I'm Alfie Jaques, a City & Guilds-qualified carpenter based in Bournemouth (BH2), and I build decks and pergolas across Poole, Christchurch and about 10 miles around. Every quote I give is written and itemised after a free visit to the garden.
Screws are the cheapest part of a deck and one of the first things to let it down. Rust streaks running from the screw heads and boards that start to lift: on a garden near the sea, a lot of that comes back to the grade of metal. Here's what the grades mean, why salt air matters so much in this part of Dorset, and what to check on a quote.
What's the difference between A2 and A4 stainless steel?
A4 contains molybdenum, which gives it better resistance to salt. A2 doesn't.
A2 and A4 are the grades stamped on stainless fixings. The Steel Construction Institute puts it like this: "Steels of grade A2 have equivalent corrosion resistance to grade 1.4301 (304). Steels of grade A4 contain molybdenum and have equivalent corrosion resistance to grade 1.4401 (316)."
That molybdenum is the whole point. Marine-grade stainless alloys contain it "to resist the corrosive effects of NaCl or salt in seawater", because chloride from salt is what causes small pits and crevices to form in stainless that doesn't have it. Grade 316 has greater resistance to that pitting than grades without molybdenum.
Does salt air reach my garden if I can't see the sea?
Often, yes. The wind carries it inland, and it's at its worst near the shore and the harbours.
Bournemouth's prevailing south-westerly wind carries sea-salt aerosol off the water. Once it settles on metal in humid air it starts corroding anything that isn't made for it, and it dries and greys exposed timber faster than in an inland garden. It bites hardest in places like Sandbanks, Southbourne, Alum Chine, round the edge of Poole Harbour and along Christchurch Harbour at Mudeford.
UK stainless stockist Aalco names grade 316 as "the optimal choice of stainless steel grade" for coastal areas. So much of the patch I cover sits close to the sea or one of the two harbours that A4 marine-grade fixings are what I price for decks here. Further inland, around Ferndown or Wimborne, it's more of a judgement call, and I'll say which I'd use and why on the quote. My pages on decking in Poole and decking in Christchurch cover the harbour-side gardens in more detail.
What is nail sickness on a deck?
It's the trade name for fixings that rust and lose their grip on the timber.
You'll see it as rust stains spreading from the screw heads, screws that have gone loose in their holes, and boards that rock or lift at the ends. The metal corrodes, the hole round it gets wet and soft, and the fixing stops holding. In salty air it happens faster with ordinary steel or plated screws than with the right grade of stainless.
If your deck is showing it, look at the frame underneath before deciding anything. Loose boards on a sound frame can often be re-fixed. Rust on the joist hangers and bolts underneath is a bigger conversation.
Is it only the screws on top that matter?
No. Every bit of metal in the deck sits in the same air, including the parts you can't see.
Joist hangers, bolts, brackets and post anchors all sit under the boards, out of sight. If those corrode, the frame weakens with no sign on the surface until it moves. On a coastal deck I use marine-grade stainless for the fixings, on a frame of Use Class 4 treated timber, which is the standard for timber in contact with the ground or kept wet. My composite vs timber guide covers how the boards themselves cope with salt.
When you compare quotes, ask what grade every fixing is, top and bottom. A quote that only says "stainless screws" hasn't told you which one. For the rest of what drives the price, see what decides the price of a new deck, and if you're adding a pergola, the same applies to its footings and post fixings.