Best Screws for Treated Timber and Decking: Grade and Size Guide
Use stainless steel screws for treated timber if the fixing will stay outdoors. A2 stainless is a sound choice for most inland UK decks, fences and garden structures. Use A4 316 in coastal sites, around salt or chlorinated water, and for high-consequence joints where replacement would be difficult. Choose a screw long enough to put at least 35 to 40 mm of sound thread into the supporting timber after passing through the top board.
The screw has to survive two environments at once: rain on the outside and a damp, chemically treated piece of timber around the shank. A bright plated indoor screw may look fine on day one, then corrode below the surface where you cannot inspect it.
Why treated timber is hard on ordinary screws
Pressure treatment protects wood from decay, but the treatment salts and retained moisture can make the timber more conductive. That increases the corrosion rate of unsuitable metal fixings. The wood itself can also release organic acids. Oak, western red cedar and Douglas fir are well-known examples, especially while wet.
Corrosion below the board is easy to miss. The first visible warning may be a dark stain, a loose board or a screw head that snaps when you try to remove it. The real cost is not the failed screw. It is lifting boards, repairing split ends and rebuilding a section that should have lasted.
Austenitic stainless steel is widely specified for timber fixings because it tolerates wet wood and common treatments much better than an indoor-plated fastener. For more detail on grade chemistry, see A2 vs A4 stainless steel.
A2 or A4 stainless for treated timber?
| Exposure | Practical grade | Why |
|---|---|---|
| Indoor dry joinery | A2 | More corrosion resistance than the job normally needs, with a clean finish. |
| Inland UK deck, fence or shed | A2 | Good general resistance to rain, damp timber and normal outdoor exposure. |
| Within 5 miles of the coast | A4 316 | Molybdenum improves resistance to chloride pitting from salt-laden air. |
| Jetty, boat, poolside or road-salt exposure | A4 316 | Direct chloride exposure is where A4 earns its cost. |
| Load-bearing or regulated structure | Engineer-specified | Material grade alone does not establish structural capacity. |
A4 is not automatically stronger than A2. The marking after the grade gives the property class for bolts, such as A2-70 or A4-80. Wood screws are selected by their own product data, geometry and intended use. If the connection is structural, use a fastener with the required declared performance and follow the design specification.
Choose the screw type before the size
Countersunk wood screws
A stainless countersunk wood screw is the everyday choice for decking boards, battens, trim and light garden joinery. The head finishes close to the surface and the coarse thread grips timber. Do not bury the head deeply. Crushing the wood fibres creates a water pocket and weakens the top board.
Coach screws
Use a hexagon coach screw where you need a larger diameter and a spanner-driven head, such as post brackets, gate hardware and heavier timber members. A washer under the head spreads the bearing load. A coach screw is not a substitute for a designed structural bolt connection, but it is far more suitable than a small decking screw for chunky outdoor hardware.
Bolts through timber
For joints that need clamping from both sides, use a bolt, nut and washer rather than relying on thread in the wood. Stainless hexagon bolts and large washers make the connection inspectable and serviceable. Protect the thread from galling during assembly, particularly in A4. Our stainless fastener galling guide gives the method.
Decking screw size guide
The screw must pass through the board and still leave useful thread in the joist. Ignore the pointed tip when judging effective engagement. The tip helps entry but carries little pull-out load.
| Job | Typical starting size | Check before buying |
|---|---|---|
| 19-22 mm decking board to joist | 4.5 or 5.0 x 50 mm | Aim for roughly 30 mm or more into sound joist after the board. |
| 25-28 mm decking board | 5.0 x 60 or 65 mm | Allow for any gap, spacer or counterbore. |
| Fence board to rail | 4.0 or 5.0 x 40-50 mm | Avoid breaking through the rear of the rail. |
| Timber bracket or post hardware | 6-10 mm coach screw | Use the bracket holes, required embedment and load data. |
A useful non-structural rule is to have the screw penetrate the supporting member by at least 1.5 times the top board thickness, provided the screw will not emerge from the far side. Dense hardwood may need a different diameter, pilot and thread pattern.
