Decking Tips and Tricks

Once again, we are coming into another decking season. Unfortunately, the increased demand is associated with an increase in decking problems.

Accordingly, it’s fitting to provide an overview of the decking products available on the market, along with a thorough evaluation of each product’s pros and cons. The aim is to prevent post-installation problems from arising.

Ensure there is good sub-deck drainage

The pooling of water beneath the sub-floor can cause the sub-deck timbers to absorb moisture.

Protect the joists with a waterproofing product

This helps keep the joists at a constant moisture content. It also improves board durability and screw fixing holding in winter.

Plan decking board gaps carefully

For 90mm boards, maintain gaps of minimum 4mm, and for boards exceeding 130mm, aim for gaps of 6mm to 7mm. This prevents boards from expanding too tightly in winter or contracting excessively in summer.

Treated pine joists

Treated Pine is a softwood and decking should be fixed with a 65mm length screw. 50mm can be used for structural hardwood joist.

Before I go into some detail about the various decking species on the market, I should explain some of the terms I refer so you will have a better understanding of the pros and cons of the products below.

Shrinkage Rates

Shrinkage can be radial or tangential. Radial shrinkage is less than tangential shrinkage within the same wood species, see diagram for reference. In most Australian species, the resource is regrowth trees where the log diameter is relatively small, meaning the shrinkage criteria is tangential.

Sapwood

In the past, a prudent sawmiller would exclude the sapwood from the product. However, because of the smaller regrowth logs they now include more sapwood. Consequently, the sapwood which is always lighter in colour is more reactive to moisture content changes, compared to the more durable darker heartwood.

Oil Coatings

Because of the smaller regrowth timber we are dealing with today, oil coatings have become very important in today’s decking world. Why? Because a good oil coating will defuse into the wood. Effectively, it acts as a waterproofing agent by keeping moisture gain/loss to a minimum, achieving a more stable decking product. However, homeowners should be warned, that to maintain a longterm decking product, regular coatings should be applied.

Decking Timbers

Currently, our warehouse is stocking the following decking: Merbau, Spotted Gum, Blackbutt, Grey Ironbark, Mixed Ironbark, Jarrah, Tallowwood, Treated Pine, Grey Box, Plantation Oak, White Mahogany, Stringybark & Flooded Gum.

I will make a comment on the main species only.

Spotted Gum

The book ‘Wood in Australia’ quotes shrinkage rates of 4.5 percent radial (where the annual rings run mainly across the end section) and 6 percent tangential (where the grain is mainly across the whole width of the end section).

My Comment: It has a durability class 1 rating when used as a decking. I suggest a quality oil coating, particularly in a 135x19mm dimension.

Blackbutt

The book ‘Wood in Australia’ quotes shrinkage rates 4 percent radial & 7 percent tangential.

My comment: With similar shrinkage rates to Spotted Gum, I suggest a quality oil coating. Also, Blackbutt decking can exude extractives (staining) an oil coating will aid in suppressing staining, particularly by swabbing the cut end grain. Currently, supply is not easy.

Ironbark

Probably the most durable species on the market. Shrinkage rates 4.5 percent radial, 7.5 percent tangential.

My comment: Because of its density it is difficult to work so your tools should be very sharp. Like most red timbers, they can leach sap stain so I suggest using a quality coating, particularly swabbing the end grain.

Silvertop Ash

Silvertop Ash is not as durable as other Australian species above, rated as class 2.

My comment: If supplied with any Sapwood, it may be attacked by borers, so I suggest an oil coating.

Treated pine

Easily the cheapest decking on the market and I have been amazed at how good the quality has improved lately. Shrinkage is 3 percent radial, 4.5 percent tangential so it is relatively stable in most conditions. Also, by law, it cannot be treated with CCA when used in public places. It is also available in long lengths.

My comment: Never forget it is a softwood so it can be easily damaged with hard-wearing shoes. Ensure if using it to select a high grade where imperfections like knots are at a minimum.

Jarrah

Like many species today, the quality is not as it used to be. Shrinkage 5 percent radial, 7.5 percent tangential. Surprisingly, Jarrah decking is not as durable as other species like Spotted Gum, with a class 2 in above-ground situations.

My comment: The resource is not as good today as previously so I strongly recommend a quality oil coating. We currently have in stock 140×19 Finger Jointed 5.7 metre lengths.

Tallowwood

Tallowood decking is a very durable species that can have greasy or slip features. Shrinkage 4 percent radial, 6 percent tangential.

My comment: It may be worthwhile washing the surface with an agent to combat any slippery features.

Composite Decking (Perma Decking)

This is made from 87% recycled materials. The product can be ordered in long lengths and is relatively easy to install and low maintenance.

My Comment: although it is stable in width, it needs expansion gaps at butt joins to allow for longitudinal movement.

Merbau

Some would say I have left the best till last, and I think its shrinkage says it all. 1.5 percent radial, 2.5 percent tangential, in other words, Merbau decking does not move around when there is a change in climatic conditions. Some will not use it because they feel it is a rainforest timber. Also, it has a reputation as being a notorious tannin bleeder. That reputation was probably made when it originally was imported unseasoned, today it is kilndried, so the leaching is greatly reduced.

My comment: Because of its low shrinkage factor I would recommend it in any situation particularly if sub-deck ventilation may be compromised like around a swimming pool. In this regard, Merbau outperforms most species. To combat the bleeding factor, apply a quality oil coating, particularly swabbing the trimmed ends prior to installation.

How to Choose The Right Decking

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