One of the key goals when designing and installing any gutter system is preventing damage to structures from the seepage of rainwater.
So that water will not flow into the building, design gutters and downspouts to be properly installed. To ensure rainwater does not flow back into structures, size overflow measures to accommodate a 1 in 100-year rainfall event. Water overflow mainly occurs during high-intensity rainfalls. When due to either poor design or blockages, the gutter system is unable to cope with the volume of water entering the gutters. No gutter system should, when overflowing, allow water to enter the walls or internal structure of a building.
The design of adequate overflow measures will vary from site-to-site depending on the choice of gutter. Structure drainage and downpipes must be considered too. The licensed contractor is responsible for complying with the relevant local building codes. Your contractor must design and install the right gutter system for each individual circumstance.
An accepted overflow measure for high fronted gutters is as follows. The installation of a gutter system with a 10mm gap between the fascia and the back of the gutter. It may be necessary to use a spacer or other proprietary trade product to establish the 10mm gap.
Be smart. Know your local building codes BEFORE you undertake a gutter tear-off and re-install project.