The verdict: displacement, not width, is what makes a crack serious
Homeowners measure width; the useful measurement is relative movement. The notebook's building-science baseline sets no crack-width threshold, and that absence is the point — what makes a crack serious is whether one side of the opening has moved against the other, and whether a rigid finish is being racked by a floor plane that is no longer flat. The mechanism is specific: crawl space supports settle, the floor above them drops unevenly, that movement transfers stress into the framing, and drywall, which cannot flex, cracks first at its weakest points, the corners of doors and windows. A narrow crack that has not changed in two years is a finishing issue. The same crack radiating outward in a stair-step or diagonal pattern from a door corner is a report that the supports below the floor have already moved.
Measure the floor rather than the plaster. The notebook puts floor-plane change at fractions of an inch, and that is enough to turn a rectangular door opening into a parallelogram — which is why a door that begins to drag on carpet or stops latching is a stronger signal than the crack itself. Two readings taken four to six weeks apart, at the same points, tell you direction and rate. A crack that measures the same deflection across both readings is a monitoring case; one that is still opening, or where the floor plane is still dropping, is a repair case. Bring both readings to the inspection and the conversation becomes arithmetic instead of opinion.
The short version
- What makes a Shreveport crack serious is displacement, not width, whether one side of the opening has moved against the other or a rigid finish is being racked by a floor plane that is no longer flat.
- The notebook sets no crack-width threshold, and it puts floor-plane change at fractions of an inch, enough to turn a rectangular door opening into a parallelogram and make a door drag.
- Two readings taken four to six weeks apart at the same points tell direction and rate: a crack holding the same deflection is a monitoring case, while one still opening is a repair case.
- Wood moisture is checked against a 19% ceiling for framing and 16% wood moisture equivalent for gypsum board, because above that line decay fungi have the condition they need.
- The screwdriver probe taps the joist with the handle and presses the tip into the grain; healthy wood rings solid and stops the tip, while a dull thud or sinking tip means hidden channel damage.
- Deferring one phase converts a $2,500-$6,000 job into a $6,000-$15,000 job, an avoidable spend of $3,500-$9,000, before adding $2,000-$5,000 per incident if a shifting frame shears a plumbing line.
The engineering core: four measurements that separate cosmetic from structural
Four measurements do most of the diagnostic work, and the notebook supplies all four. First, relative displacement across the opening: if the two sides no longer align, the finish is being stretched or crushed rather than simply separated. Second, wood moisture content, against the notebook's ceiling of 19% for framing timber and 16% wood moisture equivalent for gypsum board before finishes are reapplied — above that line decay fungi have the condition they need, and a wet joist loses strength before it ever looks damaged. Third, crawl space relative humidity against the notebook's targets: below 60% to stop fungal growth, 30–50% ideal, 40–55% the working range for a properly encapsulated space with active dehumidification. Fourth, the screwdriver probe test — tap the joist with the handle, then press the tip into the grain. Healthy wood rings solid and stops the tip. A dull hollow thud, or a tip that sinks with a spongy feel, means the interior has been eaten out and the crack above is being published by a member that has already failed.
| Signal you can measure | Threshold from the notebook | What it means | First action |
|---|---|---|---|
| Crack pattern | Diagonal or stair-step, radiating from door and window corners | Stress transferred from uneven support settlement, not a plaster defect | Structural inspection, 60–90 minutes |
| Floor plane | Change measured in fractions of an inch; a door racks out of square | Supports have shifted or lost contact with their piers | Re-level assessment before any cosmetic repair |
| Wood moisture content | Above 19% on framing, above 16% WME on gypsum board | Decay-favourable condition; strength loss precedes visible rot | Dry the source, then re-read at three joists |
| Crawl space relative humidity | Above 60% (target 30–50%; 40–55% encapsulated) | Moisture is available to fungi and to termites | Dehumidification or encapsulation scope |
| Screwdriver probe | Dull hollow thud; tip sinks with little pressure | Channel damage inside the member | Replace or sister the member, then treat it |
When to stop reading and call a structural engineer
Three conditions make the answer expensive if you keep watching. The first is a floor plane still dropping after two dated readings; every season of movement adds to the deflection a repair has to reverse. The second is any member that answers the probe test with a dull thud or a sinking tip, because it has already given up load capacity. The third is moisture that survives correction: if relative humidity stays above 60% on Hydrologic Soil Group D ground with the vents closed and a dehumidifier running, the enclosure is leaking air, the unit is undersized, or water is still arriving from below — and only the third needs a drainage answer rather than a sealing one. A 60–90 minute inspection is cheaper than the repair phase it prevents.
