How far should water drain away from a foundation?

Two distances matter. The ground itself should fall at least 6 inches within the first 10 feet measured out from the wall — the International Residential Code grading standard, roughly a 5% slope — so rain sheds away instead of soaking the perimeter. Roof water carried by downspouts should discharge even farther: the IRC sets a 5-foot minimum, but on expansive clay 8 to 10 feet is the better target, because a roof concentrates a lot of water into one stream and clay holds it. The principle is the same at both scales — get the water past the loose backfill near the slab and onto ground that leads it away.

What is the 6-inches-in-10-feet rule and how do I measure it?

It is the surface-drainage requirement in the 2024 International Residential Code (§R401.3): the finished grade must fall at least 6 inches within the first 10 feet out from the foundation, about a 5% slope. To measure it, drive a stake against the foundation and a second stake 10 feet straight out, tie a string between them, and level the string with a line level. Then measure straight down from the string to the ground at the far stake — that drop is your fall. Less than 6 inches, or any slope back toward the house, is negative grade and needs correcting. The Foundation Performance Association frames the same idea as an obvious 2–5% slope, about 1 inch of fall per foot for the first 5 feet.

Do I need a French drain or just better grading?

Grading first, almost always. Correcting the slope and getting downspouts away from the wall is the least expensive fix and addresses the most common cause of water against a foundation, so it is where you start. A French drain — a gravel-filled trench with a perforated pipe — is the right tool for a specific, narrower problem: a zone that stays soggy after you have already fixed the grade, or a spot where a high water table or an uphill slope keeps feeding water toward the house and the ground genuinely cannot be sloped away. Installed where simple regrading would have worked, a French drain is money spent on the wrong layer.

How far should downspouts extend on clay soil?

The IRC minimum is a 5-foot discharge from the foundation, but on expansive clay aim for 8 to 10 feet or more. Clay drains slowly, so water dropped near the wall lingers and saturates the perimeter at one point — the classic recipe for one-corner heave. Splash blocks alone only carry water about 2 to 3 feet and are really meant for well-drained soils, so on clay pair the downspout with a solid extension, an accordion extender, or a buried rigid line. The goal is to release roof water where the grade can lead it away, not where it pools against the slab.

What kind of fill should I use to regrade?

Use compacted sandy-clay fill, tapering down and away from the house — not pure sand. Pure sand against a slab acts like a French drain you did not mean to build: it channels water straight down to the perimeter soil and the footing, which is the opposite of what you want. A sandy-clay mix sheds water at the surface while resisting erosion. Place it in compacted lifts so it does not settle and quietly reverse the slope you just built. And keep the finished soil line below the foundation's designed level — never bring fill up over the brick line, weep holes, or siding.

Can I raise the soil against my house to fix the slope?

No — that is the most common do-it-yourself mistake in drainage work. Piling dirt against the wall to fake a slope buries the weep screed or brick weep holes (the gap that lets the wall drain and breathe), traps moisture against the structure, and can invite termites and rot. The correct move is to grade the soil away from the house with compacted sandy-clay fill, keeping the soil line below the brick line, weep holes, and siding at all times. Build the grade out and down; do not bury the base of the house to get it.

How much does foundation drainage cost?

It runs as a ladder from nearly free to a few thousand dollars, which is the point of doing the cheap steps first. Cleaning gutters and extending downspouts cost very little and are usually do-it-yourself. Regrading the perimeter or cutting a swale is moderate earthwork that scales with lot size, access, and how much soil has to move — a few hundred dollars on your own up to a few thousand with a contractor. A French drain is the priciest common option, roughly $10 to $65 per linear foot, or about $1,000 to $6,500 for a 100-foot exterior run. Treat company quotes as indicative and get a written scope for the actual work. For the full picture see our cost coverage.

Will fixing drainage repair existing foundation damage?

No. Drainage is preventive: it stops new movement and slows what is already in progress by removing the moisture swings that drive it, but it will not lift a slab that has already settled or heaved beyond tolerance. That takes an engineered structural repair such as deep underpinning. Think of good drainage as protecting the result and preventing the next cycle, not as undoing damage. If cracks are progressing or a floor visibly slopes, get an independent engineer's elevation survey before any structural work — and then fix the drainage too, so the repair stays stable.

