Read the head before you break concrete
Water against a foundation is a column with a weight, not a puddle with a mop. Each foot of head presses at roughly 0.433 psi, about 62 pounds on every square foot it can reach (9.81 kPa per metre of head, about 1,000 kg on every square metre), and it presses in every direction at once. Nine feet (2.7 m) of saturated backfill on an eight-foot (2.4 m) wall is tons of load hunting for the weakest joint in the box. Every repair below does one of three things: lowers that column, intercepts it before it arrives, or gives it a cheaper path than the one through your structure.
Start with an observation hole. A cased auger hole outside the footing line, sleeved with perforated pipe and capped, tells you in one wet week what guesswork never will: how high the water actually stands, how fast it recovers after you bail it, and whether it drops within hours of the rain stopping. Perched water sitting on a clay lens behaves nothing like a genuine table. The first disappears between storms and yields to surface work; the second sits there all spring and has to be pumped.
Map the interior evidence before anything gets sealed. Chalk the staining height on the wall, mark which cracks run and which stay dry, and note whether the floor-to-wall joint weeps at the same time as the wall face or hours later. Seepage that starts during a downpour points at surface water and roof discharge. Seepage that starts a day after and continues through a dry week points at a table that has risen. Those two diagnoses buy completely different scopes of work, and reading them backwards means excavating a wall when the fix was a downspout extension.
Uplift: pressure acting under the slab
Below the floor, head acts upward across the whole footprint, and a basement slab is thin. Four inches (100 mm) of normal-weight concrete weighs about 50 pounds per square foot (240 kg per square metre), so roughly ten inches (250 mm) of head beneath it balances its own dead weight. Anything more and the slab is being lifted rather than loaded. Sub-slab pressure therefore announces itself as a heaved crack, a floor cracked in a rough ring away from the walls, or a joint at the wall base running clear water while the wall above stays dry.
The cove joint, the seam between wall and floor, is not a bond. It is a construction joint between two separate pours, and pressure under the slab finds it first because it is the longest continuous unsealed line in the building. Hydraulic cement pressed into that joint does stop the visible water, and it also raises the head under the slab until the next weakest point opens, often a crack mid-floor or a block core two courses up. Sealing without relieving is diversion, not repair.
Where the slab is coming out anyway, put a capillary break and a vapour retarder back under it. ASTM E1745 Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs covers the material and ASTM E1643 covers installing it. Coordinate the radon side of the job while the floor is open: a sealed, gasketed pit lid with a port serves both sub-slab depressurisation and a clean pit, and retrofitting one after the concrete is closed costs several times what it costs now.
With the floor open the sheet is nearly free and can never be added afterwards, so settle the footprint now, while this is still a roll count rather than a second demolition.
The total crawlspace floor or basement wall area to cover.
Sheet spent where seams overlap before they are taped.
Vapor barrier rolls needed
2 rolls
- Area to cover (with overlap allowance)
- 1,188 sq ft
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Vapor Barrier Calculator: 2 rolls — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- The roll is fixed at a 10 ft x 100 ft (3 m x 30 m), 1,000 sq ft (93 m²) sheet. The calculator never asks what you are buying, so a 12, 16 or 20 ft wide roll, a 50 ft roll, or a reinforced 10-20 mil barrier will not divide into this count. Take the area figure from the breakdown and divide it by your own product's stated coverage.
- The overlap allowance is for flat-plane seams only. Nothing is added for turning the sheet up the foundation wall, wrapping piers and columns, sealing around penetrations, or the off-cuts an irregular crawlspace footprint produces. Measure and add those separately.
- Only the sheet is counted. Seam tape, mastic, mechanical fasteners and termination bar are not in this estimate.
- This is a quantity take-off, not a vapour-control design. It says nothing about the permeance, thickness or puncture class the barrier has to meet, or which face of the insulation it belongs on. In a cold store or an unvented crawlspace the wrong side traps moisture inside the assembly no matter how many rolls you order.
- It is not a radon or ground-gas membrane specification. Those are designed, jointed and verified systems with their own material, welding and testing requirements, and a 6-mil poly roll count does not substitute for one.
Lateral load on the wall
Against the wall face the same column pushes sideways, hardest at the bottom where the head is greatest. Saturated clay backfill adds its own lateral component on top of the water, which is why the classic failure in a concrete block wall is a horizontal crack along a bed joint near mid-height with the wall bowing inward above it. That is a structural finding, not a drainage finding. Relieving the water stops the condition worsening; it does not restore a wall that has already displaced, and installing interior drainage while ignoring a bowed wall is the most expensive error on this page.
Block walls hold water inside them. Cores fill from the footing up and stand as their own column behind the parge, so a wall can weep at a mortar joint four courses above a floor that is bone dry. Relief for that condition means weep holes drilled into the bottom course, discharging into the interior drain, or the cores keep their head and the parge keeps failing. Poured walls leak instead at cold joints, form-tie holes and shrinkage cracks, and those want injection from the appropriate face rather than a surface patch.
