I need to build a small timber (spruce) balcony that relies on support brackets. Beam strength calculations cause me a lot of confusion and inconfidence. So my question is, how thick should the structural beams/joists be, considering that the balcony is 900 mm x 1800 mm (2'9" x 5'9") in size?

The elements in question are marked as a, b, c and d on the drawing.

Should the diagonal brace (a) be thicker than the beam it supports (b), given that the diagonal carries the load of everything above it? Or is it better vice versa?

Also, which structure would be safer and more economic: the one labeled "Option 1", with a single, horizontal 1800 mm main beam (c) that relies on the brackets? Or the one below, labeled "Option 2", with several vertical 900 mm joists that do not rely on the side brackets? (I would opt for "Option 1", but I'm not quite sure why.)

Structure for balcony with diagonal braces

The structure will be covered with patio floor boards (28 mm / 1.1 inch thick, probably Siberian larch). The balcony will also have a wooden railing and a stair from the side. It is almost exclusively used as an entry to the 2nd floor of the house, so it won't receive much constant "human load", nor will it have any furniture or other heavy items. I live in Eastern/Souhtern Europe, so snow load needs to be taken into account, though.

Many thanks for helping out!

  • 1
    You really need to have this structural assembly properly designed by a qualified architect or engineer. Depending on where you are located the local building authority may even require that before giving you a building permit.
    – jwh20
    Commented Jul 26, 2021 at 14:10
  • 2
    Can 'a' be connected to the cornder 'b&d'? What is the vertical between a & b connected to (call it e)? Can e transfer load down into foundation? What does the ledger 'L' (for all the 'c') connect to? Your load calculation is based on local codes, what is the required live load? (It doesn't matter that you believe it won't receive much load).
    – P2000
    Commented Jul 26, 2021 at 14:42
  • 3
    Beam size is the least of your worries. here. You can't really construct a balcony as you've drawn it without structural steel connections to the wall (and a wall that can handle large lateral loads). Most such balconies are cantilevered, not fastened to a wall like a shelf.
    – isherwood
    Commented Jul 26, 2021 at 15:45
  • 4
    I’m voting to close this question because the OP needs to contact a structural engineer.
    – Solar Mike
    Commented Jul 26, 2021 at 15:46
  • 1
    I think "consult a structural engineer" is good advice (and possibly the best ultimate outcome here), but I see no reason to close the question because of that. This is not an uncommon framing question, and I'd welcome more insights and debate from other writers and the OP.
    – P2000
    Commented Jul 26, 2021 at 17:26

1 Answer 1


Both comments from @jwh20 and @P2000 are well pointed, I am only to provide my view on what you are expecting.

For a small balcony like this, either option has its advantages and disadvantages, but personally, I prefer option 2, as it is more geometrically/dimension stable, thus has a higher built-in safety factor.

In the balcony, the 5'9" beams must be sized to support the load yet keep the deflection in check. For this case, you are expecting a structurally graded 3"x10" or 3"x12" lumber for a uniform live load of 40 psf. The diagonals can be studs of the same grade ranged from 3"X4" to 3"x6", depending on the angle of inclination and connection details.

However, the strength of the supporting columns is a big concern here, as you are introducing new loadings into the columns that are not designed for (note that on top of the dead load and live load, wind, snow, earthquake all affect the design). Also, proper connection affects the stability and durability of this balcony. Thus, as suggested, you shall consult a structural engineer for this project, or at least, have the engineer go over your design.

  • 1
    Agree, "consult a structural engineer", because the engineer will do a site visit and consider all structural conditions that are not apparent to us or the OP
    – P2000
    Commented Jul 26, 2021 at 17:20

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