What Subframe Is Best For Composite Decking?

One of the most challenging things when thinking about building a decking area is deciding exactly what material to use for your subframe. You may think that since nobody will ever see this, is it even important? The short answer is yes.

This is in fact one of the most pivotal decisions you will make aside the deck boards themselves. In this article we will compare the three main material types that are used, along with the strengths and weaknesses of each, so that you can decide the perfect subframe material for your decking project.

We are going to talk through Wooden, Composite & Aluminium subframes and this article will cover the three main points for each material:

• Strength & Durability
• Instal speed
• Cost

Point #1 – Strength

Strength & Durability is always paramount when deciding what material you should be using. This is a large factor in deciding the ideal material for your decking area as you will more than likely be using it daily in the warmer weather. Knowing in full confidence that a strong at sturdy subframe is fitted below your deck boards, you should have absolutely no worries at all in the future.

Wooden

A wooden subframe has been one of the most common options for a homeowner to use. The main reason for this is because it is low in cost and a very familiar material for tradesmen to use. In terms of strength when initially installed, wooden joists and subframes are as strong and sturdy as any of the other options we will be covering but this still does depend on the thickness of the wood. Using the best possible timber for decking joists is important as it will ensure that the decking area lasts for a long time. Traditionally Ironbark, Cedar and Pressure-Treated Pine are the best for timber as they are known to resist termites, rot and decay.

It is very important to note that a wooden subframe is the least durable by far. We would expect a wooden subframe to maintain its strength and be fit for use for around 5-10 years. The main reason for its short lifespan, is that wood is very susceptible to rot. Wherever water can sit upon the wood, it will soak right through, weakening the subframe and giving your decking an unstable feel when walking on. It will also expand and contract and is likely to twist or split. This is because it is a natural material, and as it weathers, it will change in size and expand when it gets damp, and contract again when it dries out. This can lead to an unsightly finish of your deck, as it can expand the gaps between the boards, and give your deck an unlevelled finish.

Aluminium      

Aluminium subframes are quickly becoming an increasingly popular option to use as a frame system for decking. The main reason for this is its incredible durability and we would expect an aluminium subframe to last 20-30 years very easily. The reason for it lasting so long is that it does not rot or warp, as it does not react with or absorb water, meaning it retains its strength properties for many years. The expansion and contracting of aluminium joists are so minor (around 1-2mm over a 3m length) that you will never know the difference. This can affect the spacing of your deck boards but would only be a maximum of 1mm of movement caused by the subframe. Aluminium subframes are extremely strong, depending on the thickness of the joist that it used. For comparison, a 72mm aluminium joist has around the same strength capabilities as a 150mm thick wooden joist.

Composite

Composite subframe joists are made from very similar materials to composite deck boards. They are made from a composition of wood, plastic, and a bonding agent. Composite joists are durable and will never rot so with the correct support, we would expect a composite subframe to last around 15-20 years. However, a composite joist will overtime expand and contract a little, around 5-10mm over a 3m length. Regarding the strength of a composite joist, this is one of the weaker options, and will need to be supported on average every 500mm. This depends on the joist thickness and quality.

Plastic

Plastic joists are usually made from recycled plastic, that is melted down and poured into moulds to form a joist. Plastic subframes are one of the weakest options and will need to be supported and braced with noggins very often. However, plastic joists are very durable and will not rot, retaining their original strength for around 10-15 years. Like composite and wood, they will also expand and contract around 5-10mm over a 3m length. There are different thicknesses available, which will affect the strength. Again, as with wood, the thicker the stronger.

Point #2 – Installation Speed

The instal speed is an important factor when deciding what the best subframe material is for you. You will want to think about whether you will as a professional to help, or if this is a project that you want to work on yourself and save money.

Wooden

As discussed earlier, wooden subframes are very common and have been used for a very long time. The actual install of a wooden subframe is very quick. A 30sqm area may take around 2 days to instal if you committed a large amount of your time to it. This is because it is a familiar material and tools are readily available for the job.

Aluminium

Aluminium subframes are made of a much stronger material than wood. This means that installation is slightly different. Instead of just screwing wherever suits, aluminium joists have either brackets or designated locations where the joists are fixed together using a self-tapping screw. These are easy to use but will potentially take slightly longer to instal. We would expect a 30sqm area to take around 2.5 days to install, again this would need you to dedicate a good amount of time to the project. This is because screwing brackets can take longer than placing a screw wherever suits, like wooden decking.

Composite

Composite subframes are like aluminium to instal. You will need brackets wherever you want to join one joist to the other. As it is a soft material, screwing can be simpler than with aluminium, acting similarly to wood. It takes a similar amount of time to instal as an aluminium subframe would – around 2.5 days for a 30sqm area.

Plastic

Plastic subframes are installed very similarly to wooden. They both have similar properties and can be installed with standard woodscrews. The difference is that plastic joists are slightly weaker so will need more support on the underside of the joists. This means that installing a plastic subframe for a 30sqm area would usually take around 2-2.5 days.

Point #3 - Cost

Cost is a significant factor and probably at the forefront of your decision making when deciding what subframe would be best for your decking area. Everyone will have various budgets, sizes and requirements for their deck areas.

With any of the materials discussed, the larger the deck area, the more materials required and this usually also pushes up the cost, so we ask you to bear this in mind.

Wooden

Wooden joists are very readily available from most DIY shops, meaning the cost of this material is relatively low. It costs around £7.50 per linear meter for a structural wooden joist. A simple 30sqm deck area would use around 130 linear meters of joist, meaning it would cost around £1000 for the subframe materials (excluding supports).

Aluminium

Aluminium is a more durable material and slightly harder to purchase from regular DIY shops, meaning it is more expensive. It can cost around £12 per linear meter for a high strength aluminium joist, however as it won’t warp like timber, it does not require noggins so less material is required. A simple 30sqm deck area would use around 110 linear meters of joist, meaning it would cost around £1500 for the subframe materials including fixings. This also excludes the supports.

Composite

As composite joists are comparatively weak, meaning they have the lowest price per linear meter. Composite joists cost around £5 per linear meter. As with timber, noggins are required to ensure that it does not bow or warp. A simple 30sqm deck would use around 130 linear meters of joist, meaning it would cost around £850 including fixings for the subframe materials. This excludes any supports, which is where the cost would increase because of the quantity of them required.

Plastic

Plastic joists have similar properties to wood but are much more expensive. They cost around £13 per linear meter. A simple 30sqm deck area uses around 130lm so would cost around £1800 including fixings and excluding supports.