Coatings deserve honest framing. Damp-proofing resists moisture; waterproofing membranes of the type covered by ASTM C836 and ASTM D6135 are formulated to resist head, and both belong on the positive, soil side of the wall. An interior coating sits on the negative side of the pressure and will be pushed off the substrate by anything more than incidental damp. Where a coating is genuinely in scope, with the wall excavated, prepped and dry, get the coverage right so the film thickness the product needs is the film thickness it actually receives.
Resistance to head lives in film thickness, and film thickness is coverage rate divided across the coats specified — fix both against the excavated wall area before a pail is opened on it.
The total basement wall area to be coated.
The area one unit of coating covers in a single coat, per the manufacturer's data sheet.
The total number of coats specified for the application.
Waterproofing coating needed
32.5 gal
A coating on a basement wall is one part of a system whose other parts — drainage, a drainage board, and somewhere for water to go — do most of the work. Applied alone against real hydrostatic pressure, a coating is being asked to do something it cannot.
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Basement Wall Waterproofing Coating Coverage Calculator: 32.5 gal — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Excludes the drainage board, protection layer, perimeter drain and the sump or outfall the drain discharges to.
- Coverage rates assume a sound, prepared substrate. A rough or porous wall consumes considerably more, and voids must be filled before coating rather than by it.
- Positive-side and negative-side applications are different products for different situations; a coating designed to be applied externally will not necessarily work applied internally.
Concentrating the pressure at one point
Everything above assumes the pressure has somewhere to go. A drainage system does not remove water; it collects the head at one low point and hands it to a pump. So the pump must match the water the relief path actually delivers in the worst hour, not the square footage of the house or whatever the last pump happened to be.
Measure rather than guess. Pull the pump, let the pit fill during heavy rain, and time the rise between two marks. An 18-inch (450 mm) pit holds roughly 13 gallons per vertical foot (160 litres per vertical metre), so a foot of rise in four minutes is about three gallons a minute (11 litres a minute) arriving. Catch the wettest event you can and design above it. Inflow into a new interior system is often several times what the old pit ever saw, because the tile now drains soil that previously held its water.
Pump selection lives on the curve, not the carton. Total dynamic head is vertical lift from pit to discharge plus friction through pipe, elbows and check valve, and a pump rated at zero head can deliver a fraction of that number once ten feet (3 m) of lift and forty feet (12 m) of run sit in front of it. Switch life matters as much as capacity: heavy oversizing produces short cycles that kill float switches, so size for a drawdown that gives the motor a real run each time. On a finished basement, a second pump on a separate circuit with a staggered float earns its place the first time a switch sticks.
Turn your measured pit recovery and lift into the flow the pump has to hold at working head.
The inside diameter of the sump basin/pit.
The depth of water the basin fills before the pump should activate and clear it.
How quickly the basin fills to the 'on' level during heavy inflow.
Minimum pump capacity, GPM
6.47 GPM (minimum)
This sizes for the basin's fill rate only — also check the pump's rated GPM at your actual vertical lift height (to the discharge point), since GPM drops as lift height increases on every pump's performance curve.
- Basin volume at fillable depth
- 12.94 gal
- Same flow in L/min
- 24.5 L/min
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Sump Pump GPM Calculator: 6.47 GPM (minimum) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 6.47 GPM (minimum) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- The number is an inflow rate, not a flow a pump will deliver. Vertical lift, discharge-pipe friction, elbows and the check valve all cut into a pump's rated output, and only the lift is even mentioned on this page. Size from the pump's performance curve at your actual total head, never from its headline GPM.
- The basin is modelled as a bare cylinder. The pump body, the float, the check valve and any gravel in the pit bottom all displace water, so the real volume between the float levels is smaller than the volume shown — and an inflow rate worked back from a timed fill in a real pit is lower than what this returns.
- No margin is added. A pump rated at exactly this flow only breaks even with the inflow: it never gains on the water and never shuts off. The model also says nothing about cycle rate or how long the motor can run continuously.
- The fill time is an assumption you supply, not a derived quantity. Nothing here reads rainfall intensity, the footing-drain or roof area feeding the pit, or a rising water table, so the answer is only as good as the worst-case fill time you entered.
- This is not a drainage or waterproofing design and not a backup-power calculation. It does not cover where the discharge may legally terminate — that is set locally and sanitary-sewer discharge is restricted in many places — nor freeze protection on the discharge line, backflow prevention, or the capacity you need when mains power fails during the storm that caused the inflow.
