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Roof Drain Flow Rate

Published flow rates for Froet Roof Drains

roof drain capacity calculation


Please Note:
The below flow testing results are for Froet Drains only.
Do not use this information with any other drain. Do not assume there are any similarities with any other drains. The flow testing results are to be use in combination with your existing piping calculations, these are not a replacement of the pipe flow results.

Another important point to understand is that you should only use the GPM values up to the very low negative pipe pressures (10ft Pipe Chart) not past -5. If you notice the drain only testing and the begining of negative pipe pressures in the 10 foot pipe chart correlate closely. As the negative pipe pressures begin to increase is when the siphonic pull is begining in the pipe and can lead to damaging pulsating vibrations in the piping system as it increases.

Froet feels that the drain only values are safe GPM's values to use. You will notice some anomalies in these charts that don't seem to make sense. Until a testing lab is able to test drains again we will not be able to test to try to correct these issues.

Froet Industries feels it is important to get this information to the enginering community so we have posted as it is. This should give you information to help you design your drainage systems correctly. If you have any questions in regard to this information please call Froet Industries.

Click here to use the Roof Drain GPM Calculator
(You must have Microsoft Excel installed to view)


Froet 3” Standard Primary Drain

DRAIN WITHOUT DRAIN PIPE

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

39

89

94

108

114

123

Velocity of flow (ft/s)

0.5

4.8

5.2

5.6

5.6

5.8

DRAIN WITH 10 FT PIPE

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

47

133

266

398

411

429

Velocity in drain pipe (ft/s)

3.6

7.3

7.8

10.1

10.3

10.7

Pressure in drain pipe (in. W.C)

-0.6

-3.3

-21.8

-39.3

-40.8

-41.3

DRAIN WITH MODIFIED PIPES

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

49

136

271

293

330

327

Velocity in 1st stack (ft/s)

2.5

3.1

3.7

4.2

6.5

7.4

Velocity in horizontal pipe (ft/s)

3.1

3.6

3.9

5.1

5.8

7.3

Velocity in 2nd stack (ft/s)

6.4

6.8

7.0

7.0

7.3

7.9

Pressure in 1st stack (in. W.C)

-1.1

-10.9

-28.4

-37.1

-39.1

-37.7

Pressure in horizontal pipe (in. W.C)

-1.6

-14.6

-37.2

-38.7

-48.5

-47.5

Pressure in 2nd stack (in. W.C)

-1.5

-18.8

-40.3

-43.6

-42.7

-42.1



Froet 4” Standard Primary Drain

DRAIN WITHOUT DRAIN PIPE

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

43

150

130

190

229

248

Velocity of flow (ft/s)

0.9

3.5

3.1

4.0

4.5

5.0

DRAIN WITH 10 FT PIPE

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

45

152

317

545

680

773

Velocity in drain pipe (ft/s)

1.5

4.6

8.5

8.6

8.9

9.3

Pressure in drain pipe (in. W.C)

0.3

-1.7

-12.7

-32.7

-40.8

-50.9

DRAIN WITH MODIFIED PIPES

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

78

142

285

503

545

611

Velocity in 1st stack (ft/s)

0.3

3.6

5.4

7.7

7.8

7.4

Velocity in horizontal pipe (ft/s)

2.8

3.3

6.2

8.7

9.2

8.9

Velocity in 2nd stack (ft/s)

5.0

6.5

6.7

7.2

7.7

7.7

Pressure in 1st stack (in. W.C)

0.8

-0.8

-18.3

-37.0

-39.6

-38.9

Pressure in horizontal pipe (in. W.C)

0.3

-5.2

-29.1

-56.4

-61.3

-61.2

Pressure in 2nd stack (in. W.C)

-0.4

-7.9

-33.9

-47.6

-50.2

-49.4



Froet 6” Standard Primary Drain

DRAIN WITHOUT DRAIN PIPE

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

133

228

371

537

427

551

Velocity of flow (ft/s)

0.4

0.5

1.6

3.2

2.4

3.0

DRAIN WITH 10 FT PIPE

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

153

156

347

534

803

1062

Velocity in drain pipe (ft/s)

0.4

2.1

5.3

6.8

7.2

7.3

Pressure in drain pipe (in. W.C)

1.2

1.0

-0.8

-5.8

-14.2

-31.9

DRAIN WITH MODIFIED PIPES

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

122

208

339

526

731

975

Velocity in 1st stack (ft/s)

0.5

0.9

1.7

3.3

5.5

7.5

Velocity in horizontal pipe (ft/s)

2.4

1.8

3.4

4.6

5.6

6.9

Velocity in 2nd stack (ft/s)

5.5

7.2

6.6

7.5

8.5

8.8

Pressure in 1st stack (in. W.C)

