This article describes how to build decking structures to really last. Durable structures that last two or more times as long as normal can easily be achieved, at relatively low cost, if the failure points are designed out during construction. We list those points, and offer some decking design alternatives that solve them. If you have decking that has already rotted, you will probably spot the failure points below and may want to refer to our ‘how to repair decking‘ article to repair the damage.
Why does Decking Rot
The most common failure of wood decking is due to the incorporation of one or more fundamental flaws in to the design.
These poor design characteristics are then compounded by the EU’s ban on chromated copper wood treatments in 2004. Tanalised timber these days isn’t what it used to be, and it’s life underground or outdoors will be restricted as a result. This sadly we can’t do anything about.
The EU have also banned creosote as well – just to compound matters.
Decking Design Flaw 1 – Buried timber uprights
If you want your deck to last, avoid an early failure, and put your decking repair off till later; don’t bury cheap tanalised (Exterior grade) softwood in the ground and expect it to last.
If you really must use wood – Oak fence posts aren’t much dearer than softwood. Try to find some creosote, real creosote, if you can. If you can’t, consider treating the uprights with Smiths Clear Penetrating Epoxy Sealer first.
Better yet, why not skip the problem of buried wood rotting altogether, and support the deck on something more durable in the first place?
Brick pillars look great, are easy to build, and you can easily incorporate a damp proof course in during construction (this one has a slate dpc built into it). They are perfect for supporting horizontal members. This one and it’s partners support second hand railway sleepers (soaked with creosote when new – £10 to £20 each second hand), which themselves support an oak framed floor of my summerhouse. I expect this structure to outlast me!
Concrete fence repair stubs. Cheap at around a tenner, easily set in place and with a couple of convenient holes for attaching your superstructure to. If you do use Concrete fence stubs, do give some serious consideration to using a layer of Bituthene or other waterproofing agent (see design flaw 2 later for details) between the stub and the Joist.
Decking Design Flaw 2 – Moisture Traps on the Joists
Normal construction of decks has the planks which form the upper surface (The Decking Boards) placed across and perpendicular to a joist, which may be anything up to 2″ X 12″ on edge (the 12″ dimension vertical). This maximizes the stiffness in supporting a deck, which may extend out from the side of a house and have little or no vertical support below.
The planks are screwed or nailed to the joists. This of course punctures both wooden members. There is also a thin air space between each deck plank and joist because the wood pieces do not fit perfectly. That thin gap will also open up as the structure moves around a bit with use, age and changing weather.
Rain falls and dew condenses at night, allowing water to collect in these narrow dark spaces. Warmed by the sun, these spaces become a breeding ground for the natural bacteria and fungi which exist everywhere. Over a period of years the wood in these areas is eaten away by this microscopic life. The overall condition is generally called dry rot.
Good decking design seals this area during construction to avoid the inevitable onset of rot here, unless of course you want to use plastic alternatives for both the deck boards and the joists themselves (an expensive but not unreasonable approach – seriously consider it).
Using Bituthene® or a similar bitumen tanking product between the joist and the deck board will seal this area and prevent the ingress of moisture vapour. Bituthene® is also available as Bitutape in narrower widths.
If the deck boards being used are of a reversible design, as they often are in the UK, it is impossible to get an air tight seal using a bituminised tape, and the use of a modern polyurethane sealant, such as 3M 4200, will be required.
Treating the underside of the deck boards where they will cross the joists and the tops (or indeed the whole) of the joists themselves with Smiths Clear Penetrating Epoxy Sealer (CPES) will offer additional protection prior to applying the Bituthene or 3M 4200.
NOTE on wooden deck boards:
The flat bottom design is suitable for use with the Bituthene® sealing method, whereas the reversible design is far easier to seal properly using 3M 4200.
Decking Design Flaw 3 – Excessive Movement
Movement is bad for all structures. Decking will typically move because of changes in temperature, moisture and use of the structure. Good decking design can minimise this – and significantly aid durability.
Minimise movement and cupping of deck boards by using lots of fasteners, at least two on each point where a board is fastened to a joist, near the edges. It is very false economy to use a single screw in the middle of the board.
Ensure that if you have long runs of joists you have some means of stopping them twisting. In a house noggins (short pieces of the joist, fastened in perpendicular to the joists and between the ends) would be used for this purpose, but on a deck they represent more of a hazard due to the amount of end grain trapped against the joist.
Your decking design may well have a substructure of large timbers running between the verticals, with the joists running above them. Provided the joists are attached to this structure, this will provide that support. If your structure does not offer that support to stop the joists twisting, then give serious consideration to perhaps using a smaller joist, fastened from underneath. Either of these solutions should incorporate the gap sealing techniques described in decking design flaw 2.
Screws hold structures tighter than nails normally. If you do want to use nails, consider using ridged nails which are designed to grip the timber and resist being pulled out.
Decking Design Flaw 4 – Inappropriate fasteners
If you really want your deck to last, then you have to use the right fasteners.
Rusting steel causes a phenomenon known as nail rot in wood. This causes high porosity and early failure of the timber. Your durable deckign has been designed to last, so your fasteners should also be sourced to last, outside.
Ideally you can source stainless steel screws or nails. High quality double dipped galvanised fasteners are an alternative, and there are also many ranges of purpose built decking fasteners available.
Decking Design Flaw 5 – End grain protection
End grain is always the easiest site for rot to start in any timber. It has high porosity, and makes an easy breeding ground for the life that will cause the rot.
Seal end grain with Smiths Clear penetrating Epoxy Sealer. It will penetrate deeply in, and seal the end grain against the ingress of water.
Avoid tight gaps which may trap water next to end grain. Several design flaws are easily avoided here:
- If your deck is too large to span with a single board you will require a butted up joint – two bits of end grain in close proximity and a rot hotspot. All of your deck boards will be the same length, so arrange to have two joists at the joint perhaps an inch or two apart or so. This will allow you:
- A decent air gap – not trapping water and allowing the boards to breathe
- Well supported ends of the deck boards
- Screws fastened in a sensible distance from the end of the board (perhaps 2 inches) which bed into the middle of the joist, avoiding damage to both members.
- Where the deck boards butt up to a vertical section leave a decent gap to allow water to drain and the board to breathe. Place the joist in a sensible position to provide support whilst allowing the fasteners to be away from the very end of the board.
A little thought when doing your initial decking design and layout can give you a nicer structure that lasts far better as well. Who doesn’t want to design low maintenance decking?
Designing Durable Decking – What can you expect?
Applying the decking design techniques described above can easily double the life of any wooden decking solution. A relatively small cost and amount of time applied at the start of the project will repay handsome dividends over the course of time.
Hopefully we can encourage you to apply some or all of the decking design ideas above, and perhaps make you think about the use of hardwood instead of softwood.