#12 vs. #14 vs. #15 Commercial Roofing Screws: Strength Differences and Correct Use Cases
Featured Answer: The difference between #12, #14, and #15 commercial roofing screws comes down to diameter, strength, and application. #12 screws are typically used for insulation attachment, while #14 and #15 screws are used for membrane securement and high-performance assemblies. #15 screws are the strongest, offering the highest pull-out and tensile performance for severe wind uplift conditions [1][2].
Table of Contents
- Introduction
- What Screw Sizes Mean
- Diameter & Strength Comparison
- Pull-Out Resistance Differences
- Tensile & Shear Strength Differences
- Correct Uses for #12, #14, and #15 Screws
- FM & Miami-Dade Requirements
- Why Fastener Selection Matters
- FAQ
- References
1. Introduction
Commercial roofing fasteners play a critical role in resisting wind uplift and maintaining long-term system integrity. Screw size directly affects withdrawal resistance, plate performance, membrane security, and compliance with FM Approvals, Miami-Dade NOA requirements, and IBC Chapter 15 fastening provisions.
The three most common screw sizes—#12, #14, and #15—serve different purposes. Choosing the wrong fastener can compromise system performance or invalidate system approvals.
2. What Screw “Number Sizes” Mean
The numbers (#12, #14, #15) represent the nominal shank diameter: the higher the number, the larger the screw. Larger screws provide more thread engagement, increased pull-out values, and higher tensile capacity [3].
- #12: ~0.216–0.218 in. diameter
- #14: ~0.242–0.246 in. diameter
- #15: ~0.260–0.265 in. diameter
3. Diameter & Strength Comparison
#12 Fastener
Smallest diameter; primarily used for insulation attachment. Not typically used to secure membranes except in specialty assemblies. Provides adequate withdrawal strength for ISO and cover board attachment over steel or wood decks.
#14 Fastener
Mid-size fastener; used widely for membrane securement in mechanically attached systems and can be used for insulation in adhered roofing systems. Offers significantly higher pull-out performance than #12 screws.
#15 Fastener
Largest diameter and the strongest of the three. Used for high uplift zones, corner and perimeter securement, Miami-Dade assemblies, and FM high-performance RoofNav systems.
4. Pull-Out Resistance Differences
Pull-out resistance measures how well a screw resists withdrawal from the substrate. Larger screws generally yield better performance:
- #12: baseline pull-out resistance [1][4]
- #14: ~15–25% stronger than #12, depending on deck gauge
- #15: ~30–40% stronger than #12; highest performance for steel decks [4][5]
5. Tensile & Shear Strength Differences
Tensile and shear strength increase with screw diameter due to higher metal cross-sectional area. As documented in ASTM F606/ISO 898 fastener testing:
| Screw Size | Tensile Strength | Shear Strength |
|---|---|---|
| #12 | Good | Good |
| #14 | Better | Better |
| #15 | Best | Best |
6. Correct Uses for #12, #14, and #15 Screws
| Application | Recommended Screw | Reason |
|---|---|---|
| Insulation attachment | #12 | Cost-effective; adequate pull-out for ISO and cover boards |
| Mechanically attached membrane | #14 | Designed for securing TPO, PVC, and EPDM membranes |
| High wind zones (perimeter/corner) | #14 or #15 | Higher withdrawal values |
| FM-approved high uplift assemblies | #14 or #15 | Required to meet withdrawal resistance in RoofNav assemblies |
| Miami-Dade HVHZ | #15 | Needed for extreme wind uplift conditions |
| Insulation in adhered systems | #14 or #15 | More secure hold prior to adhesive cure |
7. FM & Miami-Dade Requirements
FM Approvals (FM 4470): Substitutions are not permitted unless the alternative fastener meets or exceeds the assembly’s tested performance. Many RoofNav assemblies call for #14 or #15 screws for wind uplift compliance [1].
Miami-Dade NOA: HVHZ assemblies frequently require #15 fasteners due to extreme uplift pressures and increased substrate density requirements [2].
8. Why Fastener Selection Matters
Even the best membrane or insulation board can fail if fastened with inadequate screws. Incorrect fastener selection can cause:
- Plate bending or dishing
- Membrane flutter
- Insulation shifting
- Loss of uplift resistance
- System failure during high-wind events [7]
(Internal link suggestion: Add link to “Essential Roofing Supply FM-Approved Fasteners.”)
9. FAQ
Are #15 screws always the best option?
No. They are strongest but not always necessary. Use them only where the assembly or uplift requirements demand it.
Can you substitute a #12 with a #14?
Only if the assembly allows substitutions. FM prohibits untested substitutions [1].
Do #14 and #15 fasteners damage insulation?
No—plates distribute load properly and prevent crushing.
Which screw is used for membrane fastening?
#14 and #15. #12 screws are generally not used for membrane securement.
10. References
- FM Approvals. Standard 4470 – Roofing Systems.
- Miami-Dade County. Notice of Acceptance – Roofing Fasteners (HVHZ).
- ANSI/ASME B18.6.3. Dimensions of Thread-Forming Screws.
- NRCA. Roofing Manual: Membrane Roof Systems – Fastener Withdrawal Data.
- SPRI & FM Global. Wind Design Guide for Mechanically Attached Roofing Systems.
- ASTM F606 / ISO 898. Mechanical Testing of Fasteners.
- FEMA. Wind-Induced Damage to Roof Systems – Technical Report.