Restore a Tiled Dome

[JAE]

I posted this in the Structural Engineers Forum as well:

We have a potential project dealing with a number of historical buildings. One building includes a dome approximately 20 feet in diameter on the roof and is constructed of:

1. A bottom concrete ring support.
2. Vertical steel strap channels which run from the concrete base ring upward on an arc to a top concrete compression ring which is approximately 5 feet in diameter.
3. Horizontal steel strap channels running around the perimeter of the dome, wired to the vertical straps and spaced about 2 feet o.c.
4. Metal lath wired to the horizontal straps.
5. A concrete base about 2 to 3 inches thick which is applied to the lath and bonded well.
6. An exterior finish made up of small ceramic tiles about 1/2" square and set in a mortar bed on the concrete base.

The issue is that the dome leaks. The metal straps all appear to be only slightly rusted (surface scale) and do not appear to be a big issue. However, some verical cracks up the dome face are apparently in line with the vertical straps.

A lot of micro cracking around the tile is also visible. The metal lath and concrete base appear to be in good condition. There is no visual deformation of the dome, just the nuisance of water getting in.

The tile cannot be removed as it is of historical significance. The owners (who have a lot of maintenance knowlege) had mentioned a product that they used on a glass/concrete pit cover. It was a very low viscosity rubber-based waterproofing sealant that worked great and apparently doesn't discolor.

QUESTION:

Would a sealant over the tile make sense? I've learned over the years that its not always wise to seal masonry, but ceramic tile? The inside of the dome can be vented or air conditioned if necessary. The rubber product is said to be able to bridge 1/16" cracks, depending on the thickness of application. It also isn't supposed to discolor.

If a sealant isn't a good idea, what else can be done? The fear is that water leaking through the substrate of the dome would rust the straps, deteriorate the concrete base, and weaken the dome. The interior of the dome is not a finished/accessible space so we could waterproof the floor inside and drain, but you still weaken the dome.

The building is in a mild environment (i.e. no severe winters) but does get some freezing.

Any ideas/thoughts?

 

[MPHenry]

JAE,

You say that the ceramic tiles cannot be removed. But can they be covered up? If a sealant is used must it be clear/transparent so that the visual tile appearance can still be seen?

If the tiles can be covered over completely then I would be looking at either a mechanically fixed EPDM rubber membrane system or applying a liquid applied rubber sealant such as Thortex Poly-Tech or equivalent. The mechanically fixed EPDM option would be the best as the fixing method would be directly into the concrete sub-base of the dome and would not depend on the current fixity of the tiles. A white on black EPDM membrane would be the most aesthetic.

Regards

Mike B-)

 

[JAE]

Thanks for your response, Mike.

No, the tiles cannot be covered up as they are very historical and an important aesthetic element of the building.

 

[MPHenry]

JAE,

I suspected that this was the case. The only thing I can think of other than a surface sealant is to try crack injection using a hyrophobic sealant. The injection process could take place from inside the dome in order to reduce the risk of damage to the tiled outer surface. Maybe a quite word with a Crack Injection Specialist Contractor might yield some fruitful results.

Regards

Mike B-)

 

[JAE]

Hey, thanks,

Just a question about the crack injection with hyrophobic (is it hydrophobic?) sealant. Does this stuff work with plaster. Apparently, we found out that the "concrete" attached to the metal lath is really a plaster.