Best Practice for building Timber Decking Frames in the UK

Oilcanfinish Outdoor Living timber decking frames are built to last. In residential Decking Frame installations the project budget doesn’t always allow for aluminium decking frames, therefore we have developed a best practice for building timber decking frames in the UK to enhance timber decking frames which gives the homeowner a maximum return on their investment. This is achieved by extending the life of a timber frame, without breaking the bank.

The alternatives to timber decking frames, such as composite decking frames or aluminium decking frames, will always outlast softwood timber, however composite frames can warp over time in with high temperature fluctuations, and aluminium is expensive.

So let’s say the project you are working on has a Timber Decking Frame budget, but you keep hearing about how they all rot after 10 years, or you have seen this first hand in your own back yard. It doesn’t have to be this way. You can build a Timber Decking Frame which will last over 20 years for a small increase in price, not nearly as much as upgrading the whole decking frame to composite or plastic joists, or aluminium.

The main difference is in the materials which are used and how they are installed. This is also what makes an Oilcanfinish Timber Decking Frames different from others.

If any of the 6 key items listed below are substituted for inferior products, they can result in the entire frame needing to be replaced within 20 years of installation, in some cases within 10 years of installation. If the recommendations we follow in our Best Practice for building Timber Decking Frames is followed, you can greatly extend the life and strength of your decking project.

If some aspect of the decking frame fails, it means the entire cost of the decking has to be paid twice over the course of 20 years (waste removal, installation charges, framing timber, structural screws, and possibly some or all of the decking).

Budget Decking Installation

£4,500 lasts 10 years

£4,500 lasts next 10 years

£9,000 Total

Oilcanfinish Decking Installation

£5,750 lasts 20 years

Above example based on a 6mt wide x 3mt deep (18 Square Metre) decking installation onto the back of a Terraced House using 50×150 Decking Joists and Trex Enhance Naturals Decking Boards with hidden fixings) maximising Decking Board usage without complex patterns. Prices correct as of February 2023.

Top 6 Reasons why an Oilcanfinish Timber Decking Frame is better

  1. We only use Use Class 4 Timber
  2. We tape our Decking Joists
  3. We only use Exterior Rated Structural Screws and Bolts and End Grain Preservative.
  4. We ensure our Joists off the ground
  5. We pour our own concrete post footings and use Post Stand Offs.
  6. We use Stainless Steel Wall Bolts with Resin Fixings

Why do we only use Use Class 4 Timber for our Decking Frames?

All timber has a Use Class Rating, from 1-5, as per British Standard EN 355-1:2006 which outlines Durability of wood and wood-based products.

Use Class 1 & Use Class 2: Internal use which is kept dry. Use Class 2 for slight risk of moisture or insect attack.

Use Class 3 : External use which is kept above ground and is coated or uncoated, such as Cladding or Fence Rails

Use Class 4: External use which has direct soil or water contact, such as fence posts or decking which is sunk into the ground.

Use Class 5: External use which will be in regular or constant contact with the ground or water, such as marine piling.

Pressure Treated Timber for sale in most Builders and Timber Merchants is Use Class 3. Use Class 4 Timber is very uncommon. In my area of South West London I have 1 local independent supplier who stocks Use Class 4 Framing Timber, in 1 size (150mm x 50mm x 4.8mt), which is Champion Timber. Our other 2 suppliers of Use Class 4 Timber are based outside of London. This is how rare it is to find it. If the merchant does not know if their Timber is Use Class 3 or 4, always consider it Use Class 3. Use Class 4 Timber is always clearly advertised as it commands a premium price.

Due to the nature of how decking is built, a void is created between the decking and the ground with limited airflow and circulation. Moisture rises from the ground into this void and creates a humid environment, especially in the UK. Oilcanfinish treat the humid void underneath decking as a Use Class 4 environment.

There are a lot of installers who will use Use Class 3 Timber, and that’s fine for them, but not for us. We want your Decking to be protected and last as long as we possibly can and if we can squeeze out an extra 5-7 years usable life span it is a significant cost savings over time.

Note this is completely different from Structural Timber Ratings (C16 or C24 or unrated) and also separate from Durability Ratings, also classed 1-5 confusingly.

Why do we use Decking Joist Tape on our Decking Frames?

