Yes, roof valleys can affect your rain gutters and cause overflow?
Roofs have many different designs from flat roofs to steep roofs with many valleys. The following information will discuss roofs with valleys and the flow of rain water down valleys.
By their design, valleys create a faster flow of water down the roof than a roof with no valleys. The problem this can create is that, in heavy rains, water flowing down valleys can over-shoot the rain gutter. This water continually landing on the ground at the same location will eventually cause harm in terms of puddling near the house or building, damage to plantings, etc.
So, what can be done to eliminate or at least minimize this problem? Following are some suggestions…
Worst case scenario is very large, steep roofs with long valleys. Rain will continue to accelerate down these valleys and overflow the inside corner of the rain gutter. You may never be able to eliminate this problem, but it can be minimized.
By installing an L-shaped deflector at the bottom of the valley, water will be slowed down and channeled half to the left and half to the right into the gutter.
For very steep and long valleys, in addition to the above deflector, install a deflector or deflectors up higher in the valley. This will spread the flow of water along the roof and reduce the acceleration of water at the bottom of the valley.
Inside Corner / Miter
Install an large inside corner (miter) that is much bigger than a standard inside corner.
Install Larger Rain Gutters
This will not work in all situations but if the size of your gutters for your roof is marginal, installing 6” or even 7” gutters could help. You will probably need a professional rain gutter installation company to look are your present gutters and advise you on larger gutters.