Curbs, Curbless Showers & Thresholds
Step 4 of the Construction Order — Containing Water at the Entry
This page explains why shower curbs exist, how curbless showers are permitted in New Jersey, and what inspectors are evaluating when reviewing thresholds and transitions.
This guide follows TCNA Handbook threshold design methods for curbless configurations and curb detailing.
Curbs are not decorative. They are a water-containment device.
Official Standards Referenced
| Standard | What It Covers | Official Link |
|---|---|---|
| TCNA Handbook | Curb and threshold detailing | tcnatile.com |
| IPC Section 417.5.2 | Shower receptor requirements | codes.iccsafe.org |
| ICC A117.1 | Accessible design (when triggered) | iccsafe.org |
| ANSI A118.10 | Waterproof membrane standards | tcnatile.com/faqs |
The Code Intent (Plain Language)
The shower entry must prevent water from leaving the wet area under normal use and during temporary drain restriction.
Whether achieved with a curb or a curbless design, the goal is the same: containment and control.
Curbed Showers (Traditional Thresholds)
When a shower includes a curb, model plumbing codes adopted by New Jersey typically expect:
- A finished curb height at least 2 inches above the drain
- A curb that is not excessively wide or unsafe
- Waterproofing that fully wraps the curb
What "2 Inches Above the Drain" Actually Means
| Measurement | Reference Point |
|---|---|
| Start | Top of the drain grate (finished) |
| End | Top of the curb (finished tile surface) |
| Minimum | 2 inches |
This is not measured from the bathroom floor. It applies only when a curb is present.
This requirement addresses splash-out and overflow risk.
Curb Construction Requirements
Per TCNA guidelines:
| Element | Requirement |
|---|---|
| Structure | Solid framing or masonry (not foam alone) |
| Width | Typically 3-4 inches minimum for stability |
| Height | 2" minimum above drain after tile |
| Waterproofing | Continuous from floor, up, over, and down curb |
Waterproofing at Curbs
The curb is a critical transition point. Waterproofing must:
- Extend from the shower floor up the inside face
- Continue over the top of the curb
- Wrap down the outside face
- Maintain continuity at corners
No fasteners should penetrate horizontal curb surfaces.
Curbless Showers (Zero-Threshold)
Curbless showers are not prohibited in New Jersey residential construction, but they are treated as performance-based designs.
What This Means for Homeowners
| Curbed Shower | Curbless Shower |
|---|---|
| Curb physically contains water | Design and drainage contain water |
| Prescriptive code compliance | Performance-based compliance |
| Standard approach | Requires additional planning |
Removing the curb increases planning responsibility—it does not remove requirements.
How Curbless Showers Achieve Water Containment
Instead of relying on a curb, water containment is achieved through:
| Strategy | How It Works |
|---|---|
| Floor elevation planning | Shower floor lower than bathroom floor |
| Extended slope | Slope continues beyond wet area |
| Strategic drain placement | Linear drain at entry captures water |
| Drain capacity | Larger drain handles flow volume |
Common Curbless Design Approaches
| Approach | Description | Best For |
|---|---|---|
| Recessed shower floor | Shower floor dropped below bathroom level | Slab-on-grade, new construction |
| Linear drain at entry | Trench drain placed at threshold | Prevents water exit |
| Full bathroom slope | Entire floor slopes toward shower | Open bathroom designs |
| Barrier-reduced | Minor bevel vs. full curb | When full curbless isn't possible |
NJ Inspector Expectations for Curbless Showers
Inspectors may accept curbless showers when they see:
| Element | Verification |
|---|---|
| Continuous slope | Water directed inward to drain |
| Adequate drainage | Linear or appropriately-sized drain |
| Extended waterproofing | Membrane continues beyond wet area |
| Planned water control | Documented design intent |
Coordination with Your AHJ
Early coordination is strongly recommended. Before finalizing a curbless design:
- Contact your local Construction Office
- Describe the proposed design
- Ask about specific requirements or interpretations
- Document the conversation
- Submit for review with permit application
Pro tip: Not all inspectors have extensive experience with curbless showers. Providing clear documentation of your design intent—including TCNA method references—can facilitate approval.
