Which screw should I use for decking?
Which screw should I use for decking?
Stainless steel — 304 inland, 316 near the coast — or a screw specifically rated for modern treated timber, which eats ordinary zinc plating.
Timber preservatives changed and fixings had to change with them. When CCA was withdrawn from most residential use, it was replaced by copper-rich systems such as ACQ and copper azole, which contain several times more copper — and copper in contact with zinc or plain steel in the presence of moisture drives galvanic corrosion. Ordinary electro-galvanised screws in modern treated decking can be visibly corroded within a year, and the tell-tale is a black stain bleeding from every screw head. Stainless or a fixing explicitly rated for ACQ-treated timber is the fix. Aluminium is worse still and should not be in contact with treated timber at all, which matters for flashings and joist hangers as much as for screws.
The options, ranked
| Option | Good at | Bad at |
|---|---|---|
| A2 (304) stainless | Corrosion-resistant, will not stain the timber, and unaffected by the copper in modern preservatives. | Softer than steel, so it snaps if driven without a pilot hole in dense timber. |
| A4 (316) stainless | The grade for coastal and pool environments, where chlorides attack 304. | Noticeably more expensive. |
| Coated exterior deck screw | Ceramic or polymer coatings rated for treated timber at a fraction of the stainless price. | The coating is the protection, and it is damaged by the driver bit and by countersinking. |
| Electro-galvanised screw | Cheap. | The thinnest zinc coating of any option; it corrodes within a season in treated timber and streaks the deck black. |
Technique that changes the outcome
- Pilot-drill hardwood and any dense decking; stainless snaps easily
- Keep screws at least 20 mm from board ends to avoid splitting
- Two screws per joist per board, and countersink so heads do not catch bare feet
- Use fixings and hangers of a compatible metal — not aluminium against treated timber
What goes wrong
- Electro-galvanised screws in modern treated timber
- Mixing metals — a stainless screw through an aluminium fitting corrodes the aluminium
- Driving coated screws so hard that the coating is stripped at the head
Compiled practical guidance rather than a published standard. Product formulations vary by brand and market, so read the label — and where the work is structural, electrical or gas, the regulations that apply where you are govern rather than anything here.
Other screw questions
| For | Use |
|---|---|
| Wood | A pilot hole matching the screw's core diameter in the lower piece and a clearance hole in the upper one — that is what makes a joint tight. |
| Masonry | A concrete screw into a precisely sized hole, or a plug and screw — and hollow block needs a different anchor entirely. |
| Plasterboard | Coarse-thread bugle-head drywall screws into timber studs, fine-thread into metal studs — and never use them for anything structural. |
| MDF | A straight-shank MDF screw with a parallel thread, always into a pilot hole — the edge splits without one. |
| Sheet metal | A self-drilling (Tek) screw with a drill point matched to the material thickness, or a self-tapping screw into a correctly sized pilot hole. |
| Chipboard | A twin-thread chipboard screw with an unthreaded shank under the head, so it pulls the joint together instead of bursting the board. |
| Plastic | A thread-forming screw designed for plastic, or a heat-set threaded insert where the joint will be undone more than once. |
| Roofing sheets | A hex-head screw with a bonded EPDM washer, driven until the washer just spreads — no further. |
| Brass hardware | Cut the thread with a steel screw of the same size first, wax the brass one, and use a driver that fits the slot exactly. |
Common questions
- Which screw should I use for decking?
- Stainless steel — 304 inland, 316 near the coast — or a screw specifically rated for modern treated timber, which eats ordinary zinc plating.
- What should I avoid?
- Electro-galvanised screws in modern treated timber Mixing metals — a stainless screw through an aluminium fitting corrodes the aluminium Driving coated screws so hard that the coating is stripped at the head
Related pages
- Which screw and pilot hole should I use for wood? A pilot hole matching the screw's core diameter in the lower piece and a clearance hole in the upper one — that is what makes a joint tight.
- Which screw should I use in masonry? A concrete screw into a precisely sized hole, or a plug and screw — and hollow block needs a different anchor entirely.
- Which screw should I use for plasterboard? Coarse-thread bugle-head drywall screws into timber studs, fine-thread into metal studs — and never use them for anything structural.
- Which screw should I use in MDF? A straight-shank MDF screw with a parallel thread, always into a pilot hole — the edge splits without one.
- Which screw should I use for sheet metal? A self-drilling (Tek) screw with a drill point matched to the material thickness, or a self-tapping screw into a correctly sized pilot hole.
- Which screw should I use in chipboard? A twin-thread chipboard screw with an unthreaded shank under the head, so it pulls the joint together instead of bursting the board.
- Which screw should I use in plastic? A thread-forming screw designed for plastic, or a heat-set threaded insert where the joint will be undone more than once.
- Which screw should I use for roofing sheets? A hex-head screw with a bonded EPDM washer, driven until the washer just spreads — no further.
- How do I drive brass screws without snapping them? Cut the thread with a steel screw of the same size first, wax the brass one, and use a driver that fits the slot exactly.
Sources
- Typical range compiled from manufacturer specifications — At One Place