When a downpipe gets blocked we have to design for overflow conditions.

This normally allows for the water to overflow either the front or the back of the gutter.

We assume the water overflows equally along the full length of the gutter.

We then use the weir formula to calculate the depth of water overflowing. shown as “h” in the above diagrams.

Knowing this depth allows us to adjust the flashing, or the distance below the top of the fascia, to prevent any water from entering the building.

However eaves gutters normally have a slope, so the overflowing edge is not level.

This raises the question, if the downstream end is the lowest, wouldn’t all the water be overflowing there?

Meaning that the code, and all our calculations to date have been wrong?

So, I decided to check it out by using computational fluid Dynamics (CFD).

This is a computer program that plots the motion of every single particle in the water, and gives us a reasonably good idea of what will happen.

Its not the best quality but I had to reduce the file size substantially for it to upload.

So, check it out here…..