Zurn Z100-89 Roof Drain Usage…

The Zurn ZC100-89 overflow roof drain is a high quality cast iron roof drain that is installed as a “backup” to the main roof drain. 

The overflow roof drain will go into service if the primary or main roof drain become blocked with debris. This “failsafe” mechanism can prevent rainwater from ponding on the flat roof. It can literally prevent the flat roof from collapsing under the stress caused by the excessive load of standing rainwater. It is a plumbing code requirement for this reason. 

The Zurn ZC100-89 overflow roof drain is used in conjunction with a primary or main roof drain for new installation applications or it used individually if you are replacing an existing unit. 

It is not generally used with a single deck plate like it is shown in the first picture above, rather it is mounted in a large double deck plate which includes the primary and overflow drains together in one unit. It can be used with a single deck plate, however deck plates would need to be used for both of the roof drains. This generally occurs when when installation space becomes an issue 

The deck plates are not engineered to lower the roof drains and so both of these drains are installed at deck level. This system works fine as long as the roof covering that surrounds the plate is thick enough to allow the drains to remain lower then the roof material. The deck plate is furnished by the drain manufacturer.   SEE HERE

ROOF DRAIN SUMP PANS  Where roof coverings or membranes are thin, lowering or “sumping” roof drains is recommended. In this case a double sump pan should be used which lowers the drains two inches below the roof deck. See here.   

Further, additional water dams can be double or triple stacked on the roof drain easily by adding several water dams on top of each other to raise the overflow dam above the roof surface. This is highly useful where a spray foam roof is being used or has been used.  SEE HERE

In both cases, the objective is to create a positive flow to the drains.