FL Number Measurement Guide for Sloped Concrete Parking Decks
Overview
This document provides guidance on measuring Floor Levelness (FL) numbers for concrete parking decks with 1% and 2% slopes. FL numbers quantify the levelness of a concrete surface and are part of the FL/FF (Floor Levelness/Floor Flatness) system defined by ASTM E1155.
Understanding FL Numbers
What FL Measures
Floor Levelness (FL) measures the elevation variations in a concrete floor over a 10-foot distance. It indicates how level the surface is relative to its intended elevation or slope.
Higher FL values = More level surface with fewer elevation variations
Lower FL values = Greater elevation variations from the intended plane
FL vs FF
FL (Floor Levelness): Measures conformity to a level plane or intended slope over longer distances (10-foot sections)
FF (Floor Flatness): Measures bumps and dips over shorter distances (2-foot sections)
Both measurements filter out the intentional design slope to measure only unintended variations.
Equipment Requirements
Primary Method: Digital Floor Profiler
Professional-grade digital profilers are the standard for accurate FL/FF measurements:
Dipstick® Floor Profiler
F-Meter
Face® Floor Profiler
These devices:
Automatically record elevation data at 1-inch intervals
Calculate FL/FF numbers in real-time
Account for intentional slopes
Provide statistical analysis and reporting
Meet ASTM E1155 requirements
Alternative Method: Manual Measurement
A 10-foot straightedge with precision measuring gauges can be used, though this is:
Less accurate than digital profilers
More time-consuming
More prone to operator error
Generally not recommended for contractual compliance testing
Measurement Timing
Critical Timing Window
Minimum wait: 24 hours after concrete placement (concrete must support foot traffic)
Optimal timing: 24-72 hours after placement
Maximum for acceptance: Typically within 72 hours, unless specified otherwise
Why Timing Matters
Too early: Risk of damaging the surface
Too late: Additional curling and shrinkage may affect readings
Industry standard practice focuses on the 24-72 hour window
Measurement Procedure
Step 1: Divide Into Inspection Lots
Standard lot size: 2,000 to 5,000 square feet
Lots should be relatively uniform in pour sequence and conditions
Very large decks may require multiple inspection lots
Step 2: Establish Survey Lines
Run survey lines perpendicular to the direction of concrete placement
Space lines 10 feet apart within each inspection lot
Lines should cover representative areas of the deck
Avoid edges, joints, and areas with known defects
Step 3: Set Up Reference Plane
This is critical for sloped surfaces:
Establish the design elevation at multiple points
Input the intended slope (1% or 2%) into the profiler software
The reference plane must match your design slope direction and magnitude
The profiler will measure deviations from this sloped plane, not from absolute level
Step 4: Collect Data
With Digital Profiler:
Walk along each survey line at a steady pace
The device records elevation every 12 inches automatically
Maintain consistent speed and contact with the surface
Ensure the device is properly calibrated before starting
Data Collection Requirements (ASTM E1155):
Minimum 20 elevation readings per survey line
Multiple survey lines per inspection lot
Typically results in several hundred data points per lot
Step 5: Calculate FL Number
The FL calculation uses a complex statistical formula based on:
Elevation differences between consecutive measurements
The number of data points collected
The intended reference plane
Digital profilers calculate this automatically. The formula accounts for:
Sample size
Cumulative elevation differences
Statistical variability
Acceptable FL Values for Parking Decks
Important Disclaimer
There is no single universal standard for parking deck FL numbers. Acceptable values depend on:
Project specifications (always check these first)
Engineer of record requirements
Owner expectations
Local building codes
Type of parking structure (public vs. private, precast vs. cast-in-place)
Intended use and traffic patterns
General Industry Practice
Based on typical industry practice, parking decks commonly fall in these ranges:
Standard Parking Decks: FL 15-25
FL 15: Acceptable for basic parking structures
FL 20: Common specification minimum
FL 25: Higher-quality installation
Context for Comparison (these are NOT parking deck requirements):
High-tolerance warehouse floors: FL 45-100+
Retail spaces: FL 35-50
Office buildings: FL 25-35
General commercial slabs: FL 20-30
Why Parking Decks Have Lower FL Requirements
Parking decks prioritize different factors:
Drainage is paramount: The intentional 1-2% slope is critical for water management
Vehicle traffic is more forgiving: Cars can easily traverse minor elevation changes
No precision equipment: Unlike warehouses with narrow-aisle forklifts
Cost-effectiveness: Achieving high FL numbers on sloped surfaces is challenging and expensive
What Actually Matters More
For parking decks, focus on:
Consistent slope: Maintaining the design 1-2% grade throughout
No ponding: Water must drain properly with no standing water
Surface finish: Adequate texture for wet traction
FF (Flatness): Short-wavelength variations that could affect drainage or create trip hazards may be more important than FL
