Converting existing interior slab to exterior balcony 1

[steellion]

A building owner wants to explore creating balconies by removing the exterior cladding and re-cladding the facade about 10' inside the building perimeter. This would convert what was an interior slab to an exterior slab exposed to the elements. Columns are on the exterior, so the balconies would not be cantilevers.

My main concerns are not having enough concrete cover for an exterior condition, and freeze-thaw cycles (this is a moderate climate with annual freezes). The best I have come up with so far is a bonded topping slab with a "traffic coating" to provide cover and waterproofing, and some sort of liquid waterproofing membrane on the underside of the slab (since the bottom bars will be critical to the structure).

I saw in the other balcony thread that some engineers are using 3/4" cover with a protective membrane for balconies. I don't think I would do this for a new design, but for a retro-fit like this, it may help with the code requirements since it will be difficult to retroactively add cover.

 

[thaidavid40]

Aside from the detailing issues you mentioned, there is the issue of live load capacity. If this was a multi-family housing building, then the floors were likely designed for a 40psf live load. Now that this might become a balcony, the required live load increases to 60psf on the converted area. Accounting for that might blow the developer's budget for conversion.

Thaidavid

 

[dcarr82775]

It is not a balcony, it would be considered a deck. Balconies cantilever, decks do not. I would have no issue using the slab with the 3/4" cover and providing a weather barriers like a traffic coating or some other waterproofing membrane.

 

[Ron]

Steellion....your approach is good. Be aware that if the slab covers occupied space, the rules change a bit with regard to waterproofing....it becomes a roof as well as a loaded structural deck. You might also consider placing a restriction on occupancy in your plans and have the building department enforce it with an installed, prominent placard, assuming you have a live load reduction from current code requirements. Further, this might be a large enough renovation/change to constitute a change of building use and a cost of construction that might interject full current code compliance.

 

[hokie66]

External balconies should be set down from the interior. If you are going to top the external part, you will reverse this, and inevitably create water problems. If you have to do this, I would put all my effort into making sure the trafficable membrane is a good one.

Waterproofing the soffit of the slab is not a good idea, as that prevents water from escaping, not what you want.

 

[TXStructural]

I would echo Hokie on this. Do NOT trap moisture in the slab with a membrane on the bottom. If this is exposed to ocean air, you will need to take measures to avoid salt migration through the slab ON ALL SURFACES. If this is an inland location, attend to waterproofing the top and edges. If it is a very humid location, you will need to deal with condensation on the underside.

Also, concrete quality will determine how durable this ends up being. For instance, if the slab is hard troweled, you may have a problem with freeze-thaw durability.