calculating size and number of downpipes

Home Forums Public Forums Drainage & Sewerage calculating size and number of downpipes

Viewing 3 reply threads
  • Author
    • #278571

      Can anyone tell me how to calculate the size and number of downpipes required for a given roof area. Materials 2 at the building design course at TAFE left me confused. I’ld like to put it into a spreadsheet so I can specify the correct size and number of downpipes for each project
      Thanking you in advance

    • #298775

      No problem. All you do is take the length times the width of the roof area.

      Then you get the pitch of the roof area (more pitch faster drainage) then you look at local codes. For example there are several ways to figure controlled flow.

      1- You can take the ten year frequency (in your area) storm is not stored on the roof for more than 24 hours. This 10 year part will give you an average if needed for actual piping below the leader so as to not over load the combination system if applicable.

      2-Make sure the depth of the water does not exceed 3″ during the storm, but you may want to slow down the drainage as not to overload the system during a deluge

      3- If the roof is level it should have 45 degree cants installed at the parapets and the flashing a least 6″ high.

      I always install 2 roof drains on ANY roof under 10,000 Sq. Ft and at least 4 roof drains on any roof over 10,000 Sq. Ft.

      Ideally the roofing contractor will also install a scupper to insure the water level cannot exceed 3″ depth in case of a stopage

      .Remember a cubic ft of water =62.4 pounds @ 39 degrees and most roofs are not designed to hold a lot of weight.

      One promenade roofs/decks the inet (strainer) must be at least twice the area of the conductor (leader piping) that this drain is connected to.

      On weather protection only roofs, the dome strainer must be 4″ above the roof surface AND must have an available inlet (dome area) 11/2 times the leader/conductor piping it is connected to

      The greater the pitch of the roof the larger area can be drained though a given sized pipe.

      When your combining a storm drainage system with a waste or sanitary system your going to have to interpolate the square footage into fixture unites (7.48 gallon=1 Fu or 1 Cu Ft) Depending on the size of the total drainage area would be the key factor in deciding what constant to use interpolation of Sq. ft in FU and visa versa.

      The mineral surface also comes into play as gravel roofs would offer more restrictions and the back up by the combination gravel stop/retaining ring could cause severe restriction and debris build up around the drain proper.

      Once you plug these variables in the rest is easy.
      The main thing is to have Fun and enjoy.

    • #298776

      ok my mistake, I need to specify that I am looking for the formula for the Victorian/Australian regulations based on ARI, the roof catchment area, the gutter discharge area etc

      Thanks Karinise

    • #298777

      Sorry there I mistook this posting from the USA .
      Why not try this site and contact one of my Aussie counter parts

      If I can help in any way please feel free to contact me or Loz

Viewing 3 reply threads
  • You must be logged in to reply to this topic.

Pin It on Pinterest

Share This