When to drill a pilot hole in timber
Drill a pilot hole near board ends, in hardwood, with larger coach screws and anywhere splitting would be expensive. The pilot should normally follow the screw's solid core rather than the outside thread diameter. Use the actual screw as a visual check because thread geometry varies.
- Softwood, away from an edge: many smaller wood screws can be driven without a full-depth pilot, but a clearance hole through the top board improves clamping.
- Hardwood or dry timber: drill a pilot close to the screw core diameter and consider a clearance hole through the top component.
- Coach screws: drill a full-depth pilot. Use a larger clearance hole through any bracket and place a washer under the head.
- Near an end or edge: pilot drilling is cheap insurance against splitting.
Keep the driver square and stop when the head is seated. High speed and overdriving damage the recess, heat the fastener and crush the timber. The correct driver is easier to identify with the fastener head and drive glossary.
Five mistakes that shorten deck life
1. Using indoor screws outdoors
A yellow or bright plated screw is not automatically suitable for treated timber. Check the coating and application declaration, not the colour.
2. Choosing A2 beside the sea
Salt-laden air can reach well inland. Use A4 for anything within 5 miles of the coast, and for direct salt or pool exposure. Browse the Simfix marine collection for chloride-resistant options.
3. Fixing freshly treated, dripping timber
Let treated timber drain and condition as directed by its supplier. The British Stainless Steel Association notes that CCA-treated timber should be aged for at least seven days before stainless fixings are installed. Follow the treatment supplier where its waiting period is longer.
4. Mixing incompatible metal hardware
A stainless screw through a galvanised bracket can create a bimetallic couple when wet. The risk depends on metal area, electrolyte and exposure. Keep hardware systems compatible and isolate dissimilar metals in aggressive sites. The principles are explained in our galvanic corrosion guide.
5. Treating every joint as cosmetic
Deck boards, handrails, ledger connections and structural posts do not carry the same consequence. For safety-critical timber work, use the specified structural fastener and design, not a general-purpose wood screw selected from appearance alone.
Cost over the life of the job
Stainless screws cost more per box than basic plated screws. The comparison changes once labour is included. Replacing corroded deck screws means clearing the area, finding heads hidden by dirt, extracting broken shanks and potentially damaging boards. Spending a few pounds more on the correct grade is usually cheaper than one repair visit.
Continue through the stainless fixing series
A practical treated-timber set
- 5.0 x 50 mm A2 countersunk wood screws - a useful starting size for common inland decking and garden joinery.
- 5.0 x 40 mm A2 countersunk wood screws - shorter fixings for thinner boards, trims and fence work.
- 6 x 40 mm A2 coach screws - a spanner-driven option for light brackets and outdoor hardware where the dimensions suit.
Frequently asked questions
What screws should I use for pressure-treated wood?
Use screws specifically suitable for treated timber. Austenitic stainless steel is a dependable option. A2 suits most inland UK outdoor work, while A4 316 is better near the coast or chlorides.
Is A2 stainless steel suitable for decking?
Yes for most inland UK decks with ordinary rain exposure. Use A4 316 within 5 miles of the coast, near pools, or where salt spray and de-icing salts are present.
What length screw for 22 mm decking?
A 50 mm screw is a common starting point because it leaves roughly 28 mm below a 22 mm board. Check the point, any gap and the board supplier's fixing specification.
Should I pre-drill treated timber?
Pre-drill near ends and edges, in hardwood, and for larger screws. A clearance hole through the top board also helps the head pull the board firmly to the joist.
Can I use stainless screws with galvanised brackets?
Sometimes, but wet contact between dissimilar metals can cause galvanic corrosion. Use a compatible system or isolate the metals where exposure is persistent or saline.
Next step: record the timber thickness, required embedment and exposure before ordering. Then select by diameter and length in the decking, garden and timber collection.
