By placing a scupper 2"- 4" above the roof surface at the lowest point near the existing roof drain
A clogged drain most likely.
Both and but normally a master plumber would fix the problem by removing the existing drain lead/copper /cast iron and install a new drain and remove the membrane to the existing slab water proof the slab with either roof cement or flashing cement then install 4 PSF sheet lead over the drain flange/clamping gravel stop collar and you will now have a leak proof job
They are secondary drains that work in tandem with the primary pipes to prevent overflows.
A mansard roof is difficult to install due to its shape and structure. What makes a mansard roof so difficult to install is that there are 4 sides all of which have different slopes.
SMD
Take your snake on the roof and clean the drain from the vent.
To install the Asphalt Arc30 year roof depends with the materials that you will use.
A scupper allows water to exit the roof, usually into a gutter drain.
Then rain falls on a roof and if the roof works properly the rain will drain off of the structure.
One can install roof flashing by various steps. You can find these steps on several websites. A website you can look on for installing roof flashing is YouTube.
Metered roof drains are installed on flat roof systems. Their primary purpose is to allow a small amount of water to drain through them, and allow for water to pool on a roof top. This pooling allows for the sewage treatment plants not to be overwhelmed by the rain water. The metered roof drains must be spaced 50 feet from the roof edge and 100 feet from another metered roof drain. Each metered roof drain has 4 weirs built into its body. By removing weirs you can control the amount of water that will drain. The more weirs removed the faster the roof will drain. The roof must be engineered to hold the weight of the pooling water. More information can be found on the web sites from the manufacturers of metered roof drains. Zurn, Mifab etc.
Install the sill flashing first, according to the manufacturer's instructions. As a rule, flashings are nailed into the curb and sometimes cemented to the roof with plastic roof cement, but not nailed to the roof.