A guide to using the correct screws for your timber deck.

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Timber Screws

With so many screws out there, each with its own unique features, it can be confusing to know which ones to choose. It is vital to choose the correct screws for the correct applications to ensure your deck remain strong and durable.

Screws come in many types with different thread patterns and head shapes, varying in length and purpose. Each type serves a specific function. Despite their seemingly simple appearance, screws have intricate features and uses. An important factor to consider is that the screws you choose should be able to handle the environment it is used in. Using the wrong screws in certain environments can lead to serious problems like structural failure.

With this in mind, you might ask, how does one select the appropriate screws for a certain project such as a deck?

In this blog, we will discuss some important facts you need to know when choosing the correct kind of screw.

Understanding timber screws

When dealing with screws, the main worry is preventing corrosion, which can result from factors like wood moisture, humidity, saltwater, chemicals, and weather. To aid in choosing the right screws for particular tasks, like building a timber deck, a classification system was created. It takes into account three crucial factors: Atmospheric Corrosivity Classes, Service Classes, and Wood Corrosivity Classes.

Let’s discuss it in more detail:

Atmospheric corrosivity classes for screws

Atmosphere corrosivity Classes

Atmospheric corrosivity classes categorize the corrosion risk based on factors like humidity, air pollution, chloride content, and whether the connection is indoors, outdoors shielded, or outdoors exposed. Here’s how the categories are defined:

  • C1: Suitable for use in dry environments without exposure to chloride and very low pollution levels, like deserts and central arctic regions.
  • C2: Appropriate for relatively dry settings (more than 10km from the coast), such as small towns and rural areas with minimal pollution.
  • C3: Designed for use in environments with low humidity (3km – 10km from the coast), such as urban and industrial areas with moderate pollution.
  • C4: Intended for use in environments with high humidity (0.25km – 3km from the coast), like urban and industrial areas with a high pollution level.
  • C5: Reserved for environments with extremely high humidity (less than 0.25km from the coast), such as industrial areas with severe pollution.
Service classes for screws

Service Classes.

The service classes relate to the temperature and humidity levels where a wooden structure is situated, linking these conditions to the moisture content within the wood. Here’s how they’re categorized:

  • SC1: Suitable for use in insulated and conditioned buildings.
  • SC2: Intended for use in sheltered conditions (not directly exposed to rain or precipitation) in uninsulated and unconditioned structures.
  • SC3: Designed for use on elements directly exposed to weather but not constantly submerged in water.
  • SC4: Reserved for elements immersed in soil or water, such as foundation piles and marine structures.
Wood corrosivity classes for screws

Wood Corrosivity Classes.

Corrosion caused by wood varies based on factors such as wood species, treatment, and moisture content, affecting only the part of the connector inserted into the wood. Wood is classified based on its average moisture content, with certain levels contributing to metal connector corrosion. Here’s the breakdown:

  • T1: Suitable for screws used on untreated or treated wood (regardless of pH level) with a moisture content of 10% or lower, primarily for indoor applications.
  • T2: Suitable for screws used on untreated or treated wood (regardless of pH level) with a moisture content higher than 10% to 16%, specifically for outdoor protected applications.
  • T3: Suitable for screws used on untreated wood only (with a pH level of less than 4) with a moisture content of 16% to 20%, designed for outdoor use.
  • T4: Suitable for screws used on any untreated or treated wood (with a pH level of 4 and over) with a moisture content of 16% to 20%, suitable for outdoor applications.
  • T5: Suitable for screws used on any untreated or treated wood (with a pH level of 4 and over) with a moisture content of less than 20%, specifically for outdoor applications immersed in soil or water.

Types of materials for timber screws

The type of material used in screws greatly influences their core strength, resistance to corrosion, and overall performance. Here’s a breakdown of the two main types:

Stainless Steel Screws:

Made from an alloy of steel containing at least 10.5% chromium by mass, stainless steel screws form a passive layer of chromium oxide on their surface. This layer acts as a shield, preventing corrosion and staining. Stainless steel screws are highly resistant to rust and corrosion, making them ideal for environments where moisture or corrosive elements are present, such as outdoor construction projects and marine applications. an example would be the Countersunk Deck screw.

Stainless steel screws

Carbon Steel Screws with EVO Coatings:

Carbon steel screws, composed of iron and carbon, are typically more affordable than stainless steel screws but are prone to corrosion. To improve their corrosion resistance, carbon steel screws can be coated with various materials, with one common option being EVO (Electrodeposition Coating). EVO coatings create a protective layer over the carbon steel, shielding it from corrosion and extending the screw’s lifespan. These coated screws are often chosen for applications where cost is a consideration, but corrosion resistance remains crucial. An example would the Cone-shaped counter head deck screw. This type of screw is available in five basic colours.

