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Old 01-27-2019, 10:25 AM
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Has anyone done a cantilever addition? If so, can give me some idea of what it cost just for the structure? I think this may be a good solution to an issue I have with a small bedroom. From what I can see, I could gain a few feet and that would make a world of difference in small bedroom that is only 9 x 13. I would just bump out the back bedroom at the end of the house. The height from the ground is about 30''. I would sister new joists/beams to the originals that are running in that same direction to create the bump out. No plumbing or heating is involved, would just have to wire for electrical outlets. I would reuse an semi new existing window and add another on the other end.

The roof eve/overhang is 18", I would even consider just bumping out to the roofline and that still, would give us the room we need.


Last edited by NYTom; 01-27-2019 at 10:46 AM..
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Old 01-27-2019, 10:40 AM
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I have never done a cantilever addition but have lived in two houses in which one of the former owners did so (the house I'm in now is one of them; they added approximately 2 ft of interior space to one bedroom.)

The most important thing to make sure the contractor does is to properly insulate the cantilever. Most cantilevers are inadequately insulated which can easily result in the room being colder in winter, even if the cantilevered wall includes a baseboard. As part of the re-siding job on this house last year I told the contractor to open up the cantilever and insulate it according to the diagram shown at this link. Click to zoom and print out and give to the contractor:


The information at the "Description" tab is helpful too.

One thing I would add is to make sure you are using solid soffit covers (not ventilated) on the underside of the cantilever where "soffit closure" is indicated. Unlike your roof eaves, where the more ventilation the better, you want the opposite here.

In previous winters the room with the cantilever was always 4 or 5 degrees colder than the rest of the house. The cantilever was re-insulated as per this diagram in early September. So far this winter, the room is the same temp as in the adjacent hallway where the thermostat is, except for those single-digit nights when it was 2 degrees colder. But that room is also on an outside corner which accounts for that.

ETA: If you are bumping out to the existing roofline: Depending on what style of roof you currently have there, make sure the contractor makes proper provision for adequate fascia for the new gutters. The former owners of my house didn't, and as a result there wasn't enough of a drip edge. The siding contractor had to retrofit new fascia all along that section of roof because although the prior contractor had stuck the gutter back on above the cantilever, it was dumping a good amount of water down the inside of the wall rather than into the gutter itself. Unfortunately enough water did come out of the downspout to make it appear as if the gutter was working properly, but rotted-out sheathing and moldy fiberglass batts under the old siding proved differently. Lots of repair work was needed there.

Last edited by BBCjunkie; 01-27-2019 at 10:49 AM..
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Old 01-27-2019, 10:54 AM
4,333 posts, read 5,255,431 times
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Thank you, that is good info. The room is normally colder than others, I think because it's north facing and may not be properly insulated. I figured with the new wall I can get that part fixed.

You must spread some reputation around before giving it to BBCjunkie again.
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Old 01-28-2019, 09:03 AM
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I agree with BBCjunkie, properly insulating a bump out with bottom air exposure is almost impossible and there will be a tremendous heat loss. You would need to incorporate additional heating into that alteration and I’m guessing it would be both costly and perhaps not blend architecturally with the rest of your house.
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