Building the relief path
The trench does the real work. Perforated tile laid in a washed-stone envelope gives water a route with almost no resistance, and the envelope, not the pipe, carries most of the flow, which is why stone gradation matters more than pipe diameter. Use clean washed stone with no fines. A crushed product carrying dust binds with silt in its first season and the trench turns into a slow wick instead of a drain.
Set the invert where the pressure acts. Interior tile belongs at or below the underside of the slab, bedded alongside the footing, never perched on top of it, because a line above the footing relieves nothing under the floor. Exterior tile sits at the footing base with continuous fall to the pit or to daylight. Perforations face down so the pipe takes water at the lowest point in the trench rather than after the trench has filled. Bedding and backfill for thermoplastic drainage pipe follow ASTM D2321 Standard Practice for Underground Installation of Thermoplastic Pipe for Sewer and Other Gravity-Flow Applications.
Filtration is the part that gets skipped. In silty and fine-sand soils the envelope needs a geotextile separating stone from soil, chosen against filtration criteria of the kind set out in AASHTO M288 Geosynthetic Specification for Highway Applications. Wrapping the trench is more durable than a sock pulled over the pipe, which clogs at the perforations and cannot be cleaned. Add sweep cleanouts at corners and on any long run, because on the day the tile silts, access decides whether the repair is a jetting visit or a second excavation.
A footing drain, top to bottom
- Cohesive backfill cap — clay-rich soil placed over the granular fill so rain sheds away instead of pouring down the loosest ground anywhere on the property Trench Excavation & Backfill Volume Calculator
- Geotextile wrap — taken right around the envelope rather than socked over the pipe, since a blinded sock cannot be jetted while a blinded wrap is still mostly open stone
- Washed stone envelope — this is what actually conveys the flow, which is why gradation decides more than pipe diameter does and why a dusty crushed product blinds inside one season French Drain Gravel Calculator
- Perforated line — holes turned down and bedded beside the footing rather than on top of it, with sweeps and cleanouts wherever the run changes direction Perimeter Weeping Tile Pipe Segment Calculator
- Footing and subgrade — the elevation datum for everything above: an invert set higher than this relieves nothing at all under the slab
Size the washed-stone envelope from trench width, depth and run before the truck is ordered.
The total length of the drain trench.
The width of the trench, typically wide enough to work in comfortably.
The depth of the trench, below the frost line in cold climates if the drain must run year-round.
The outer diameter of the perforated drain pipe.
Gravel backfill needed
2.758 yd³
- Trench volume
- 2.67 yd³
- Pipe volume (subtracted)
- 0.16 yd³
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French Drain Gravel Calculator: 2.76 yd³ — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
The pipe is drawn to scale inside the trench, because the gravel quantity above is the trench volume minus that circle. On a narrow trench with a big pipe the subtraction is a large share of the answer, and this is where you see that.
What this calculation does not cover
- The trench is modelled as a constant-depth rectangular prism, so a run laid to fall is only covered if the depth entered is the average depth over the whole length. A 15 m (49 ft) run at the usual 1 per cent fall finishes 150 mm (6 in) deeper than it starts, and a trench entered at its shallow upstream depth actually holds about 16 per cent more gravel than the figure returned.
- Gravel is taken as filling the trench to the full depth entered, from the trench floor to ground level. Any soil or turf cap placed over the stone has to come off the depth before it is entered — a 100 mm (4 in) topsoil cap on a 460 mm (18 in) trench is more than a fifth of the volume this page returns.
- The answer is a volume, not a tonnage, and the 10 per cent added covers ragged trench walls and settlement rather than compaction to a stated density. A quarry weighs stone out rather than measuring it, so converting needs the bulk density of the material actually supplied; clean drainage gravel is commonly quoted near 1.5 tonnes (1.7 tons) per cubic metre, which puts the default 2.81 yd³ order at roughly 3.2 tonnes (3.5 tons).
- No check is made that the pipe fits the trench. A 200 mm (8 in) pipe entered against a 100 mm (4 in) trench width is accepted and answered normally at 0.31 yd³, and where the pipe would displace more than the trench holds, the result is floored at zero rather than reported as impossible geometry.
- Pipe diameter enters this calculation only as displaced gravel, never as capacity. Whether the drain carries the water depends on the catchment area feeding it, rainfall intensity, the gradient and the infiltration rate of the surrounding soil — stepping from a 100 mm to a 150 mm pipe over 15 m moves this order by about 0.21 yd³ and settles none of those questions.
Sending relieved water where it cannot return
Discharge design decides whether the pump ever rests. Water dumped four feet from the wall runs back down the granular backfill, re-enters the tile and arrives at the pit again, a closed loop that runs the motor continuously and reads on site like an inflow problem. Carry the discharge clear of the backfill zone to a point that falls away from the building, and confirm where the far end is permitted to terminate.