EVO coated carbon steel screws

Most recommended screws for timber decks

Understanding the key factors and recommended choices can simplify the decision-making process for purchasing screws, especially because there is such a variety. Let’s explore the top recommendations for screws to use in timber deck construction, ensuring both structural integrity and aesthetic appeal.

Countersunk screw

Countersunk Screws

A countersunk screw is a type of screw with a conical shape head that’s designed to be flush with or below the surface of the material it’s driven into. It is often used for aesthetic or safety purposes. There two types of countersunk screws namely:

SCI HCR Countersunk Screws

This stainless steel screw, is top-notch when it comes to resisting corrosion. It’s rated in the highest corrosion resistance class (HCR), making it ideal for enduring tough environments. Specifically, it can withstand high levels of atmospheric corrosion (C5) and wood corrosion (T5).

What sets it apart is its composition. It contains high amounts of molybdenum and nickel, which are key in providing maximum corrosion resistance. Additionally, the presence of nitrogen ensures that it maintains excellent mechanical performance over time. So, you can count on this screw to hold up well, even in challenging conditions.

SCI A4|AISI316 Countersunk Screws

This screw has a special thread design, called an asymmetrical umbrella thread. This fancy name describes an uneven thread pattern that provides superior grip compared to regular screws.

Another interesting feature is the elongated reamer cutter. Imagine a tiny drill bit at the very tip of the screw. This cutter helps create a small pilot hole before the threads engage with the wood. This pre-drilled hole makes screwing the screw in much easier, especially in dense or hard materials.

the screw has under-head cutting ribs. These are little bumps located just below the screw head. As you screw it in, these ribs help cut into the wood fibers, creating a cleaner and tighter fit.

All these features come together to create a screw with superior torsional strength. In simpler terms, this screw can handle a lot of twisting force without breaking.

It is made from a high-grade stainless steel known as AISI316. This type of steel is incredibly resistant to rust and corrosion, making it perfect for use in harsh environments.  Think salty sea air, constantly wet decks, or even very acidic wood – this screw can handle it all.

It is ideal for outdoor use in highly aggressive environments. Suitable for wooden boards with a density of < 470 kg/m3 (without pre-drill) and <620 kg/m3 (with pre-drill).

Cone shaped concealed head screw

Cone-Shaped Concealed head Screws

This screw, as the name suggests, has a head designed to be hidden within the material it’s screwed into. The head itself is not flat, but rather tapers down to a point resembling a cone. This cone shape allows the screw head to sink below the surface of the material, creating a smooth and aesthetically pleasing finish.

Cone Shaped Concealed Head Screws

It’s made from a special type of stainless steel (AISI316) that’s incredibly resistant to rust. It is specifically designed to excel in challenging environments. Whether you’re working with wood that’s been treated with chemicals, or in areas with extremely high moisture levels (like a constantly wet deck), this screw can handle it.

It has a unique under-head thread that turns in the opposite direction (left-handed) compared to regular screws. This special feature helps the screw dig into the wood more effectively, creating a superior grip that won’t loosen over time.

These crews are available in two materials namely:

KKT A4 | AISI316 – The stainless steel version is ideal for very aggressive environments, for acidic, chemically treated wood and very high internal moisture (T5).

These screws can be used for outdoor applications with highly aggressive environments. Suitable for wooden boards with a density of <550 kg/m3 (without pre-drill) and <880 kg/m3 (with pre-drill).

KKT Colour -This is the carbon steel version screw with coloured anti-rust coating (brown, grey, green, sand and black) for outdoor use in service class 3 on non-acid timbers (T3).

These screws can be used for outdoor applications. Suitable for wooden boards with a density of < 780 kg/m3 (without pre-drill) and < 880 kg/m3 (with pre-drill).

Are you in the process of building a deck?

Here at Sabie Poles, we don’t just supply top quality Poles and Timber products, we are your one stop for all the materials you need to successfully complete your project. Thus, if you are building a deck, we have you covered with quality decking boards and deck screws. We have the following decking available.

Pine deck with pool

Softwood Decking

We supply Pine Wood decking. This timber product is a more affordable option. It is pressure treated with CCA according to  SANS (South African National Standards) 10288 regulations. This treatment method enhances its durability extensively and makes it suitable for use in most environments, especially on the coast. To learn more about our softwood decking products, please click on the following link: Pine wood decking

Hardwood deck

Hardwood Decking

The density of our hardwood decking ranges between 800 kg/m3 and 1300 kg/m3. They are highly durable and naturally resistant to decay and insects damage and further treatment is optional. Both Countersunk Screws and Cone-Shaped Concealed head Screws work perfectly well on these decking boards however, pre-drilling is required. To learn more about our hardwood decking products, please click on the following link: Hardwood Decking

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