Curbless Shower Conversion: Subfloor Engineering for Chicago-Area Homes

Most curbless shower projects on the North Shore are not primarily a tile decision. They are a floor structure decision that happens to have tile on top of it.

The transition from a tub or curbed shower to a zero-threshold entry looks straightforward until the floor is opened. In a typical Chicago-area pre-war home with wood framing (common across Wilmette, Winnetka, Glencoe, and Kenilworth) the bathroom floor structure is a set of 2x8 or 2x10 joists carrying subfloor and tile. Getting that floor to drain toward a center point or linear drain without a raised curb requires either recessing into the structure or building up around a tray. Both approaches work. They arrive at different structural requirements, cost profiles, and timelines.

This post covers the engineering layer that most bathroom remodeling articles skip.

The Two Paths: Recess vs. Tray

Path 1: Recessing the Subfloor

A traditional mortar-bed curbless shower needs roughly 3.5 to 4 inches of total floor depth below the finished shower floor to create proper pitch to the drain. That depth typically does not exist above the joists without cutting into them.

A linear-drain installation needs less (approximately 1.5 to 2 inches) because the drain runs along one wall and the floor slopes in one direction rather than four directions toward a center point. The shallower recess means less structural involvement, but some joist cutting is still common.

Per Fine Homebuilding, the moment any joist is notched or cut for drain access or floor recess, the modification must be reviewed against the span table for that joist size, and fire blocking must be addressed in pre-war balloon-frame construction where wall cavities run full height from sill to roof. That review should happen before demolition, not after.

If joists are already undersized or have been previously notched (a common find in pre-war North Shore bathrooms where old cast-iron drain work has been patched and adjusted), sistering is often required before the curbless conversion proceeds. Sistering typically runs roughly $150-$325 per joist depending on joist size and access conditions.

Path 2: Building Up with a Linear-Drain Tray

The alternative approach avoids joist modification entirely. A prefabricated linear-drain tray is installed at the shower entry, slightly raising the shower floor above the bathroom floor level. The slope runs toward the linear drain at the perimeter.

NAHB CAPS standards specify that interior floor transitions should not exceed 1/4 inch for accessible design. A properly installed linear-drain tray can meet this threshold at the entry, producing a floor that reads as level to a walker or wheelchair while still draining correctly.

This is often the faster path in pre-war homes. It avoids the structural review sequence, keeps the floor cavity intact, and is generally more predictable in cost. The tradeoff is a slightly raised shower floor relative to the rest of the bathroom, which some homeowners prefer to avoid.

What Pre-War North Shore Framing Adds to the Decision

North Shore housing era matters here. Wilmette, Winnetka, and Kenilworth built heavily in the 1890s through 1920s. Balloon framing is the dominant construction method in homes of that era: studs run full height from foundation sill to roof rafter, with no platform break at each floor. When walls are opened, fire blocking must be added in the wall cavity. This is code-required and is not optional.

The open-cavity problem is well documented: in a fire, unblocked balloon-frame walls act as chimneys, accelerating spread. Any remodel that opens walls in a pre-war North Shore home requires fire blocking regardless of whether that was the homeowner’s intent at design stage.

Beyond framing, pre-1940 bathroom floors in this region commonly have:

• Cast-iron drain lines that benefit from camera inspection before any work is planned around the drain location
• Galvanized supply pipes that corrode from the inside over 40-50 years; in homes that had lead service lines, corroded galvanized pipe can trap and release lead particles, per the EPA
• Subfloor rot under the existing tub, which is the single most common find in a pre-war bathroom gut. Repair scope ranges from minor joist patching to full replacement; the cost generally falls between $700 and $3,500 depending on how many joists are affected

None of these are visible from the outside. They are scope items that a curbless shower conversion in this housing stock should plan for, not hope to avoid.

The Grab-Bar Question: Blocking Before Tile

A curbless shower conversion is frequently part of a broader aging-in-place bathroom plan. If grab bars are part of that plan, the blocking must go in before tile is set. This is a sequencing issue that affects the entire project schedule.

