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Most building codes and FHA financing requires all homes
to have positive drainage. Just what is positive
drainage? Positive drainage means that all water on your
property drains away from all structures. Sounds good
but in reality there is much more to it than just water
draining away from structures.
In order to allow for adequate flow away from a
structure even during heavy rains a minimum 10% slope in
the first 10 feet out from the structure is required.
This means that when you are 10 feet away from your
structure the ground you are standing on should be 10”
lower than the ground adjacent to your structure. Or the
slope drops a little less than 1” for every foot.
Remember to put gutters and down spouts on, besides
helping to get the water away from your structure they
also prevent the water dripping from the roof from
eroding the soil away next to your structure.
It doesn’t do much good to have positive drainage if the
water has no where to go once it gets away from you
structure. Areas that drain the water off your lot once
it is away from you structure must have a minimum of a
3% slope or 3” fall for every 10 feet. Most US cities
will not allow sod on swales of 3% slopes or less.
Instead concrete is required due to the unavoidable
puddling of water in the drainage area. Puddling makes
for constant muddy area and dead grass.
Here are some suggestions to help with drainage swales.
Always grade as wide of a flat bottom as possible. This
will accommodate more water and lead to less ponding and
erosion problems. A swale that is graded like a V with
no flat bottom can be nothing but problems. In a V swale
with minimum drainage silt and sediment, the fine dirt
that washes with water, settles in the small bottom and
eventually fills in the swale, making for perpetual
clean out costs. In a V swale with substantial drainage
the water flows thru fast in the small bottom and erodes
away the sides carrying them down stream. This
eventually leads to unsightly eroded banks that are
straight up and down like a cliff.
Always take your soil conditions into account when
considering drainage. Sandier materials that allow water
to soak in fast can have must less “positive drainage”
than clay type materials. Clay materials are much more
problematic when water sets especially near your
foundation. The swelling and added weight of these
materials when they are constantly wet can cause costly
foundation problems.
It is wise to contact a qualified dirt person familiar
with your area to help suggest remedies to resolve your
drainage problems. Proper surface water drainage is by
far the best way to move water. If this cannot be
accomplished with grading the material on your lot other
means of drainage such as french drains or small storm
sewers can be considered. Hopefully this will be a last
resort.
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