Why New Basements Leak: The Seven Causes, in the Order I Look for Them

Warranty Water From a builder

Why New Basements Leak: The Seven Causes, in the Order I Look for Them

A wet new basement is almost never a mystery. There are seven causes, they announce themselves in a predictable order, and six of them are outside. Here is how I walk a wet basement, and what each finding means for the warranty.

Short version: Check, in order: surface water (grading and downspouts), window wells, the sump pump, the weeping tile and its outlet, the membrane and the top of the foundation, foundation cracks, and finally slab moisture or condensation that is not a leak at all. Most wet basements in new houses are the first three, which are cheap. Water penetration through the basement is a two-year Tarion item on a covered home, and a warranty item under any decent builder’s contract.

First, is it a leak?

A damp basement in the first year is sometimes the house drying. A new slab and new concrete walls hold a lot of water and release it for a year; a tight house with the HRV switched off holds it. Condensation on cold walls and pipes looks like seepage. The tell: condensation is spread out and appears in humid weather; a leak is local and appears after rain or snowmelt. Run the HRV, a dehumidifier for the first summer, and watch where and when the water shows up before anyone starts digging. Mould during construction covers what happens when drying is ignored.

The seven causes, in order

  1. Surface water at the wall. Backfill settles in the first year and the grade that sloped away now slopes in; downspouts dump beside the foundation; a patio or walkway pitches toward the house. Nine times out of ten this is the first thing I find, and the fix is a wheelbarrow of soil and a downspout extension. Lot grading and drainage requirements is the standard.
  2. Window wells. A well with no drain to the weeping tile, or a drain blocked by leaves and gravel, fills in a storm and the window becomes a drain. Every well needs a drain connected to the tile and a cover in a wooded lot.
  3. The sump. A pump that failed, a float stuck on the pit wall, a check valve that let the discharge run back, a discharge that froze, or a power outage in the storm. If the house depends on a sump, it needs a backup pump and a high-water alarm. Weeping tile, drainage and sump pumps explains the system.
  4. The weeping tile and its outlet. Tile laid on top of the footing instead of beside it, crushed in backfill, laid without a filter in clay so it silted up, or an outlet to daylight that is buried or has no screen. This one needs excavation or a camera inspection to confirm.
  5. The membrane and the top of the wall. Damp-proofing stopped short of grade, a drainage membrane punctured by backfill or not lapped over the tile, or a gap at the top of the foundation where the sill plate meets concrete and the siding does not cover it. Common at the garage and at the front porch slab.
  6. Foundation cracks. Vertical shrinkage cracks in poured walls can weep and are usually sealed from inside with an injection; they are a warranty item. Horizontal, stepped or widening cracks are a different conversation; cosmetic vs serious defects says which. ICF walls crack far less, which is one reason we build them, but openings still deserve a look.
  7. Slab moisture and plumbing. Water coming up through the slab points to a high water table and an overwhelmed tile, or to a plumbing leak under the floor. A plumber’s pressure test before anyone blames the foundation.

What to do, this week

  • Photograph the water, the location and the weather; note when it appears
  • Walk outside in the rain and watch where the water goes
  • Pull the sump lid and pour a bucket in; watch the float and the discharge
  • Check every window well drain with a hose
  • Report it to the builder in writing the same week; water penetration is a year-two Tarion item on a covered home, and the clock on the forms is running. Tarion’s coverage by year has the deadlines.

Who pays

On a Tarion home, water penetration through the basement and foundation is covered for two years from possession, which puts most of the list above on the builder. Grading settlement after the first year and problems caused by your own landscaping are yours. On an owner-built home there is no Tarion and the trades’ own warranties apply, which is why the photographs from the foundation stage matter. Under a good builder’s own warranty, the whole list is theirs for the warranty period; ours is seven years.

Preventing it on the next house

Everything on this list was decided before the backfill: tile beside the footing with stone and a filter, membrane lapped over it and run to grade, window well drains tied in, a sealed sump with a backup, downspouts to daylight, and a grading plan somebody actually built. An ICF basement adds a warm, dry wall that does not condense. ICF foundation pros and cons and the stage checklists are the two pages to read before the excavator arrives.

Related guides on this site

weeping tile drainage sump pump ontarioLot grading and drainage rulesICF foundation pros and conscosmetic vs serious construction defectstarion warranty 1 2 7 year coverage explainedMould during constructionDeficiencies and warrantyfirst year new home maintenance

Sources and further reading

Questions people ask

Why is my new basement wet?

In order of likelihood: grading and downspouts sending water to the wall, window wells without working drains, a failed sump pump, weeping tile problems, a membrane gap, foundation cracks, or slab moisture. Most new-house leaks are the first three.

Is a leaking basement covered by Tarion?

Water penetration through the basement and foundation walls is covered for two years from possession on a Tarion-enrolled home. Report it in writing and file the forms on time.

Can a basement crack be fixed from inside?

Vertical shrinkage cracks in poured concrete are routinely sealed from inside with an injection and it holds. Horizontal, stepped or widening cracks need an engineer's opinion first.

How do I tell condensation from a leak?

Condensation is spread over cold surfaces and appears in humid weather; a leak is local and appears after rain or snowmelt. Run the HRV and a dehumidifier the first summer and watch when and where the water shows up.

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Reviewed and updated October 2026. Costs are Ontario ranges, not quotes; code references are to the Ontario Building Code in force at that date.