The cost model, and the arithmetic of waiting
The notebook prices the failure curve, not the crack. Caught in years one and two, when the signal is a floor bounce or a dragging door and the correction is targeted support work, the band is $2,500–$6,000. Left into years three to five, when beams sag across multiple bays and diagonal cracks tear open at door and window corners, the band is $6,000–$15,000. Left into years five to ten, the notebook puts severe structural compromise, extensive rot and shifting that can rupture under-floor plumbing at $15,000–$30,000 or more. Show the arithmetic: the mid band is roughly 2.4–2.5 times the early band ($6,000 ÷ $2,500 = 2.4; $15,000 ÷ $6,000 = 2.5), so deferring one phase converts a $2,500–$6,000 job into a $6,000–$15,000 job and the avoidable spend alone is $3,500–$9,000. Add plumbing at $2,000–$5,000 per incident if a shifting frame shears a line. On a slab the crack repair can be the cheapest line on the invoice and the plumbing access the most expensive.
Truth table: which crack ever pays to leave alone
| The crack you are looking at | Mechanism in the notebook | Repair band | Leave it and monitor? |
|---|---|---|---|
| Shrinkage crack in a slab, no displacement | Concrete curing; no support movement | Cosmetic sealing only | Yes — measure width quarterly |
| Diagonal crack at a door corner, door still true | Early support settlement, fractions of an inch | $2,500–$6,000 targeted support | No — inspect, then repair |
| Stair-step crack across a wall, door racks | Uneven support drop transferring stress into framing | $6,000–$15,000 | No — structural, not cosmetic |
| Crack beside a damp stain or mould | RH above 60%; an active moisture path | Correct water first, then structure | No — never seal over active moisture |
| Crack above a joist that answers with a dull thud | Hidden channel damage, intact exterior | Replace or sister, then treat | No — failure is already underway |
Local calibration: what Shreveport clay does to a crack
Northern Louisiana changes which cracks matter, because the clay is the engine. NRCS map units for Caddo Parish put depth to water table at about zero inches in the Latanier and Moreland clay units, with saturated permeability of 0.00–0.06 inches per hour in the Latanier and 0.01–0.06 inches per hour in the Moreland, both in Hydrologic Soil Group D — slow-draining ground with water already at the surface. The Moreland series runs 60–90% clay in its lower horizons with a coefficient of linear extensibility of 0.09–0.12 in the upper 40 inches, and published testing of northern Louisiana Moreland clay records a liquid limit of 79, a plasticity index of 51 and an activity of 1.3, naming soil heave as a primary structural concern. That is a soil that swells with the water table and shrinks when it drops, which is why cracks here run in seasonal cycles rather than a straight line. A crack that widens after a wet spring and closes in August is doing exactly what the clay beneath it is doing, and that cycle, not the width on any one day, is the thing to document.
| Shreveport condition | Measured value (NRCS Caddo Parish mapping) | How it acts on a crack |
|---|---|---|
| Depth to water table, Latanier and Moreland | About 0 inches | Soil stays near saturation; heave loads persist |
| Saturated permeability | 0.00–0.06 in/hr (Latanier); 0.01–0.06 in/hr (Moreland) | Water drains too slowly to relieve the soil |
| Hydrologic Soil Group | D | Subsurface drainage must be designed, never assumed |
| Moreland clay content and COLE | 60–90% clay; COLE 0.09–0.12 | Repeated swell-shrink opens and closes cracks seasonally |
| Moreland plasticity | Liquid limit 79; plasticity index 51; activity 1.3 | High volume change; heave named a primary structural concern |
Decision matrix: the response is not the repair
| If this is true | Then do this | Because |
|---|---|---|
| Crack static across two readings four to six weeks apart, no displacement | Monitor and seal cosmetically | No support movement has been recorded |
| A door racks or the floor plane is still dropping | Book a 60–90-minute structural inspection | Deflection, not width, is the failure metric |
| RH above 60% with no visible damp | Correct moisture before any structural spend | Moisture control is the key to mould control |
| The probe sinks or rings hollow on any joist | Replace or sister the member before finishes | Load capacity has already been lost |
| The crack returns within months of patching | Inspect the foundation, not the paint | Rigid finishes cannot flex with a moving floor |
Contractor spec-handoff checklist
- Two dated floor-plane readings at the same points, four to six weeks apart, with deflection recorded in fractions of an inch.
- Wood moisture content at three joists, each with date and meter identity; flag anything above the notebook's 19% framing limit.
- Crawl space relative humidity logged at the far end of the space against the 60% control line, with the 30–50% target noted.
- Crack locations drawn on a floor plan, each marked static or moving, with the pattern noted as vertical, diagonal or stair-step.
- Probe-test results per joist: solid ring, dull thud, or tip sinking.
- A scope that names adjustable steel columns on engineered footings and joist sistering where members have failed, with moisture work sequenced afterwards — structure first, plastic second.
- For a structural scope, confirm licensing: structural residential work requires a licensed Residential Building Contractor and home improvement above $7,500 requires registration, both verifiable on lslbc.gov.
- Ask for the engineer's name and licence number behind any structural opinion; call (318) 206-4167 for a local inspection referral.
