Questions about decking screws

ESSVE answers frequently asked questions about decking and decking screws.

Does the size of the deck have any significance for the choice of fastener?

Yes and no! If you build the deck correctly from the start, with a stable framework and the recommended spacing, the answer is no. But if you cut corners with this, there may be greater forces in play in a larger timber deck, as the wood moves over longer lengths. If you want extra security in your structure, stainless steel is the better choice.

There are also other factors to take in to consideration. Movements in a large decks may be greater, for example if you attach one side to the wall of the house while the rest of the deck is only supported on ground slabs or footings that don’t reach down to frost-free depth. In wintertime, the parts resting on the frozen ground will experience an increase in height, while the height of the side attached to the house remains unchanged – it’s important to make allowances for this!

How does the height of the deck affect the choice of fastener?

You can build a high yet stable deck, but there will be a greater risk of movements if a deck is built high off the ground. If you install a timber deck on high posts, we recommend that you use stainless decking screws due the greater risk of movements.

The same recommendation applies to timber decks where the load-bearing structure rests directly on the ground, in order to minimise the risk of corrosion.

A2, A4 or CorrSeal, which quality should I choose?

It is important to choose correct quality of the decking screws according to where and how you are building your timber deck. Make sure you always check the recommendations from your timber supplier. Below is a brief recommendation.

Stainless A4

Stainless A4 screw is made of unhardened acid-proof steel with the highest corrosivity class C5. This screw can be used for all wood decking structures, but should definitely be used in aggressive environments, e.g. by pools and jetties, and for timber decks in coastal locations near salt water. Stainless A4 screws are also recommended if you’re building a deck with tropical hardwoods.

Stainless A2

Stainless A2 is made of unhardened steel with corrosivity class C4. We recommend this screw for structures near lakes or brackish water.
We also recommend it for railings, fences and steps for your wood deck, since it provides good resistance to corrosion and movements.


ESSVE’s CorrSeal surface treatment with corrosivity class C4 is an alternative to the anti-corrosion surface treatments A2 and A4. This surface treatment offers good corrosion protection (rust prevention) – well suited to our Nordic climate. You can use CorrSeal-treated screws for stable and low decking structures in environments not considered to be aggressive. 

To sum up: a stainless screw is not hardened, which makes it tougher. This means it can absorb movements in the wood or structure better than standard hardened screws.

Which distance should I have between decking boards?

Timber is a hygroscopic construction material. It senses the humidity and temperature of the ambient air, and constantly strives to harmonise with the ambient climate. The moisture ratio not only varies between individual pieces of wood in a batch of timber – it also varies in the cross-section of a piece of timber. Timber dries from the outside in. If no special measures are taken, this means that the inner parts of the piece of timber will be considerably moister than its surface after drying at the sawmill. When drying, the free water first leaves the cell cavity of the fibres.

When the moisture ratio then reaches fibre saturation, at approx. 30% moisture ratio, the moisture in the cell walls begins to leave the timber, and this is one of the reasons the timber begins to shrink. The timber begins to shrink from the outside in. This means that the decking you install will ‘move’ constantly throughout the year and the size of these movements will depend on the amount of water the deck is exposed to. This explains why you need to leave a gap between each decking board. 

To sum up: the decking you install will ‘move’ constantly throughout the year and the size of these movements will depend on the amount of water and the temperatures the deck is exposed to. This explains why you need to leave a gap between each decking board. 

How do I prevent fasteners from breaking?

It’s very important to follow recommendations from national policy makers when building your deck, so you do not include risks of movement in the build which the fasteners are unable to tolerate. Important aspects include distance and closeness between underlying joists and binders.

It is also important to consider the environment in which the screws will sit, as well as the load the structure will carry. For example: a deck designed for standard use with outdoor furniture and barbecue is not necessarily capable of holding a jacuzzi or pool.

Who can I ask if I have other questions?

If you have any questions about decking screws or our other fasteners, please contact our local ESSVE distributor. The staff there have the backing of the expertise of our product developers. They’ll join forces to find answers to your questions!

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