1.0

1.8

0.9

-1.5

-12.1

-31.5

Pressure in horizontal pipe (in. W.C)

-2.3

-2.1

-1.6

-9.2

-21.2

-44.0

Pressure in 2nd stack (in. W.C)

0.8

0.3

-2.4

-22.8

-41.1

-40.2



Froet 3” Bi-functional Drain

DRAIN WITHOUT DRAIN PIPE

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

120

168

225

212

244

313

Velocity of flow (ft/s)

4.3

4.0

4.5

4.4

5.1

5.3

DRAIN WITH 10 FT PIPE

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

105

220

137

157

261

522

Velocity in drain pipe (ft/s)

8.0

8.9

8.9

8.8

8.7

9.3

Pressure in drain pipe (in. W.C)

9.0

7.4

5.8

-4.2

-7.4

-24.8

DRAIN WITH MODIFIED PIPES

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

118

154

221

261

287

423

Velocity in 1st stack (ft/s)

3.8

6.0

6.0

6.0

7.1

8.6

Velocity in horizontal pipe (ft/s)

5.5

5.8

6.0

5.7

7.6

8.3

Velocity in 2nd stack (ft/s)

6.4

6.9

7.2

7.0

6.9

7.8

Pressure in 1st stack (in. W.C)

6.8

2.6

5.0

-4.0

-14.7

-16.9

Pressure in horizontal pipe (in. W.C)

-13.0

-12.2

-12.0

-11.8

-10.2

-9.1

Pressure in 2nd stack (in. W.C)

-3.1

-14.1

-16.7

-15.8

-28.4

-32.1



Froet 4” Bi-Functional Drain

DRAIN WITHOUT DRAIN PIPE

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

83

155

240

306

334

367

Velocity of flow (ft/s)

3.5

4.3

5.7

6.2

5.7

6.1

DRAIN WITH 10 FT PIPE

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

117

187

254

312

497

671

Velocity in drain pipe (ft/s)

4.3

7.0

7.2

7.8

8.5

8.9

Pressure in drain pipe (in. W.C)

5.5

5.2

4.4

1.2

-15.6

-29.6

DRAIN WITH MODIFIED PIPES

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

118

166

239

286

470

558

Velocity in 1st stack (ft/s)

3.8

3.5

5.1

5.4

7.9

7.9

Velocity in horizontal pipe (ft/s)

2.3

3.8

3.7

6.9

8.6

8.8

Velocity in 2nd stack (ft/s)

5.1

6.0

6.8

5.7

7.0

7.1

Pressure in 1st stack (in. W.C)

4.1

3.9

2.0

3.3

-15.1

-28.7

Pressure in horizontal pipe (in. W.C)

-13.2

-13.0

-13.2

-11.8

-8.4

-18.7

Pressure in 2nd stack (in. W.C)

5.5

1.4

-9.1

-19.4

-38.2

-38.4



Froet 6” Bi-Functional Drain

DRAIN WITHOUT DRAIN PIPE

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

118

202

402

616

714

870

Velocity of flow (ft/s)

1.8

4.5

6.2

7.8

3.3

3.2

DRAIN WITH 10 FT PIPE

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

120

192

397

604

704

866

Velocity in drain pipe (ft/s)

2.4

3.2

8.2

8.8

8.9

8.6

Pressure in drain pipe (in. W.C)

7.4

7.6

6.9

5.5

5.3

5.6

DRAIN WITH MODIFIED PIPES

Water head (inch)

1"

2"

3"

4"

5"

6"

Flow of water into box (gpm)

117

210

401

596

916

1164

Velocity in 1st stack (ft/s)

3.2

2.9

4.2

5.0

7.0

8.6

Velocity in horizontal pipe (ft/s)

0.5

3.2

5.2

6.5

7.3

6.9

Velocity in 2nd stack (ft/s)

4.2

5.9

6.4

8.0

8.9

7.9

Pressure in 1st stack (in. W.C)

15.5

17.6

17.3

18.1

14.2

-5.4

Pressure in horizontal pipe (in. W.C)

-13.2

-13.2

-13.3

-13.8

-11.7

-10.6

Pressure in 2nd stack (in. W.C)

5.9

6.0

1.9

-11.0

-22.4

-34.3




InformationDear Architects, Engineers, Building Owners, Wholesalers, Contractors and Manufacturer Representatives,

It has come to our attention that there is a rumor going around in the industry that our patent is no longer in force. That is totally false .Our patent is and will continue to be in force until November 6th, 2021. The installation of a product that infringes on our patent could present a liability issue to any entity involved in the sale or use of said product. We have made a PDF copy of our patent available for any one 's use to determine if a given product may or may not infringe on our patent.
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