Foundation Drainage: Grading and Drain Systems (2026)

Getting water away from the slab is the cheapest, highest-return foundation work there is — and grading is where it starts, before a single foot of drain pipe goes in the ground. Water that pools against a foundation soaks the perimeter clay and swells it, while running water can wash out the soil under the footing; that is the why, covered in full on our poor drainage and grading page. This page is the how: how to measure the grade around your house, how to regrade it correctly, how to get gutters and downspouts to carry roof water clear, and how the common drainage systems — swales, French drains, catch basins, and subsurface drains — actually compare so you spend on the right one.

Get the grade right first

Before you price a drain system, check the ground. The single number worth memorizing is the surface-drainage standard in the 2024 International Residential Code (§R401.3): the finished grade must fall at least 6 inches within the first 10 feet measured out from the foundation — roughly a 5% slope — so rain sheds away instead of soaking in. For paved or otherwise impervious surfaces that abut the house, driveways and patios, the slope away should be at least 2% (about 1/4 inch per foot). The Foundation Performance Association frames the same target as an "obvious" 2–5% slope, about 1 inch of fall per foot for the first 5 feet.

You do not need a survey crew to check it. Measure it yourself in ten minutes:

Drive a stake against the foundation and a second stake straight out, 10 feet away.
Tie a string between the two stakes and level it with a line level so the string is dead horizontal.
Measure straight down from the level string to the ground at the far stake. That vertical drop is your fall over 10 feet.

Six inches or more, sloping away, is positive grade — what you want. Less than 6 inches, or any slope back toward the house, is negative grade, and it needs correcting. A genuinely flat perimeter is its own version of the problem: with no fall, water sits rather than sheds, so it usually needs to be re-graded into a gentle slope or given a swale to lead water off.

To fix negative grade, build the soil up and out into a positive slope using compacted sandy-clay fill — not pure sand — tapering down and away from the wall. The reason to avoid pure sand is counterintuitive but important: sand against the slab behaves like an accidental French drain, funneling surface water straight down to the perimeter soil and the footing, which is the opposite of shedding it. Place the fill in compacted lifts so it does not settle and quietly reverse the slope a year later. And observe the one rule that governs all of this: never raise the soil above the brick line, the weep holes or weep screed, or the siding. The grade is built by sloping the ground away, not by burying the base of the house.

Gutters and downspouts

A roof is a large, efficient water-collection surface whose entire job is to take rain off hundreds of square feet and concentrate it into a handful of streams. Where those streams land decides whether the roof protects your foundation or attacks it.

Start at the top: keep the gutters clean. A clogged gutter overflows at the eave and dumps that concentrated water straight down the wall, which makes any downspout work moot. With the gutters carrying water to the downspouts, the question becomes where the downspouts let go of it. The IRC sets a 5-foot minimum discharge distance from the foundation, but on expansive clay that is a floor, not a goal — 8 to 10 feet (or more) is preferable, because clay drains slowly and water dropped near the wall lingers and saturates one spot.

The hardware you use to get there matters, because each option has a real reach limit:

Splash blocks carry water only about 2 to 3 feet and are really meant for well-drained soils. On clay they rarely move water far enough on their own.
Downspout extensions and accordion extenders are inexpensive and DIY-friendly, and they push the discharge out onto positive grade where the ground can lead it away.
Buried rigid pipe is the durable solution where a surface extension would be in the way. Use solid Schedule 40 PVC sloped at about 1/4 inch per foot so it drains fully and does not silt up, daylighting well away from the house.

The objective is unglamorous and specific: stop concentrating roof water against the foundation, and release it where the grade carries it off. A surprising share of "one wet corner" foundation movement traces back to a single clogged gutter or a downspout emptying at the wall.

Drainage systems compared

Once the grade is positive and the downspouts are extended, most homes are done. Where a zone stays wet anyway — a chronically soggy stretch, an uphill slope feeding water toward the house, or a low spot that will not drain — you step up to a built drainage system. They are not interchangeable; each solves a different problem. Surface grading and swales are the cheap first move; a French drain is for the zone that stays soggy after the grade is right.

