Does anyone know what the standard height measurement is between the last
shingle on a roof and how low or high the gutter should be? I had a builder
tell me that the gutters are too low after the second layer of roof shingles
were installed over the first. and that they needed raised up.
Place a straight-edge along the roof, extending over the top of the gutter.
If it touchs the gutter, and raises off the edge of the roof, then the
gutter is probably high enough. If the straight-edge lays flat on the roof
and passes over the top of the gutter, then water rolling off the roof is
likely to do the same - shoot right over the gutter.
It is my understanding that in areas subject to heavy snowfall, that the
front edge of the gutter should be in-line with the roof plane. This allows
snow that builds up on the roof to slide off onto the ground, instead of
piling up on the gutter and ripping it off the building.
We get our fair share of snow in the winter, so I used this method when I
installed my own gutters. It seems to work well, even in heavy rains. The
only exceptions are the areas where the roof valleys channel the water into
a single area. In a heavy rainstorm, these concentrated areas to tend to
flow out past the gutter. I can't really raise the gutters up much, so I'm
thinking it has more to do with the gutter screens blocking the flow more
than the height of the gutter.
There are a lot of misconceptions about gutters and gutter heights within
the construction community. The important rule to remember is, that with
very few exceptions, water is ruled by gravity ... water falls down. This
means that gutters at the edge of a roof can never be too low. In fact,
they can only be too high.
The age-old rule in Architectural Graphic Standards is that the outer top
edge of the gutter should be below the fall line of the roof. This is so
that snow, ice, and debris will slide off of the roof over the gutter.
Minimally, this prevents moisture from snow and wet debris from entering
the structure and rotting the edge of the roof. Maximally, it prevents
sliding ice from deforming or ripping the gutter off of the building.
Lightweight roof debris is blown about by the wind, and keeping the gutter
below the fall line will reduce the amount of twigs and leaves the gutter
catches. And finally, if the gutter does happen to fill up with debris,
moisture from the debris will contact the fascia board instead of the more
absorbent roof sheathing and shingle edge.
The exact minimum dimension below the fall line is adjusted depending on
the slope of the roof. The steeper the roof, the smaller the dimension
needs to be (from one inch for a flat roof to as little as one-quarter
inch for roofs with a 12 in 12 pitch or steeper). To determine the
correct height of a gutter, lay a board flat on the sloping roof so that
it extends several inches beyond the roof edge. The bottom of this board
is in line with the fall line of the roof. The outer top edge of the
gutter should typically be one-half inch below the bottom of the sloping
Like plumbing waste lines, gutter runs are minimally sloped, so that
debris will flow to the downspout. The minimum slope is one-eighth inch
per foot along the run. To attach the gutter at the correct sloping
height, first attach the gutter location farthest away from the
downspout(s) at the minimum dimension below the fall line as previously
determined. This will be the high point of the gutter run. Then attach
the gutter at the downspout at the low point that is determined by the
minimal gutter slope for the run of the gutter (one-eight inch per foot,
or ... at least one inch down for every eight feet of run). The midpoint
attachments should be in line with this slope. It is important not to
have any low points that could collect water and debris, so string a
taught line between the previously attached high and low endpoints to
accurately determine the mid-slope attachment locations.
Installing the correct roof drip edge is very important. The bottom of
the drip edge should extend away from the fascia at a downward forty-five
degree angle to ensure that the water drops away from the fascia and into
the gutter below. Gutter shape is also important. The top of the fascia
side of the gutter should be at least one-half inch above the top of the
outer edge so that when the gutter overflows, water will not flow back
onto the fascia.
One of the few exceptions to downward water flow can occur where there is
a concentrated flow. This routinely happens at the bottom of a steep roof
valley where there is a build up of water from a large roof area. The
flow rate is further increased when metal valley flashing is used. At the
bottom of the valley, the concentrated stream of water will overshoot the
gutter. But the gutter should not be raised to solve this problem.
Instead, a metal fin is routinely installed vertically from the top of the
outer edge of the gutter at a valley corner. This fin extends at least
one inch above the fall line of the valley (which is shallower that the
fall line of the roof), and at least six inches horizontally from the
outer corner of the gutter. The fin should be designed to 'break away' if
it is ever hit by an ice flow.
PARTI\\' :: architecture
I agree that too high is worse than too low, and that includes having any
'metal fin' that raises the edge too high. My neighbor had a fin of about 2
inches added to the entire length of one of his gutters, and around the
ends. I never did exactly figure out why he did this, I assume because he
thought the water was over-shooting the edge. Bottom line is when the
downspout plugged up, the water backed up in the gutter, couldn't roll over
the front edge so it went the other way - over the back, under the shingles
and into his house!
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