Photo of Rain Water beading on Decking Joist Tape

Water Beading on Joist Tape

Decking Joist Tape provides 2 main advantages:

  1. Water which does not run down the sides of the joists beads on the tape (as in the photo). Then it evaporates naturally. What it doesnt do is absorb into the top of the wood.
  2. As the screws go down through the tape into the timber decking joist, the tape seals around the screw preventing water from travelling down the screw thread into the centre of the wood.

The tops of the joists, specifically between the decking boards, and the centre of the joists are two places where most decking frames start to initially rot and deteriorate.

We use Trex Protect Joist Tape or Walther Strong Joist Tape.

Why do we only use Exterior Rated Structural Screws and Bolts on our Decking Frames?

We regularly remove old decking frames prior to starting our work. The points where the old decking has failed are always the same, the timber joists have rotted out in all the same places. Sometimes the frames are so weak we do not even need a pry bar to take them apart, we can pull them apart with our hands.

The Bolts used to assemble your decking frame matter. If they are not rated for exterior use and also classed as structural, several problems can occur.

The screws will break. There is a big difference between structural screws and standard screws, there is also a big difference between nails and screws. Nails will bend but rarely break. Screws will break if they are not fit for purpose. Structural Screws will not break provided they have been installed at the correct frequency and in the correct locations. What you are looking for is screws which conform to BS EN 14592:2008+A1:2012 for fixings intended for load bearing structures and in the UK you can start with Carpenters Mate.

The screws will rust. If the screws rust prematurely, the rust will negatively affect the wood it is supposed to be holding in place and instead weaken the wood around the screw, resulting in the exact opposite result which was intended.

At a minimum, your structural screws should have either a Hex Head or Washer Head to pull the two pieces of timber together. Standard Countersunk screws will not do the job. The structural screws we use are either 6.7mm width or 8mm width.

Why do we ensure our Decking Joists and Frames are off the ground

Photo of Decking Joists elevated off the ground level to permit airflow

Elevated Decking Frame to allow Airflow & prevent Wood Rot

There are several reasons why your decking frame needs to be off the ground.

Airflow. Your decking frame will get wet, from both rain and humidity. It does need time to occassionally dry, as much as is possible in the UK. Airflow under the decking and through the frame allows the timber to fully dry. You do not want it turning into a sauna down there as moisture will keep coming out of the ground.

Wood Rot. You want some separation between damp soil and earth and your timber decking frame. Soil is always damp and will cause the timber to rot prematurely, regardless of what Use Class rating the Timber has. Use Class 4 timber will last longer, but will still eventually rot, just not as quickly as other timbers.

Why do we pour our own new Decking Post footings?

When we pour our own footings we know exactly how deep they are, what mix has been used of Cement and Ballast, and what the footing pours are sitting on. We have removed footings which are only a few cm thickness and we have seen posts set into concrete or postcrete. Often these posts are going through the cement and sitting onto the soil or clay underneath meaning they can move over time. We set our posts onto the tops of the footings we pour, using Post Shoes to keep the bottoms of the posts off the ground. Posts in contact with the ground and soil are a weak point in a decking frame.

Posts should always be separated from the ground and should be installed using a Post Stand Off, which is a Galvanised Steel Shoe which keeps the post a few cm off the ground to allow water to run off the post, down the shoe to the ground, preventing water from gathering at the base of your post and causiong premature rot. See Simpson Strong-Tie Post Stand-Off for examples of these.

Why do we only use Stainless Steel Wall Bolts with Resin Fixings for our Decking Frames?

Stainless Steel Wall Bolts do not rust. We combine these with Stainless Steel Washers and Nuts. The alternative materials available, such as BZP Steel or Zinc Passivated, will rust over time in an exterior environment, regardless of what it says on the packaging.

There are numerous types of bolts which are faster to install, or supposedly stronger, such as Concrete Bolts which are threaded and bite into the material they are being installed into. Other options include Wall Plugs, Steel Threaded screws which have a plastic insert and Shield Anchors with steel wall inserts. All of these items require the fixing to expand into the material around it and bite into, or grab into the material. As we are working in the UK, most of the time we are attaching the Decking Wall Plates, or Ledger Boards, onto Brick walls.

We do not want to expand into either soft bricks or into Engineering bricks, as the bricks will, over time, crack and break apart. We use Resin as it forms a bond with the brickwork without the need for anything to bite into the brick work. The Resin also seals around the hole to prevent water ingress through the sides of the screw or anchor into your home.