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Flat Roof Drainage

bpemberton's picture


I was looking to get some opinions on flat roof drainage systems The roof will be covered with a white PVC membrane from IB Roof Systems.

I am looking for the greenest system available to me.

Is siphonic drainage applicable on a residental home with approximately 3500sqft of roof? or is that strictly only for large scale commerical buildings? Haven't been able to find much information on it on the web.

Siphonic Drainage (post #190731, reply #1 of 6)


You've made a good start for a sustainable roof by choosing IB Roof Systems, a carbon neutral company. It is among the longest lasting roofing systems on the market and has been offering residential lifetime warranties for over thirty years.

To answer your question on siphonic drain systems

Siphonic Drain systems were designed for roofs 10,000sf and up. Roofs smaller than that should be able to drain roof water effectively by following local building codes. It is always a good idea to get as much water off the roof as possible there may be some standing water on lower-sloped roofs. Ponding water on IB Roof Systems membrane, however, does not effect the performance or the lifetime residential warranty.

To find out more about IB Roof Systems Green initiatives go to . I have attached thier green overview as well.

IB_green_overview.pdf593.29 KB

Thank you for responding. (post #190731, reply #2 of 6)

It's nice to see suppliers and manufacturers provide information when called upon.

A Great Place for Information, Comraderie, and a Sucker Punch.

Remodeling Contractor just outside the Glass City.


flat roof drainage (post #190731, reply #3 of 6)


Flat roofs are the standard in residential construction where I live.  it is not necessary to have any elaborate system of drainage. We don't, in residential or commercial.

 The successful draining of water depends upon how well the roof was designed.  I presumed the house has already been constructed.  The biggest mistake is not having enough of a roof slope, even though the roof is supposedly flat.  And the second mistake is the direction of the drainage.  the third issue is where the water will end up.

In homes here, the rainwater is directed into horizontal spouts around the perimeter of the roof, through the use of proper roof pitch and inclined planes against horizontal surfaces.    

If your roof is not pitched correctly and the water does not flow where it can be collected, then the roofers, could add crickets or other inclined planes,  to properly direct the water.  I do it all the time when I must install a new material on a flat roof.  

Then I direct the water to the perimeter of the roof, where it flows into horizontal spouts.  The water spills on the ground, as standard practice here.  But you could collect that water, through downspouts.  I have even seen decorative metal chains attached to the spouts and then the water gently flows down the chain to the garden below.

Since your emphasis is on building green, then collect the water and direct it to some use, like a garden.









White is poison to a picture!  Use it only in highlights.

Peter Paul Rubens. 1577-1640.

I would add: don't use white in architecture either, except rarely and sparingly.

Playing Devil's advocate (post #190731, reply #4 of 6)

here a little, but what's "green" about 3500 sf home? The whole concept of sustainable design is one, to get off of the McMansion kick and start building homes that are reasonable and efficient. If you have a family of 12 and plan for 8 bedrooms, then this might be reasonable.

Not wasting resources is infinitely more friendly to the environment than "greenwashing" the same wasteful excessiveness that has become so popular and ubiquitous in recent decades.

Although flat roof drainage (post #190731, reply #5 of 6)

Although flat roof drainage is taken care of by the builder and rain water quickly and effectively into gutters if it does not then a few steps need to be performed. Rectify the possible inaccuracies in the construction and use good waterproofing materials. Consider using a better design fall that is steeper which can be created with the help of sloping decks, firrings, pre formed insulation boards and concrete and screeds.

Flat roof drainage (post #190731, reply #6 of 6)

Your profile say's near the Mexico border.  Which side?

Is this an existion structure?  Is the roof truly flat-zero pitch?  Is this Adobe style architecture? Are there parapets around the roof area?  Is the drainage system by canales?

Most codes today require a min. of 1/4" per foot pitch.  As has been previously stated, for proper drainage the roof should be pitched.  This can be properly  accomplished by using tapered rigid insulation.