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RE: Gas Tight Seal

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Yes, you will need to have a gas tight seal at the perimeter.  The concrete
roof will move on top of the walls.  You probably will have to seal the
bottom of the concrete deck as well.  Check to see what the potential and
the concentration is for hydrogen sulfides.  

A cast in place concrete roof will probably not be tight enough to be gas
tight.  Generally these decks require a flexible seal at the perimeter and a
plastic or an elastomeric coating on the under side of the roof.  The under
side of the roof can be done in T-Lock or a plural applied  elastomeric
polyurethane system or a hydrocarbon-modified urethane.  Specify systems
that are specifically used for waste water treatment with the chemical
exposures this particular plant will have.  Check out Polibrid, Armatec, and

I have had to repair and replace concrete domed roofs due to gas leaks and
hydrogen sulfide attack.  There are also some metal roof manufacturers who
specialize in digester roofs.  Look them up on the AWWA web site.  Crom is
one that comes to mind.

You will also have to protect the walls and column for hydrogen sulfide
attack.  You will have to use a spray applied elastomeric for the existing
concrete down to about 2 feet below the minimum operational water line.
Remember that now you will be trapping and concentrating gasses that
previously escaped.

Harold Sprague, P.E.
The Neenan Company

-----Original Message-----
From: Randy Russ [mailto:rruss(--nospam--at)]
Sent: Wednesday, October 21, 1998 8:02 PM
To: Structural Chat
Subject: Gas Tight Seal

I am putting a concrete roof on an existing wastewater digester. I am using
a center column for the roof and am simply resting the slab edge on the top
of the esisting wall on a bearing pad. 

I am concerned with gas escaping. Do I need to provide any type of gas
tight seal or is the weight of the 18 inch roof slab enough.Has anyone
encountered this situation?

Randy Russ, P.E.