Termination rules vary. Whether sump discharge may enter a storm sewer, must go to daylight, or is prohibited from the sanitary sewer is governed by the local plumbing code and municipal ordinance, and penalties for a sanitary connection are real in jurisdictions that inspect for them. Establish that before trenching, not after the pipe is buried.
Details that generate callbacks are few and predictable. A check valve installed without the small weep hole below it lets the pump lock on an air pocket after a dry spell. A valve set too high leaves a tall column of water to fall back at every stop, roughly a gallon per cycle for each ten feet of inch-and-a-half pipe (near four litres per three metres of 40 mm pipe). An exposed above-grade discharge freezes solid during a January thaw, precisely when the tile is running hardest. A freeze-relief opening near the rim joist and a run that leaves the building below frost depth answer both. Discharge sizing is a friction problem, and an undersized run quietly steals capacity from the pump curve.
Every metre of undersized line is friction the motor pays for on every cycle, so set the diameter from the flow you actually measured instead of matching whatever riser the last installer left behind.
The sump pump's rated discharge flow rate.
The maximum velocity allowed in the discharge pipe to limit friction loss and water hammer risk — commonly around 1.5 m/s (about 5 ft/s).
Minimum discharge pipe diameter
1.822 in
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Sump Pump Discharge Pipe Sizing Calculator: 1.82 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 1.822 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Returns an INSIDE diameter, and pipe is sold by nominal size. Wall thickness is the difference: a heavier schedule at the same nominal size has a smaller bore, so stepping up one nominal size can still leave you under the diameter calculated here. Check the published inside diameter for the material and schedule being bought, not the size printed on the pipe.
- Can return a diameter smaller than the pump's own discharge tapping, and that is not permission to reduce it. The port size is set for the impeller behind it, so necking a 1-1/2 in (38 mm) outlet into a 1-1/4 in (32 mm) line adds head the pump was never rated against and runs it off its curve. Treat the port size as the floor this result sits on top of.
Head you can delete instead of pump
Reducing the column beats pumping it. A thousand square feet (93 m²) of roof sheds about 620 gallons (2,350 litres) for every inch (25 mm) of rain, and a downspout emptying at the foundation delivers that straight into the backfill trench, the loosest and most permeable ground on the property. Extending leaders past the excavation line removes more head from a typical basement than any interior work will.
Grade the first stretch of ground away from the wall with real fall, and cap granular backfill with cohesive soil so surface water sheds rather than soaking into the chimney of loose fill around the house. Required fall over the first several feet is set by the residential code adopted in your jurisdiction, so read the edition in force rather than working from habit. Settled backfill that has formed a saucer against the wall is ordinary on houses past ten years old, and it is corrected with soil, not with more stone.
Window wells are miniature reservoirs standing against the wall. Each one wants a drain tied to the footing tile or the interior system, gravel that has not silted to the top, and a cover, or the well fills and puts direct head against a window never built to resist it. Patios and drives that have tipped back toward the house do the same thing on a larger scale, and they are worth checking before anyone prices an excavation.
Proving the pressure is gone
Commissioning is not switching the pump on. The system has to be loaded, watched under load, and handed over in a state the owner can maintain, because a drainage system that silts up unnoticed fails at the least convenient moment available to it.
Expect the pit to silt through the first season. A fresh stone envelope releases fines until the trench settles, and those fines end up in the pit, where they wear impellers and jam floats. Schedule a clean-out after the first full spring and annually after that. The honest measure of the work is a dry cove joint through a week of thaw with the pump cycling on a rhythm instead of running flat out: the head has been relieved, not merely relocated.
- Run a hose into the pit and into the tile at a cleanout; confirm flow arrives at the pit and the level falls faster than it rises.
- Cycle the float by hand through its full travel and check the tether cannot foul the pit wall, the discharge riser or the cord.
- Confirm the check valve holds on shutdown and that the weep hole below it passes water on start-up rather than trapping air.
- Kill the circuit at the panel and watch the backup pump or battery unit pick up and complete a full cycle under real inflow.
- Label the dedicated circuit and confirm receptacle protection against the electrical code adopted locally.
- Seat and gasket the lid, then record the date for a silt inspection before the first spring thaw.
Takeoff before the first cut
Quantities and checks worth settling on paper while the floor is still whole and the excavator has not arrived.
- Peak inflow, gallons per minute — Measured from pit recovery during the wettest hour you can catch, never from floor area.
- Total dynamic head — Vertical lift plus friction through pipe, elbows and check valve; read the curve at that figure.
- Washed stone envelope — Trench width by depth by run, clean gradation, no fines, plus waste for settlement.
- Perforated tile and fittings — Full perimeter run, corner sweeps, cleanouts; invert at or below the slab underside.
- Discharge run and outfall — Length, elbows, freeze relief, and a termination point the local code actually permits.
- Backup power path — Second pump or battery unit on its own circuit, proven under load at handover.
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