HydroMax® Design
Piping Layouts – Factors to consider
1. Roofs at different elevations may be affected by wind driven rain
There is a risk that Roofs A and C will be affected by wind driven rain. This will contribute more flow to roof A because of vertical surface run-off and provide less flow to Roof C.
In this rainfall event, if Roof B and C were connected, Roof C would suffer air ingress and disrupt flow from Roof B.
The design must incorporate separate piping systems for each roof which may be affected by wind driven rain.
2. Roofs with different surface types should be drained on separate siphonic piping systems as run-off coefficients will be different. (Refer to ASPE/ANSI 45 Siphonic Roof Drainage Design Standard clause 6.6.2)
TPO or PVC single ply membranes will provide speedy run-off to the Mifab HydroMax® siphonic roof drains.
Gravel ballast roofs will have relatively slow flow as water has to filter through the stones.
Green roofs should in most cases retain water.
3. Lengths of horizontal runs
For concept design, an n approximate rule-of thumb for possible length of horizontal run relative to total height from roof to discharge is a ratio of 1 unit of height = 12 to 14 units of Horizontal run. This is an approximation and many factors come into the hydraulic calculation with number of bends etc. affecting this ratio.
Siphon Breaks
The siphonic system needs to end and transition to gravity flow or discharge to the atmosphere.
All siphon breaks on primary systems require an increase to gravity sized pipe and re-introduce air. Depending on overflow discharge location, you may want to control velocity by reverting back to gravity sized pipes.
Note: If discharge velocity is not considered an issue (flexible routing allows control of discharge location) it is possible to discharge overflow without increasing to a gravity sized pipe. Please consult with your local HydroMax® design partner for further information.
