OK, we’re finally ready to get quotes on the exterior spiral staircase that’s going from the upstairs balcony to the roof-top deck.  The balconly was engineered with a giant beam that spans the house and cantelevers out right under where the center of the spiral should land.  There’s also additional 4X blocking and strapping right where the center post of the stairs would hit the balcony, so structural support is not the concern.

I’m really worried about water-proofing the connection point at the balcony.  First, the balcony and deck material is a built-up system of diamond lath, latex-modified mortor, fiberglass mat, resin, troweled-on texture and then a color coat- they call it “Ultra-Flex 2000”. 

What’s the best way to seal the spiral’s base plate to the balcony surface?  I’m assuming I’m going to drill through the deck for the lag bolts, so that’s where I’m concerned.  I’m affraid that even if I used the best marine sealant (3M 5200) it would eventually dry out and the tiniest crack would allow water penetration.

Would it be possible to not fasten it at the bottom and just set it in a bed of sealant and let the weight of the stair do the work?  Of course it would be fastened at the top though.

Any thoughts or experience?

Thanks!