Hot-Dip Galvanized or Stainless — and Nothing Else
On a timber dock built from modern pressure-treated lumber, every fastener and connector should be hot-dip galvanized or stainless steel. That is not our preference; it is the stated position of the American Galvanizers Association, a trade body with every reason to favour zinc, which still names stainless alongside it.
The reason is the wood. The AGA records that wood preservative formulations were revised in 2003, and that the change increased the corrosivity of the wood when in contact with metal parts. The two most common treatments, alkaline copper quaternary (ACQ) and copper azole (CA), are both active corrosion materials — copper is in the name. Against them, the AGA recommends only two systems:
Only two corrosion protection systems are recommended for use with these pressure treatment chemicals: hot-dip galvanized steel and stainless steel.
The distinction that matters at the store is hot-dip. “Galvanized” on a box is not the same claim. The AGA’s point is that hot-dip galvanizing lays down a thicker layer of zinc than other zinc-coated fasteners, and the ASTM specification for hot-dip zinc on hardware is ASTM A153. A bright, electro-plated screw driven into treated decking is a short-lived decision on a structure meant to last decades — and, as the next section explains, nobody can honestly tell you how short.
Why We Will Not Tell You How Many Years Galvanizing Lasts
Because the people who make it will not either. The AGA states plainly that there is no simple rule governing the corrosion rate of zinc in fresh water: the rate is controlled largely by impurities in the water, and relatively small differences in water conditions produce substantial changes in rate. Any contractor quoting you a lifespan for galvanized hardware on a lake is quoting a guess.
What the AGA does say the data support is one trend: hard water is much less corrosive to zinc than soft water. In moderately hard or hard water a scale of calcium carbonate and basic zinc carbonate forms on the galvanized surface and slows the corrosion down.
That is not New Hampshire’s water. In its 2017–2019 survey of 50 randomly selected lakes, NHDES found that only 19.6% met its expected acid neutralizing capacity of 20 mg/L, with 80.4% below it. Buffering is not the same measurement as hardness, and we will not pretend the survey gives a hardness reading for your lake. But it describes a state whose lakes are, overwhelmingly, poorly buffered — which is not the water in which you should count on the protective scale the galvanizers describe.
The cold, by contrast, is not the enemy people assume. The AGA states that hot-dip galvanized coatings show no significant difference in corrosion rate at temperatures below −40 °F. Whatever else a New Hampshire winter does to a dock, the cold itself is not what corrodes the hardware. The water is.
Stainless, Aluminium and Galvanized in the Same Joint
The obvious upgrade — stainless bolts through a galvanized bracket — is not automatically an upgrade. The AGA reproduces a British Standards Institution table rating how much additional corrosion galvanized steel suffers when it touches another metal. Immersed in fresh water, contact with austenitic stainless steel is rated 0 to 2, where 2 means fairly severe additional corrosion with protective measures usually necessary. Aluminium is rated 1: slight or moderate, tolerable in some circumstances. Copper is 1 to 2.
Three practical consequences follow, and they are the ones we apply on site:
- Keep a joint to one system where you can. Stainless fasteners through stainless brackets, galvanized through galvanized. Where a mixed joint is unavoidable, the AGA’s own advice for very humid conditions is electrical isolation between the metals.
- Aluminium docks change the question. On an aluminium seasonal dock the frame is the large metal in every joint, and galvanized hardware against it sits at a rating of 1 in fresh water. Use the hardware the dock’s manufacturer specifies rather than whatever fits the hole.
- Copper runs downhill. The AGA warns that even runoff from copper or brass surfaces can carry enough dissolved copper to cause rapid corrosion of zinc. Brass ladder fittings and copper-capped posts drip onto whatever sits below them.
The Waterline Is a Permitting Line for Hardware Work
Swapping galvanized hardware for stainless is a change of materials, and in New Hampshire when you do it matters as much as what you fit. NHDES publishes an exemption for changing only construction materials on an existing legal dock, and it holds only if all three conditions are met: the dock’s size, location and configuration are unchanged; no work is done on any portion of the structure below the water surface at the time of repair; and no work is in a tidal area.
