How to choose the correct screws for decking and outdoor projects

What the homeowner wants to see is a perfect installation, not rust lines and stains formed shortly after installation. How to choose the correct screws for decking and oiutdoor projects is crucial planning information to ensure your project doesnt suffer from damage to wood and steel, Rust dripping onto paving resulting in staining, and fixing failure potentially resulting in permanent damage to the structure which was just installed.

As a consumer, one of the main issues is a lack of information of which product to use combined with a lack of clarity on product packaging regarding levels of corrosion resistance, suitable installation use, and usable life.

Issues using incorrect fixings

Corredere (latin) Corrosion, for Fixings the oxidisation of iron causes rusting.

These are the 4 main types you will need to be aware of when planning your garden construction, whether it is a Garden Building, Decking, Pergola, Fencing, Seating or Garden Furniture, or whatever else it is you are building.

  1. Armosperic Corrosion, the main causes are when the iron in metal is exposed to too much moisture (water) or oxygen Metal corrodes when it reacts with another substance such as oxygen, hydrogen, an electrical current or even dirt and bacteria.
  2. Chemical (Dry) Corrosion occurs on a location where a given metal is under stress or isolated from air circulation. It occurs whenever a gas or liquid chemically attacks an exposed surface, often a metal, and is accelerated by warm temperatures and by acids and salts. This includes Galvanic (Bimettalic) Corrosion, when 2 dissimilar metals are immersed in conductive solution together. Avoid by separating materials, use the same materials, coating both materials, installing a sacrificial anode.
  3. Contact corrosion, metals of different types are in contact with eachother (A2 and Galvanised for example)
  4. Extranneous Rust comes when objects that rust coming into contact with stainless steel. This includes using tools which have previously been used with Carbon Steel which are then used on stainless steel leaving some carbon behind (such as Impact Driver Bits), sparks when using an angle grinder, or water containing rust dripping onto stainless steel.

So, what products are out there? Which products should you buy?

Stainless Steel A2 and A4

  • A2 304 Stainless Steel – interior and exterior uses, kitchens and bathrooms, humid areas, high chrome content
  • A4 316 Stainless Steel, marine grade, – interior and exterior use near salt water in coastal areas, high chrome content
  • Recommendation: There are very situations where A4 is required, unless you live near the sea (or near a chemical factory)

Hot Dipped Galvanised vs Electroplated Galvanised

  • Galvanised via Hot Dipped treatment (zinc with galvanised coating) exterior use
  • Galvanised via electeoplating (bright, shiny, zinc yellow passivated) bad for Redwoods, Spruce, Treated Timber leaving black marks around screw head leading to rust. These are recommended for interior use, not in bathrooms or kitchens.
  • Recommendation: Do not use Electroplated screws for exterior installations. They will rust very quickly.

Ruspert/Silverised vs Painted/Plastic Coated

  • Ruspert (trademarked brand), Silverised exterior use
  • Painted or Plastic Coated Carbon Steel (!) Carbon steel does not have enough chromium to form a chromium oxide layer. This allows oxygen to bond with the iron in carbon, resulting in iron oxide, or rust. 
    • The impact driver bit you use to install them will scratch off the exterior paint or plastic coating and the carbon will be exposed seconds after install them… Just don’t use them. 
    • You will find boxes of 2500 Coated Carbon Steel Decking Screws for £20.00, keep shopping. Note this is a popular screw choice due to its price, it results in lower decking installation quotes. The rust formed on these screws will increase the breakdown of the wood at the cut ends, we see this weekly. Generally the screws are Green or Brown. When removed the entire thread is covered in rust which destroyed the wood around it, specifically Redwood Decking.
  • Recommendation: For tasks such as wood to wood structural fixings look for treatments such as Ruspert, Silverised, C2 (Timco), or Wirox (Spax).

