Rainfall on a sloping roof
Rainfall on a sloping roof

The Plumbing Codes have a lot of stuff on this.

But for those of us who like to delve into things, and work out how things were derived. I will attempt to offer an explanation.

The crucial thing to understand is:-

  Rainfall measurements are taken in inches or millimeters falling on a horizontal surface. The angle of the rain is not important. All that matters is the quantity of rain over a given area.

So when thinking about this, we need to calculate the area on a horizontal plane, where the rainfall would have fallen, if the roof wasn’t there. The roof intersects this amount of rainfall.

However to do this, somewhere along the line, someone has to dream up at what angle the rain is falling.

Fortunately for us, the powers that be have come up with an angle of 2:1 as shown in the diagram.

Just like anything to do with rainfall, there is no standard rainfall event. All we can do is base the design on averages, and figures pulled out of the air. For instance we design eaves gutters on a rainfall event that may, or may not, occur once in every 20 years. And a rainfall angle of 2:1 is as good as any, and in fact, as you will see later, this makes the calculations much easier.

Looking at the diagram, a roof from A to D also intersects the same amount of rain as the main roof.

In fact any roof between rainfall lines B and C, will intersect the same amount of rain, and therefore have the same catchment area.

But what is really interesting,  it doesn’t matter what the roof does to get from point A to point D. It can go up and down. or round and round. As long as the starting point is A, and the ending point is D, it will have the same catchment area.

Roof with vertical drop
Roof with vertical drop

Now, to determine what the real catchment area is, we must determine the area of the slope effect that must be added.

For a straight roof the Architect has normally shown this slope on the drawings. But if there are vertical drops, or different slopes we take the average as shown in the diagram. because this will intersect the same amount of rain.

Now the hard part, we have to do some mathematics.

We know the rain falls at an angle of 2:1, therefore in the diagram above, the length of the “slope effect added”, is half the “vertical rise” ( 2:1 remember). This is also true for the roof areas, that is, the area of the slope effect is half the area of the vertical rise, as both these lengths are multiplied by the same roof width to find the area.

So all we have to do now is find the area of the vertical rise. If you can remember your trigonometry, the vertical rise area = (roof plan area) * tan( roof slope).

Catchment area (CA)  = roof plan area + 1/2 (vertical rise area)

= roof plan area  +  1/2 *( roof plan area * tan (roof slope)

Ah, on second thoughts, its probably just as easy to look up the “slope factor” in the Plumbing Code, or simply measure the area from the Architects Elevations.

Any questions?


  1. Roger says:

    This lawyer thanks you for your website, appreciate the humour! Wonderful too that the Architect will usually pick up the overflow over the entry door!

    What is it with this inability to understand that if the front of a gutter is higher than the fascia then when the gutter is blocked water is going to enter the building!

    My situation: Carport on the Gold Coast: Combined roof area 70sqm collecting into ONE 7m long (high faced) gutter. ONE downpipe 100mm x 75mm. Downpipe exits by sealed joint into 90mm pipe to road. Did I mention it is overhung by a poinciana tree constantly dropping fine twigs, seed capsules, leaves.

    What happened: every heavy rainfall: water penetrated into carport soffit. The pipe to the road was blocked and water backed up to the gutter, overfilling it completely, slots no use at all. Water actually pushed under pressure through the gap between the roof tiles and fascia into the space.

    Solution by previous owner who could not understand what was going on: increase the FALL in the gutter to the DP by raising one end even further above the level of the fascia. He even FOLDED the back of the gutter back over the fascia to achieve this!

    Result: even more water penetrated the soffit: literally cascaded from the joints in the fibro. Whole structure rotted and needed to be replaced.

    Proposed solution by well known roofing product chain: buy a new gutter in the same high faced design! Only this time make the front even higher! (Story for another day: However, new gutter cannot be hung with Suspension Clips at a level where the front is lower than the fascia. This is only achievable with standard GP bracket. Installer failed to bend the bracket fingers over – next rain gutter fell off).

    In the wise words of a retired builder to me recently “don’t use high faced guttering”.

    • lol.
      Yea appreciate your humour also.
      Its a common problem. In your case the best solution is to set the gutter off the fascia.
      Manufacturers now provide clips for doing such things.
      This allows the water to flow over the back of the gutter but go between the gutter and the fascia, and not into the building.
      Also installing a leaf guard on your gutter may be a good idea.
      I can see you are referring to my comments on this website

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