Accessibility Standards: When They Apply
ICC A117.1 Accessibility Standards
Accessibility standards apply only when legally triggered, such as:
| Trigger | Example |
|---|---|
| Accessible dwelling units | Multi-family requirements |
| Public accommodations | Commercial bathrooms |
| Fair Housing Act | Covered multifamily buildings |
| Voluntary compliance | Aging-in-place planning |
Standard Residential Remodels
Standard residential remodels are not automatically subject to accessibility requirements, even when curbless designs are chosen.
However, if you're planning for aging-in-place or future accessibility needs, consider:
- Blocking for future grab bars
- Wider doorways
- Reinforced bench locations
- Proper slope for wheelchair access
Curbless Shower Construction: A Planning Framework
Critical Planning Equations
For a true curbless (zero-threshold) shower:
| Calculation | Formula |
|---|---|
| Required drop | Distance to drain (ft) × 0.25" |
| Perimeter height | Drain height + required drop |
| Mortar bed thickness | Varies from drain to perimeter |
Example Calculation
For a shower with drain 4 feet from entry:
- Required drop: 4 ft × 0.25" = 1 inch
- Shower floor must be 1" lower at drain than at entry
- Entry must be flush with bathroom floor
Step-by-Step Build Sequence
- Assess structure — Determine if floor can be recessed
- Plan elevations — Calculate all heights before starting
- Rough plumbing — Set drain at correct height
- Framing adjustments — Notch or lower as approved
- Subfloor prep — Verify flatness and structural integrity
- Slope mortar bed — Build to calculated elevations
- Waterproofing — Extend beyond wet area minimum 12"
- Drain integration — Match drain to membrane system
- Flood test — Verify containment before tile
- Tile installation — Maintain transitions to adjacent flooring
Waterproofing at Thresholds
Regardless of curb type:
| Requirement | Why It Matters |
|---|---|
| Continuous across threshold | Water can't escape at transition |
| Proper termination | Membrane must end correctly |
| No fasteners through horizontal | Penetrations create leak paths |
Thresholds must be treated as part of the waterproofing system, not as trim.
Extended Waterproofing for Curbless
For curbless showers, waterproofing typically extends:
- Minimum 12 inches beyond the wet area
- Into the bathroom floor area
- With proper termination or transition
Common Threshold Failure Points
| Failure | Cause | Prevention |
|---|---|---|
| Leaks at curb base | Waterproofing stopped at floor | Wrap membrane up and over curb |
| Curb corner failures | Inadequate corner treatment | Pre-formed corners or reinforcement |
| Curbless water escape | Inadequate slope planning | Calculate elevations before construction |
| Fastener penetrations | Screws through curb cap | No horizontal fasteners |
| Waterproofing stops at opening | Membrane ends at shower edge | Extend beyond wet area |
Questions Homeowners Should Ask
Before threshold design is finalized:
- Is a curbed or curbless design better for my space?
- What TCNA method will be used?
- How will water be contained without a curb?
- Has this design been discussed with the inspector?
- How will waterproofing be handled at the threshold?
- Will a flood test verify containment?
Resources for Further Reading
| Resource | Topic | Link |
|---|---|---|
| TCNA Handbook | Threshold and curb details | tcnatile.com |
| ICC Plumbing Code | Receptor requirements | codes.iccsafe.org |
| ICC A117.1 | Accessibility standards | iccsafe.org |
| NJ DCA | Local code requirements | nj.gov/dca/codes |
Key Takeaways
- Curbs exist to contain water—the 2" rule relates to drain, not floor
- Curbless showers rely on performance, not exemption from requirements
- Early AHJ coordination is essential for curbless designs
- Thresholds are part of the waterproofing system
- Planning prevents late rejection
Next Step in the Build Phase
Framing for Benches, Seats & Niches How structural planning supports long-term performance and inspection approval.
Whether curbed or curbless, water containment is non-negotiable.