Above are two examples of decking projects I have recently completed. Decking still continues to be popular and since its popularity in the early 1990s, when the basic deck for many was a simple single level surface comprising timber boards covering an outside area, normally tannalised decking boards, a great many new products have come onto the market. However a deck is only as good as the sub frame. I have seen decks that had sagged either in one corner or in the middle, which meant taking up the deck to get to the posts. The reason for this failure is usually at ground level; the posts have rotted, or sunk as in one case because the post was simply sunk into the ground without any concrete. Generally best practice is to avoid putting sub frame posts into the ground and to lay down concrete foundations, or slabs for smaller decks, and onto this sit your posts in metal feet.
Another factor to take into consideration is plant growth under the deck. Plant growth holds moisture around the sub frame leading to eventual failure. Plants need light so the obvious remedy is to lay a black damp proof membrane down at ground level to eliminate this hazard. This needs to be cut to size with cut-outs around posts and secured using stainless steel staples. Then small holes need to be introduced to avoid the pooling of water, a dank pool of water below a deck is heaven for mosquitos.
So what is tannalised timber?
Tanalised timber is hydraulically pressure treated with copper, which prevents insect attack, and biocides, namely propiconazole and tebuconazole, which act against the formation of brown rot (a type of wet rot). There are colour compounds added as well. The proceed is known as Tanalith E, and has been around since the 1940s. Tanalised timber comes in many grades. uc1-3 is for above ground and uc4 is suitable for below ground. Also strength grades are usually stamped on the wood with a ‘C’ followed by a number, C16 or C24 for instance. This is important for the bearers of elevated decks.
How do I prevent the end grain, that is the end of the post, in or at ground level, from soaking up the water from below?
The question of sealing the end grain is an important one. If left untreated, particularly if it’s a cut end, it becomes the weakest link in the armoury against insect attack and rot. Many years ago I was taught a trick by a boat builder who had been in the trade all of his working life; he simply boiled down candles and dipped the end of the timber into the melted wax, then with the wax still pliable he rubbed a piece of wood over the wax, working it into the end grain, thus filling in any potential route that water could penetrate.
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Now the boards.
As I stated earlier, the range of decking boards has grown to include recycled plastics, composite of plastic polymers and wood, hardwoods and softwoods. Softwood is usually pine if purchased from one of the large retail sheds, or redwood if from independent suppliers – still a member of the pine family but certainly a superior product.
As a matter of course I do not lay a timber deck unless it has an anti slip surface, in the form of a grit impregnated plastic strip rebated into the top side of the board. It is more expensive for sure, say 50% more or so per meter run over ordinary timber decking, but the argument over the cost is a no brainer if you unfortunately find yourself on your back unable to get up because you have broken/twisted something! It has happened.
There are clear anti slip products on the market but I don’t like them. I have found them to be OK for a couple of years at most and then there’s the job of removing the old stuff before you can reapply it. It’s not too dissimilar to anti slip paint on boat decks but the difference is that with a boat deck you can change the colour or simply paint over it again. I have found that with the clear anti slip products once water gets under it you can end up with a mosaic of blotches where the wood has discoloured.
With regards hardwoods, sure, there are plenty about, but my conscience prevents me from entertaining this. Most hardwoods are from tropical climates which is typically where rain forests are, and with the global demand for such products ever increasing, and the remaining resource ever decreasing, I do not want to be a party to the eventual collapse of these forests. For sure hardwood has its place in the carpenter’s or furniture maker’s itinerary of materials, but not for a deck. I ask you to think long and hard about the future and the environmental impact of stripping out rain forests around the world so that we can have hardwood decking . If after this thought you simply must have hardwood, please ask someone else to do the job.
It goes without saying that the fixings used in laying a deck need to have a comparable life span. It’s no good having a deck board that’s going to last 15/20 years or longer and fastenings that degrade in 5 years, so I use stainless steel in coastal areas and propriety deck screws inland. All in all, given that a deck will be exposed to UV light, rain, frost, and anything else our changing climate wants to throw at it, it has to be pretty well bullet proof to the elements.
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