Screw Coatings Explained: Galvanized vs Stainless vs Coated (Which to Use Outdoors)

Using the wrong screw coating outdoors is one of the most common and expensive mistakes in residential construction. Standard zinc-plated screws corrode within one season in ACQ pressure-treated lumber. Here is exactly which coating you need for every application.

Screw Coating Comparison Chart

Why ACQ Pressure-Treated Lumber Corrodes Standard Screws

ACQ (Alkaline Copper Quaternary) treatment replaced CCA (chromated copper arsenate) in 2004. ACQ contains significantly more copper than CCA — and copper is highly corrosive to standard zinc coatings. A standard zinc-plated screw in ACQ lumber will show significant corrosion within one season and structural failure within 2-3 years.

The same applies to CA (copper azole) treated lumber. Any pressure-treated lumber sold since 2004 requires ACQ-rated, hot-dipped galvanized, or stainless fasteners.

Hot-Dipped Galvanized vs Electroplated

These are not the same despite both being "galvanized." Electroplating applies a thin zinc layer (0.2-0.3 mil) via electrical process. Hot-dip galvanizing dips the fastener in molten zinc producing a coating 1.0-2.3 mil thick — 5-10x thicker than electroplating. Only hot-dipped galvanized (HDG) is rated for ACQ lumber contact.

Stainless Type 304 vs Type 316

Type 304 is the standard grade for most outdoor applications — decks, fences, exterior trim. Type 316 adds molybdenum which dramatically improves resistance to chloride corrosion. Use Type 316 for anything within 1 mile of saltwater, in swimming pool enclosures, or any application involving road salt exposure.

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Where to Buy Stainless & Galvanized Screws

Canada:

United States:

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Frequently Asked Questions

What is the difference between galvanized and stainless screws?

Galvanized screws are steel coated with zinc - either electroplated (thin, not for outdoor use) or hot-dipped (thick, rated for ACQ lumber). Stainless screws are an alloy through their entire cross-section with no coating to wear off. For most outdoor construction hot-dipped galvanized is adequate. For coastal environments use stainless Type 316.

Can I use regular screws in pressure-treated wood?

No. ACQ and CA pressure-treated lumber contains high levels of copper which is highly corrosive to standard zinc-plated fasteners. A standard screw will show visible corrosion within one season and structural failure within 2-3 years. Use only ACQ-rated polymer-coated, hot-dipped galvanized, or stainless steel fasteners in any pressure-treated lumber sold since 2004.

What is the difference between Type 304 and Type 316 stainless screws?

Type 304 stainless is the standard grade for most outdoor applications. Type 316 adds molybdenum which dramatically improves resistance to chloride corrosion from salt. Use Type 316 for any application within 1 mile of saltwater, swimming pool enclosures, or areas with heavy road salt exposure.

What does hot-dipped galvanized mean?

Hot-dipped galvanizing dips the fastener in molten zinc producing a coating 1.0 to 2.3 mil thick - 5-10 times thicker than electroplating. Hot-dipped galvanized is rated for ACQ pressure-treated lumber contact. Electroplated or bright zinc screws are not rated for ACQ contact despite also being called galvanized.

Which screw coating is best for outdoor use?

For general outdoor use in non-coastal climates ACQ-rated polymer-coated or hot-dipped galvanized screws are the best value. For coastal environments use stainless Type 316. Never use standard bright zinc or electroplated screws outdoors - they corrode within 1-2 seasons.

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