System	What it does	Best for	Rough cost (indicative)
Surface drainage / swale	A shallow graded channel that carries runoff across the lot and away from the house	The cheapest first option — a flat or gently sloping perimeter that needs water led off	Low — earthwork; a few hundred to a few thousand dollars by area
French drain	A gravel-filled trench with a perforated pipe that intercepts standing and subsurface water and routes it away	A zone that stays soggy after grading is corrected; a high water table or hillside feeding water in	~$10–$65 per linear ft; ~$1,000–$6,500 for a 100-ft exterior run
Catch basin	A grated box set at a low spot that collects pooling water and feeds it into a pipe	A persistent low point or yard depression that holds water after every storm	Low to moderate — basin plus the pipe run it feeds
Subsurface / footing (curtain) drain	A deeper perforated drain near the footing that relieves water and pressure below grade	Below-grade walls or a stem-wall region holding back saturated soil	Higher — depth and excavation drive the cost

Read the table as a sequence, not a menu. The least expensive intervention that solves your specific water problem is the right one, and for most slab homes on a correctable grade that is grading and a downspout extension — not a trench. A French drain installed where the grade simply needed fixing is the most common over-spend in this whole subject.

Find your problem zones

You cannot fix what you have not located, and the best time to locate a drainage problem is while it is happening. Walk the perimeter after the next hard rain and look for where water actually goes:

Standing water against the wall. Mark every spot where water pools at the foundation rather than running off. Those are your problem zones, and they tell you precisely where the grade is negative or flat.
A bowl-shaped depression under a downspout. A scoured-out hollow at a discharge point is a classic sign of erosion from concentrated roof water — the downspout is both saturating that spot and carving away the soil that supports the footing nearby.
Soil pulled away from the slab. A visible gap between the soil and the foundation tells you the clay there has dried and shrunk, the dry-side counterpart to the wet-side pooling you are mapping.

Photograph what you find and date the photos, so you can compare year over year and confirm a fix actually worked. The same walk feeds the broader seasonal routine in our maintenance checklist.

Cost and sequence

Drainage work rewards doing the cheap, high-leverage things first, and the costs below climb in exactly that order. Treat any single figure — especially company quotes — as indicative: real numbers depend on lot size, access, and how much soil has to move. Evaluate the grade first; it is the least expensive fix and addresses the most common cause of water at the foundation.

Step (cheapest first)	What it addresses	Rough effort / cost (indicative)
Clean and maintain gutters	Overflow dumping concentrated water at the wall	Low — a few hours or a modest service call
Extend downspouts (8–10 ft on clay)	One-corner saturation from roof water	Low — inexpensive parts, DIY-friendly
Regrade the perimeter to ≥6 in / 10 ft	Negative or reversed grade soaking the perimeter	Moderate — sandy-clay fill and labor, by area
Cut a swale	A flat run or low spot that needs water led off	Moderate — earthwork and grading
French drain	A zone still soggy after grading is corrected	~$10–$65/lin ft; ~$1,000–$6,500 per 100-ft run

The sequence is the savings: most homes get the bulk of their protection from the first two steps, so you rarely have to start with the expensive earthwork or a trench. Stage watering on the dry side of the same plan with our foundation watering guide, and for the full repair-versus-prevention numbers see the prevention overview. One caution before you re-landscape: if water is collecting under the slab rather than around it, the cause may be a sub-slab plumbing leak feeding water from below, which no amount of grading will fix — rule that out first.

FAQ Note

The FAQ below answers what San Antonio homeowners ask most about draining water away from a slab — how far water should drain, the 6-inches-in-10-feet rule and how to measure it, whether you need a French drain or just better grading, how far downspouts should reach on clay, what fill to regrade with, the danger of raising soil against the house, what it costs, and whether drainage can repair existing damage. For the full menu of causes behind the water problem — clay, drought, plumbing leaks, and tree roots — see the poor drainage and grading page and the signs of a sinking foundation.

Get Matched With a Vetted San Antonio Foundation Specialist

If water is pooling against your slab, a downspout is dumping at the wall, or cracks are tracking from your door corners, the right next step is a measurement, not a sales call. We'll match you with a vetted San Antonio specialist and point you to an independent engineer who can confirm whether drainage alone will protect the house or whether the foundation has already moved enough to need structural work. The match is free, the quote is no-obligation, and we don't take a fee from you. We screen for sealed-engineer diagnosis and for a documented drainage and moisture-management plan alongside any structural repair — because on Texas clay, the cheapest fix is almost always the water, and the durable fix needs both.

Foundation Drainage: How to Grade and Drain Water Away From a Slab