For a seasonal dock that settles the timing. Re-hardware it on the bank in autumn, after the pull, and it is above water by definition. Do the same job in July with the legs standing in the lake and the below-surface fittings are outside the exemption.
On a permanent pile or crib dock the below-water connections never come out. Work on them is not a materials-only change, and in-kind repair of an existing legal dock is instead classified as a minimum impact project under Env-Wt 513.25 — unless the dock holds a current voluntary registration, in which case repair of a duly registered non-tidal dock is among the statutory exemptions (Env-Wt 308.02(j)). That registration lasts five years or until the property changes hands, whichever comes first. The permit wizard’s repair branch walks through it.
And one rule catches people who only meant to tidy up. Env-Wt 513.22(b)(4) requires a seasonal dock to be removed from the water before any paint, stain or other preservative coating is applied, and kept out until it is dry. A can of zinc-rich touch-up on scratched galvanizing is, on a plain reading, paint.
Cheap Hardware Is a Short Decision on a Long Structure
The cost of a fastener is almost never the fastener. It is the labour and access needed to reach it again — and on a lake, reaching it can mean a dive, a barge, an ice-damaged structure or a permitting conversation. So we spend the stainless where the next visit would be expensive, and we do not spend it where it would not:
- Stainless: connections at or below the waterline, pile and crib connections on permanent docks, lift and canopy attachment points, and ladder and railing fixings — the ones people put their weight on without thinking.
- Hot-dip galvanized: framing hardware above the water on a seasonal dock that comes up on the bank every autumn, where every fitting is inspected and replaceable each year.
- Never: bright, electro-plated or unmarked fasteners anywhere in treated lumber.
The autumn pull is when this is cheapest to put right, because the whole dock is in the dry and photographed. Our winter preparation guide covers the rest of that visit, and the ice damage guide covers what the spring inspection finds when it was not done.
Questions
Galvanized or stainless for my dock?
Either, but only those two in treated lumber. Hot-dip galvanized for above-water framing you can reach and replace; stainless where the connection is hard to reach, at or below the waterline, or would need a permit to get at later.
Are ordinary zinc-plated deck screws good enough?
No. The galvanizers’ own trade association recommends only hot-dip galvanized or stainless with modern copper-based treated wood, because hot-dip carries a thicker layer of zinc. Look for ASTM A153 on hot-dip hardware.
Can I mix stainless and galvanized?
With care. Galvanized in contact with stainless, immersed in fresh water, is rated 0 to 2 for additional zinc corrosion in the BSI table the AGA reproduces. Keep a joint to one system, or isolate the metals.
Do I need a permit to replace dock hardware?
Often not, if the work is done above the water surface on an existing legal dock with the footprint unchanged. Below-water work falls outside the materials-only exemption. This is a summary, not legal advice; NHDES is the authority.
Can I touch up the dock while it is in the water?
Not a seasonal dock. Env-Wt 513.22(b)(4) requires it to come out of the water before any paint, stain or preservative coating is applied, and stay out until the coating is dry.
Treated-wood, fresh-water, low-temperature and dissimilar-metal positions per the American Galvanizers Association, Performance of Hot-Dip Galvanized Steel Products, including its Figure 15 (after BSI PD 6484:1979, Table 23); lake buffering per NHDES, Summary of New Hampshire’s Lakes, Ponds and Reservoirs; coating and repair classification per Env-Wt 513.22 and 513.25 and Env-Wt 308.02(j); materials-only exemption per the NHDES Docks in Non-Tidal Areas FAQs. Retrieved 8 October 2026. All figures with their sources: our NH dock permits reference.
Orientation, not legal advice. Whether a particular repair is exempt depends on the dock’s legal status and the work actually done. NHDES administers these rules and is the authority.