Applications & Head Types: Choosing the correct screws for decking and outdoor projects

Here are the screws we use for Cladding & Fencing & Decking projects:

  • Nails
    • Cladding & Slat Fencing – Lost head nails
    • Featheredge Fencing – Clipped head or Round head nails
      • The issue with lost head is also its advantage. Lost heads are tapped down so they are almost invisible while clipped or round head have a small round circle which is visible. The round head pulls the cladding towards the support batten, the lost head does not. Style-wise the increased demand for hidden fixings does slightly reduce the force holding the cladding in place
    • Nail Shank – ring shank for added holding ability, less nail movement
    • Nail Material – stainless steel or hot dipped galvanised, we use stainless steel wherever possible
  • Screws
    • Cladding – Spax Stainless Steel Fassade installed with a Wera Stainless Steel driver bit
    • Fencing – Jacksons Fencing Screws for Timber Fencing
    • Decking projects – Trex Decking and Millboard decking have their own screws which we use. We also use these screws for Hardwood Decking Projects

Here are the screws we use for Structural Projects such as Pergolas, Carports, Decking Projects:

This is where load bearing is the most important aspect for safety and durability reasons. We use Spax High Force Washer Head or TIMco A4 Stainless Index Screws depending on how exposed the screw head will be to water (TIMco A4 for high, direct exposure, Spax for indirect exposure). We find the Spax screws visibly pull the materials together.

Installation Notes

  • SS is Non-hardened so easier to strip the heads, go slow, do it once, then leave it. Especially with Pan Head screws.  Removing and reinstalling the screws is a mistake.
  • Don’t use impact drivers unless on very low vibration settings with brand new, clean bits which are made of the same materials being installed. High vibration tools make small cuts in the  protective coating on the screw or bolt right at the head which is especially damaging on applications which are horizontal (decking) where the screw head will “catch and hold” water.
  • Extranneous Rust – We use Stainless Steel driver bits to prevent extranneous rust getting onto our Screws. This is when a small amount of rust on the bit gets onto the screw when installed. It also comes from wrenches, pliers, sockets, and spanners. Use stainless tools for stainless fixings.

Other Treatments to be aware of:

Re-galvanising Spray 

If you are cutting galvanised brackets, door latches, etc… First remember this gives off poisonous fumes so wear the correct mask, do it outdoors, and use a hand saw vs an angle grinder or cut off saw. Second, file down the burrs and then respray the cut end with re-galvanising spray. You can get this is a small spray can from most tool and material suppliers, I got mine from Screwfix. Watch your overspray and let it dry before installing.

Lithium Grease

Spraying exterior metal with a Lithium Grease, such as WD-40 White Lithium Grease will not only keep moving parts lubricated but also protect it against rust. Handy for garden tools as well. Again, a small spray can with nozzle from Screwfix will last you a long time.

Powdercoating

This is something you cant do at home. Powdercoating is a method of painting which results in a consistent, strong, protective finish to metal. You often find this used as a finish for Car Parts  or in our case, Exterior Brackets for Fencing, Pergolas, Sleeper Beds and it provides an additional layer of protection to the Galvanised product underneath. You can find specialist companies which will Powder Coat your steel products. Powdercoating is not an alternative to galvanising, but an enhancement to it.

Painting

There are several paints which are specifically designed for steel. Zinnser, Andura, and Hammerite being some examples. Automotive and Marine Grade paints are ideal. These do not last as long as they used to, probably with the removal of hardful ingredients we used to all ignore. Ensure what you are painting is as clean as it can be, free of any flakes or peeling. Use a sander/polisher to fully clean out any pitted areas and paint your object in a clean dry area free from dust. Remember the work area has to remain clean, dry, and free of dust until the paint has set. Car Bodyshops used to spray water on the floor prior to painting to keep dust down, not sure if they still do, but thats what I do.

Resources and Links

British Stainless Steel Association https://bssa.org.uk/

Exterior Screws, Bolts, and Nails

Spax https://www.spax.com/uk/

Timco https://timco.co.uk/

Jacksons Fencing https://www.jacksons-fencing.co.uk/

Stainless Steel Tools & Exterior Brackets

Wera https://www-de.wera.de/en/

S3i https://www.s3i.co.uk/

Simpson Strong Tie https://www.strongtie.co.uk/