Special Considerations for Sloped Surfaces
Challenges with Sloped Deck Measurements
Measuring FL on intentionally sloped surfaces presents unique challenges:
Reference Plane Setup: Must precisely match the design slope
Slope Direction: Measurements perpendicular vs. parallel to slope may differ
Complex Geometry: Multiple slopes or drainage patterns complicate setup
Standard Limitations: ASTM E1155 was primarily designed for relatively flat floors
Best Practices
Clearly establish the design slope vector before measurement
Verify reference plane setup with the structural engineer
Take measurements in both directions (parallel and perpendicular to slope) if specified
Document the reference plane parameters used in calculations
Consider multiple inspection lots if slope direction changes
When FL Testing May Not Be Required
Many parking deck specifications do not require FL/FF testing at all. Instead, they may specify:
Alternative Quality Control Methods
Slope Tolerance Measurements
Verification that slopes fall within ±0.25% to ±0.5% of design
Measured with surveying equipment or laser levels
Visual Inspection
Looking for obvious depressions or high spots
Checking for ponding after rain or water testing
Straightedge Testing
Maximum gap under a 10-foot straightedge (e.g., ¼" maximum)
Simpler pass/fail criteria
Surface Finish Requirements
Texture depth measurements
Visual surface quality standards
Recommendations
Before You Measure
Review your project specifications thoroughly
Look for specific FL requirements
Check if FL/FF testing is even required
Note any acceptance criteria or tolerance ranges
Consult with the design team
Structural engineer of record
Owner's representative
Construction specifications writer
Verify testing requirements
Is third-party testing required?
What documentation is needed?
Are there specific accreditation requirements for testers?
Professional Testing Services
For contractual compliance or dispute resolution, consider:
Hiring a certified floor testing specialist
Ensuring testers have proper equipment and calibration certificates
Obtaining detailed reports with statistical analysis
Using ACI-certified concrete flatness and levelness technicians
If Specifications Are Silent
If your specifications don't mention FL numbers:
Discuss with the owner and engineer what quality control is expected
Consider FL 20 as a reasonable baseline if you need to establish a standard
Focus on functional requirements: proper drainage, no ponding, safe surface
Document actual conditions with photos and basic measurements
Get owner acceptance based on functional performance
Common Issues and Troubleshooting
Low FL Numbers
If measured FL is below specification:
Verify reference plane setup: Incorrect slope input is a common error
Check measurement technique: Ensure proper profiler operation
Review lot boundaries: Consider whether measured area is representative
Assess functional impact: Does the deck drain properly? Are there trip hazards?
Disputes Over Results
Different operators can get slightly different results
Weather conditions during measurement can affect readings
Early-age vs. later measurements may differ due to curling
Have a qualified third party conduct independent testing if needed
Remediation Options
If FL is unacceptably low:
Grinding: Can improve surface in localized areas
Overlays: Self-leveling toppings or thin polymer overlays
Acceptance with documentation: If functionally adequate
Rejection and replacement: In extreme cases only
Documentation and Reporting
What to Include in FL Test Reports
Date and time of testing
Concrete age at testing
Weather conditions
Equipment used (make, model, calibration date)
Reference plane parameters (slope percentage and direction)
Inspection lot locations and sizes
Survey line layout
Number of data points collected
Calculated FL (and FF) values for each lot
Statistical summary
Comparison to specification requirements
Photos of testing in progress and overall conditions
Record Retention
Maintain FL test documentation with:
As-built drawings
Construction quality control records
Building owner's operations and maintenance manuals
Conclusion
Measuring FL numbers on sloped parking decks requires careful attention to reference plane setup and understanding of what the numbers actually mean for your specific application. Always start with your project specifications, consult with qualified professionals when needed, and remember that functional performance (proper drainage, safe surface, durability) is ultimately more important than achieving any particular number.
Key Takeaway: Before investing time and money in FL testing, verify that it's actually required for your project and understand what acceptable values are for your specific situation. When in doubt, consult with the structural engineer and owner.
For More Information:
ASTM E1155: Standard Test Method for Determining FF Floor Flatness and FL Floor Levelness Numbers
ACI 117: Specification for Tolerances for Concrete Construction and Materials
Your project specifications and structural drawings
Disclaimer: This guide provides general information based on industry standards. Always follow your specific project specifications, local codes, and engineer's requirements. When in doubt, consult with qualified concrete professionals or testing laboratories.
