How To – Connecting Cast Iron And PVC Waste Line

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    Whether you are replacing damaged pipe in your home or you want to make use of Cast Iron and PVC pipe to reduce noise there will come a time when you need to join both materials.

    Only a few years ago this would have meant using a special PVC pipe and lead to seal the joint but today there are less toxic materials and methods that you can use to create a tight and dependable seal between your Cast Iron Pipe and PVC Waste Line.

    The first thing to remember is that water runs down hill and when it does it will always look for the easiest route. This means that when flowing down hill the top pipe should be a male fitting and the bottom pipe should be a female fitting. This way even if the seal was to fail the basic shape of the connection will allow the water to flow.

    This same idea is used when applying siding on the walls or shingles on the roof of your house. You want the mechanical shape of the connection to be the primary way water is persuaded to flow and then use a sealer to aid in the connection.

    So now you understand that a vertical stack pipe made of cast iron should be connected into your basement waste line by having a larger PVC connection in the basement that the cast iron pipe will fit into.  If necessary you may need to use a larger piece of pvc with a bushing in that location to surround the cast iron pipe with the pvc pipe at the connection.

    Once you have made the physical connection you will need to seal the joint. Years ago you would have used liquid lead around the joint but today you can use a compound like soil seal which is kinda like an industrial grade joint compound.  This will seal the connection fairly well by filling in the area around the joint where the cast iron meets the pvc.

    In addition you should follow up the sealer by installing a rubber coupling. These couplings must be placed on the pipe before you make the physical connection and then forced down over the joint and held in place with a stainless steel hose clamp.

    Since there is no real pressure in a waste line the rubber coupler connection could be used without the compound sealer but if you are working in a living space it is best to use both products to reduce gas leaks and increase the life of the joint.

    If possible you should secure the pipe to studs on both sides of the connection to reduce flexing. Metal strapping is preferred especially on the cast iron pipe. Use more strapping then you would normally to reduce problems due to settling.

    That is pretty much it.

    As a final note you should inspect these connections more often then you would standard pvc to pvc pipe connections. Eventually over time the materials will break down but because the rubber is not in direct sunlight you should have many years before it degrades.

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