Drainage Design for Re-Roofs in Florida

[BuildingFlorida]

Hello.

For reroof projects on flat roofs, does the Florida Building Code require redesign of the drainage system...drains and scuppers. I cannot find a specific code section stating so.

This project is in Miami Dade County so HVHZ pertains. Section 1514.4.2.2 mentions reroofing, but it only refers to reduction of drains and scuppers.

I'm not certain whether the IBC has different requirements.?

Or...is this a situation where the building official makes the call?

Thanks in advance for your assistance.

 

[brokenkeys]

The reroofs I've done included drainage review/ modification in the scope so I don't recall the requirement triggers but quickly scanning the existing building code (706) it has the same language as 1511. My quick-and-dirty reading of that is, unless exempted otherwise, FBC-B 1511.1 & FBC-EB 706.1 state you have to comply with Chapter 15. 1514.4 says you have to drain it to prevent collapse (ASCE 7 & 1611, which also refer to FM Global 1-54 [which is a great resource by the way]).

The good news is you're in an HVHZ which has a higher roof load (assuming it was built to HVHZ standards) which means it can support more water/ larger drains, which helps address issues with pre-2017 code designs (which had a habit of being under-drained).

 

[brokenkeys]

Minor correction to my above notes; HVHZ reroofs are in 1521 (1511 is outside HVHZ). I've only quickly glanced at it, but I don't see most of the exceptions included in 1511, so I'll go ahead and say you need to review the drainage (instead of my previous implications that maybe, possibly it wasn't required).

 

[BuildingFlorida]

Thanks for the comments.

I spoke with a building official colleague about this. He said that there is no requirement in the FBC that says a redesign is needed with the drains and scuppers as long as the system is operating properly. Not operating means that the drains or overflow scuppers are clogged or damaged and not draining.

 

[BuildingFlorida]

I forgot to mention...

I agree that the intent is to prevent collapse, no matter what the codes say.

Regarding this, the bottom of the overflow scupper location comes into question.

1514.4.2 Overflow Drains and Scuppers
Where roof drains are required, overflow drains or overflow scuppers sized in accordance with Florida Building Code, Plumbing and ASCE 7, Chapter 8 with commentary shall be installed with the inlet flow line located not less than 2 inches (51 mm) or more than 4 inches (102 mm) above the low point of the finished roofing surface, excluding sumps. Overflow scuppers shall be a minimum of 4 inches (102 mm) in any dimension and shall be located as close as practical to required vertical leaders, conductors or downspouts. Overflow drains and scuppers shall also comply with the Florida Building Code, Plumbing, and Section 1611 of this code.

For many older buildings in Florida, the roof live load is about 30 psf and this corresponds to about 6 inches of water.

If the intent is no collapse, then the bottom of the scupper should be a maximum of 6" off the structural system (i.e slab, deck, etc.).

For example, if there is a minimum of 7" of LWIC for R19, then the code has the bottom of the scupper at 9"... 7" + 2".

So, if the roofing system fails, then the system is holding 9" of water instead of 6".

So...another question...does this section assume that the roofing does not fail?

If it does fail, then the roof will hold more than it's likely designed to handle.

It seems that the bottom of the scupper should be set to the maximum amount of water that the roof can hold, to account for a roofing system failure.

 

[ICE]

It seems that the bottom of the scupper should be set to the maximum amount of water that the roof can hold, to account for a roofing system failure.

A safety factor sounds better than reaching the maximum. Taken to the extreme, the roof structure might be able to hold up fifteen inches of water....what then? EPS is not likely to hold much water, but the LWIC EPS could have a drainage system built in.

 

[brokenkeys]

The structural sections don't take into account head height, which is used to determine the drain/ scupper flow rate.

If, for example, the bottom of your overflow is at 2" above roof, then you only have 2" of head height for the primary drain and 4" head height for the overflow. Code says primary and overflow have to be separate. Primary and overflow scupper openings are obviously separate openings, however one jurisdiction I work in interprets that as the bottom elevation of the overflow has to be 0.5" above the primary head height (per their interpretation 0.5" is the minimum to accommodate construction tolerances, unless you can prove otherwise, which is essentially impossible). So, in that jurisdiction, if the primary head height is still at 2", add 0.5" of separation and the overflow head height is cut down to 3.5". Dropping from 4" to 3.5" head height on a 16" scupper reduces the GPM by roughly 70, which is probably a non-issue in this unrealistic example, but depending on the project, maybe it is.

Conversely, if your overflow is 4" above roof then you have 4" of head for the primary but only 2" left for your overflow before it gets too heavy for the roof to support. 2" of head might not provide enough flow, requiring additional overflow drains.

Outside the HVHZ you can go down to 20 PSF, which equates to about 4" of ponding, which doesn't leave much room for head heights, so you need more and/or wider drains. Many of the older roofs I've run across were designed around the tables in the pre-2017 plumbing code but skipped the structural part of roof drainage (or weren't designed based on anything at all) and additional drains had to be added and/or existing scuppers had to be widened.

As I mentioned FM Global 1-54 outlines all this and it's a much easier read than a typical code book.