API 650 - Floating roofs - Emergency roof drains

[Wilbri]

Refererring to API 650 - Para. C.3.8.2 Emergency Roof Drains. The following is stated:

"Double-deck roofs shall have a minimum of three open-ended emergency roof drains designed to provide drainage to prevent
sinking the roof during severe rainfall events. Emergency drains are prohibited on single-deck floating roofs. Elevation of the
emergency overflow drains shall be such that the outer rim cannot be completely submerged. These drains shall discharge at
least 300 mm (1 ft) below the bottom of the roof and shall consist of open-ended pipes, braced as necessary to the roof structure.
The drains shall be sized to handle the rainfall specified by the Purchaser, with a minimum diameter of NPS 4. The drains
shall be sealed with a slit fabric seal or similar device that covers at least 90% of the opening that will reduce the productexposed
surfaces while permitting rainwater passage. The drains shall be fabricated from Schedule 80 pipe, or heavier, and fittings
with 6 mm (1/4-in.) thick roof deck reinforcing plates."

Can anyone kindly provide me with a reason as to why emergency roof drains are prohibited on single deck floating roofs?

[linus]

Central decks of Double Deck roofs are very stiff and do not deflect. In case of single deck the flexible central deck membrane deflects under the rainwater load. This could cause backflow of contents on to the roof if the head over the emergency drain is less than upward thrust of contents in deflected shape.

Regards,

[linus]

Central decks of Double Deck roofs are very stiff and do not deflect. In case of single deck the flexible central deck membrane deflects under the rainwater load. This could cause backflow of contents on to the roof if the head over the emergency drain is less than upward thrust of contents in deflected shape.

Regards,