How Far Should Downspouts Drain From the House?

People ask how long a downspout extension should be as if there were one right number. On a Northeast Florida lot the useful answer is: far enough that the water cannot come back. That depends less on feet than on where the water ends up — and on most Jacksonville houses, the place it ends up now is the strip of soil against the foundation.

What a downspout delivers

Gutters gather the whole roof into a handful of downspouts. A single downspout can carry the rain from 400 to 600 square feet of roof, and every inch of rain on a 2,000 square foot roof is more than a thousand gallons of water. Jacksonville storms drop 2 to 4 inches at a time. Put those together and each downspout is a hose running full for the length of the storm, aimed at one spot.

Why a splash block is not far enough here

A splash block moves the water about 18 inches. On a lot in the North, that water might soak into deep soil. Here it lands on sand over a hardpan layer 12 to 36 inches down, which water barely passes. The sand fills, the water stops on the hardpan and runs sideways — and on a lot that has settled even slightly toward the house, it runs straight back to the slab. Near the river and the coast, where the water table sits about 2 to 4 feet down, the ground may already be full before the storm starts.

So how far is far enough?

Far enough to reach ground that slopes away from the house and can carry the water off — or, better, all the way to a legal outlet. In practice that means three options, from the simplest to the most complete:

1. A surface extension that carries the water several feet out onto ground sloping away from the house. It is the cheapest test: if the wet strip by the wall dries out, the downspouts were the cause. The trouble is that it lives on the lawn, where the mower and the dog find it.
2. A buried line to a pop-up emitter on your own lot, well out in the yard or in a swale, with a lid that lifts when water arrives and closes when the storm stops.
3. A buried line to the street, the swale or a dry well. This is underground downspout drainage in its complete form: solid pipe below the sod, cleanouts at the turns, and the outlet marked on the plan.

Why the pipe is solid, not perforated

The point of a downspout line is to carry roof water away from the foundation, not to let it soak in along the way. Perforated pipe belongs in a French drain, where the job is to collect water; a downspout line is a carry line.

Where it may legally end

Florida's reasonable-use rule does not let you redirect your water onto a neighbor's lot, and a downspout line that pops up at the property line does exactly that. The lowest legal point is usually the street, a swale on your own lot, or a dry well where the water table is low enough for one to drain — near the river and the coast it often is not, and we test before recommending it. In communities with an HOA, the pond easement can be an option where the HOA allows it.

And how far should a French drain be from the foundation?

It is a related question, and the answer is the same idea: the drain goes where the water is moving, which is often along the side yard or across the back of the lot, not hard against the wall. If water stands right beside the foundation, the downspouts and the grade are checked first, because they are usually the cause and the cheaper fix.

What it costs

Underground downspout drainage runs $300 to $800 per downspout in our area, depending on the distance to the outlet and whether the line crosses a driveway or a paver walk. A typical house with four downspouts is $1,500 to $3,000, and on many houses it is the whole fix. See downspout & underground drainage, add up your own house with the drainage cost calculator, or call (904) 561-1816 for a free inspection within 48 hours.