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I'm a long time follower of this forum and enjoy reading the great advice provided by everyone. I renovate commercial properties and lease them out and recently encountered a roofing issue that I have not seen addressed in prior threads so I thought I'd see if anyone here has insight on the issue.

I'm renovating an 18,000 sq ft warehouse in South Texas with a 4 inch flat concrete deck with about an inch of coal tar covered in 1/2 inch gravel. Despite being 70 years old, the roof appears to be in pretty good condition and only leaks in a couple spots where there are vent penetrations and where bolts penetrate for HVAC pads on the roof.

That said, I'd like to add an extra water-proofing layer to extend the roof's life. In a few years there is a good chance a second floor will be built over the flat roof, so I don't want to do a complete re-roof only to have to tear it off for the new construction.

I've researched a lot of coatings for the roof and a good solution appears to be a 100% silicone coating. A few of the products that seem like a good fit are Gaco S-20, Eterna-Kota, and Henry Tropicool coatings.

I've read that to prep the roof I should remove the gravel, power-wash the hardened coal tar, and then apply the coating. Gaco suggests applying their epoxy primer first, then the silicone. Eterna-Kote and Tropicool do not mention applying a primer and seem to allow for direct application of the silicone. I plan to do an adhesion test as a preliminary measure.

My question is if this approach appears solid. Are there any obvious issues with applying the silicone directly to the coal tar? Can the silicone withstand the occasional hail storms we get without the ballast as a protective layer?

There are some areas along the perimeter of the roof where water ponds in the rare days we get a lot of rain, so I'm interested in how well the silicone withstands the ponding. The literature for all the products guarantee against leaks from the ponding.

Anyhow, I'm grateful for any thoughts, suggestions, and advice.
 

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silicone over coal tar

the tar doesn't actually hold together to itself so I don't see how a primer would help. An adhesion test is a good idea.

It is difficult to get good coverage because of the rough surface. It takes a lot of product. If you spud the roof to a smooth surface that takes a lot of work too.

If you do coat it I suggest two coats or better anywhere water ponds. Should do fine. EternaKote is cheaper in this area than GACO with the same warranty.
 

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What to do

******* the gravel off ,this will wash the roof while it removes gravel. Then flood the roof with emulsion, them lay in polyester, and let it cure good , really dry, then power wash lightly again.
Then top coat with silicone. That will be a good roof.
Be fussy with who’s product you use, I would never use henrys.
Hope this helps, I am contractor in cal.
 
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