central one into the two that will get supports; this way you turn it into a single mutually supporting structure.

As an alternative to tying the central one into the two outliers, since it now seems this backs onto a wall, you might be able to add support from the wall above. Options include adding a wooden strut (sort of like a knee brace in reverse), using chain, or cable threaded through eye bolts. I would be tempted to use two diagonal cables here to help resist racking.

I don't have a joinery details as that's something I'm trying to figure out here

You have to use joinery or any of the (many and varied) designs of metal plates/hangers that are available on the market. Although this isn't actually a binary choice, you could use both systems together. Re. knee braces or corbels don't attempt to reinvent the wheel — find a good design and simply copy it.

Also re. joinery, if you choose any sort of lap joint (where part or all of the 'joists' lie on top of the lintel/beam) remember to factor in that you gain or lose a little height as a result.

With metal hangers, it's tempting to use the minimum recommended number of fasteners to attach these sorts of things, something that is apparently very commonly seen in commercial construction3. I strongly recommend resisting the urge to save a couple bucks by using fewer screws! Given the design you've chosen and the potential for weather to stress the structure significantly, you do not want to under-build,


1 Possibly also wet leaves and/or snow load if relevant to your location.

2 Note this is still the case even if you do proper old-school joinery to attach them, like a pegged mortise-and-tenon joint.

3 Even when they use nails which literally cost pennies!

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I'm planning to lay down a lightweight flat rain roof on top of the boards

If you add some pitch to the roof you can use the rafters for support instead of adding bracing below, keeping the entire area under the roof open, and the roof will also do a better job shedding water away from the house.

Let's call the vertical members studs and the horizontal ones joists to correspond with parts of a building. If you extend the studs up above the joists, you can connect the tops of the studs to the far ends of the joists with rafters which will support the roof, creating triangles that will be quite strong.

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