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We have a 1st floor cantilevered balcony using 2x 225mm glulam beams, any ideas for water/weather proofing the beams?

Gill Wills | Posted in General Questions on

The house is being built using 180mm sips with a block/stone and cedar cladding outer. Triple glazing, wood pellet boiler, solar thermal andMVHR. We aim to be as energy efficient as poss.

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  1. Nick Zees | | #1

    How far do they protrude out and how far in? 2x225mm? Is that 2x12 (") nominal (1.5x11.25") How are they anchored? Openings sealed?

    Why haven't you used treated lumber?

    Is it old or new install?

  2. James Morgan | | #2

    It's not easy to construct a cantilevered balcony without making a thermal bridge, also it's common for the cantilever to be a source of water leaks to the interior. The first problem can sometimes be addressed by wrapping the inside of the beams with foam insulation for a foot or two. The safest way to protect against the water leaks is to make the balcony deck into an overhanging roof, this will also protect the protruding beams themselves from water damage. Even so I hope you specified waterproof glue.

  3. David Meiland | | #3

    As far as I know, the only good way to waterproof cantilevered wood beams is to have custom sheet metal boots made for them. Each boot has a wall flange that is integrated with the WRB, and then has sheet metal that protrudes from the wall through the cladding to cover the top of the beam, and partway down the sides. You need to design and fabricate them carefully and then make sure they are installed at the right point in the sequence.

    Frankly, if you can avoid this type of detail, do so.

  4. GBA Editor
    Martin Holladay | | #4

    You wrote, "We aim to be as energy efficient as possible." If that's true, then you shouldn't have any cantilevered beams supporting an exterior balcony. It's a really bad idea to insert big deliberate thermal bridges through your thermal envelope.

    Moreover, almost all attempts to jury-rig some type of flashing detail to prevent water penetration results in failure and eventual rot. You can get beam rot, or wall rot, or both. Experienced builders have seen both.

  5. Nick Zees | | #5

    One of the rules that have to be followed is that length of two-by remaining in the house (and properly attached to joists, assuming it's your case), has to be at least 2x as long as length sticking out of the house. A minimal outside pitch is advised too, so water doesn't just accumulate and creep into the house.

    I would say that anything other that PT lumber is not a good idea. You will need to waterproof the wood and keep doing that every few year, just like you would maintain non-PT lumber affected by outdoor elements.

    Encapsulating flashing (just on 3 sides, I wouldn't do the bottom) is not a bad idea, but then you are inviting moisture through screw holes and not letting it dry to the above.

    What kind of decking do you plan on the top of balcony?

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