ADA 2010 Standards section 609.8, adopted as the residential design benchmark even though ADA applies to commercial facilities, requires that grab bars and their mounts withstand 250 lbs of force in any direction. Standard drywall and tile will not hold it. 2x10 blocking must be framed into the wall before cement board and tile go up.

The practical implication: blocking locations must be decided at design stage, not added later. A homeowner who finishes a bathroom and then decides to add grab bars two years later faces tile removal and repatching. A homeowner who specifies blocking during the remodel pays very little extra and has a wall that can support bars anywhere in the blocked zone.

This is one reason the accessible bathroom design sequence is different from a standard remodel. The decisions run backward from function to structure: where does the user need to grab, what does the wall behind that location need to contain, and does the subfloor under the shower entry need modification for zero-threshold access. Those questions are answered before any finishes are selected.

Permit and Review Expectations by Municipality

Curbless shower conversions on the North Shore intersect the permit question at several points:

Plumbing permit: Any drain move triggers a plumbing permit in every North Shore village.

Structural permit: Any joist modification triggers a structural permit and may require an engineer’s stamp depending on the village.

Wilmette threshold: Per the Village of Wilmette, projects over $25,000 or involving structural work require a licensed architect or engineer stamp. A curbless conversion with joist modification and doorway widening in a pre-war home often exceeds both thresholds.

Doorway widening: Pre-1980 bathroom doors commonly measure 28-30 inches. A 32-inch clear opening (the accessible minimum) requires a 36-inch door. Widening a non-load-bearing doorway runs $600-$2,000; a load-bearing one requires an engineered header. In balloon-frame homes, most interior partitions are non-load-bearing, but confirmation requires a structural review before framing begins.

Winnetka’s permit authority, the Village of Winnetka Community Development, also operates a Landmark Preservation Commission with a demolition delay ordinance on homes identified as potentially significant. This does not typically affect interior bathroom work but becomes relevant if the remodel touches exterior walls.

What This Means for the Project Sequence

A curbless shower conversion in a pre-war North Shore home typically runs this sequence:

1. Camera inspection of existing drain lines before design is finalized
2. Decision on drain system (linear vs. center, tray buildup vs. subfloor recess)
3. Structural review of joist span if recess is planned
4. Permit submission including plumbing and structural drawings if joist modification is required
5. Demo and subfloor assessment (rot and prior patching are found here, not before)
6. Joist work, fire blocking, and subfloor repair if needed
7. Drain rough-in
8. Cement board and waterproof membrane installation (allow 24-72 hours of cure time before tile setting begins)
9. 2x10 blocking in grab-bar zones and wall framing for doorway widening
10. Tile, glass, and finish work
11. Grab bar installation into blocking

Frameless shower glass is measured after tile is complete and fabricated to fit, typically 2-4 weeks. It routinely holds the punch list on accessible bathroom projects. The timeline for a full curbless conversion with doorway widening in a pre-war North Shore home runs roughly 10-18 weeks once permits are in hand.

North Shore Villages: What Differs Locally

Kenilworth’s Building Review Commission consideration applies to properties on the Village’s Historic Survey list, primarily exterior and demolition decisions, not interior bathroom work. But Kenilworth’s lot coverage rules and historic stock mean any project touching exterior walls runs through a different review layer than Northbrook or Glenview.

Glencoe’s Village Building Department maintains a House File for each structure with original permit records. For bathroom work in a home with a pre-war permit history, that file can flag whether prior plumbing or structural work was permitted or not, which is relevant when assessing what a camera inspection might reveal.

Evanston requires all contractors to be registered with the City before pulling any permit. This is a homeowner-protection step that neighboring Wilmette and Skokie do not impose in the same way.

Planning Your Conversion

If a curbless shower is part of an aging-in-place plan, the structural decision (recess vs. tray, and how the doorway is handled) is the first conversation to have, not the last. Those decisions govern everything downstream: permit requirements, joist review, demo sequence, and the grab-bar blocking plan.

See our accessible bathroom remodeling services for how Delta - Bathroom and Kitchen Remodeling handles the full design-build scope from structural assessment through finish installation. The doorless shower pros and cons guide covers the decision layer if you are still weighing whether a curbless design is right for your home.

If you are ready to discuss your specific home’s structure and what a curbless conversion would require, contact us for a consultation.