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Joists are used to create a subframe of your composite decking, slightly raise the decking boards off the ground and ensure a flush finished project. Joists are required to install composite decking, as these are what give your decking its strength and stability.

Joists are joined together in a grid-like formation that your decking boards sit on top of and are screwed to the joists with a hidden fastener system. If you opt for wooden or composite joists when completing your project, the choice is yours. However, there are some aspects to consider when choosing to put your composite decking on wooden joists

building joist

Longevity

As we all know by now, composite material lasts much longer than wood material due to its resistance to the elements. Therefore, your composite decking, when maintained correctly, will last for decades. Whereas your wooden joists will usually only last for several years, potentially a decade in some cases.

It is important to note that your wooden joists will rot and warp long before your composite decking boards need replacing. Therefore, you will need to replace your wooden joists and remove all your composite decking boards, even though your composite boards are still in great condition. This is, of course, hugely inconvenient, time-consuming and costly.

This is why it is recommended to use composite joists when fitting composite decking as their lifespan will be the same and they won’t be as susceptible to moisture damage such as rotting.

We understand that composite joists are costlier in comparison to wooden joists but choosing wooden joists will essentially end up costing you twice. You have to think long-term when it comes to composite; it is an investment and pays for itself over time with low maintenance and longevity. 

If you still decide to opt for wooden joists due to budget, then it is essential to make sure you have chosen pressure-treated wood as this will have a longer lifespan in comparison to non-treated wood.

Pressure-treated wood has been soaked in chemicals in order to preserve its longevity and prevent rotting and insects from eating it. Although treated wood will last longer than non-treated wood, its lifespan is not as long as composites, and you will still have to replace your joists before the complete lifespan of your decking. 

Overall, yes, you can put composite decking on top of wooden joists; however, it is not highly recommended and may cause you issues down the line.

If you have an existing subframe made from wood that has been standing for years and are looking to only replace your decking, it is not advised to put brand new composite decking on an existing wooden frame, as it will not retain the structural integrity needed to stabilise the decking for decades to come.

Lots of homeowners try to do this to save money and avoid building a new subframe but all it does is cause problems in a few years when the subframe is rotten and corroding and causes collapse, which damages the decking and is dangerous to use. Essentially, your subframe is the most important part of your project and should be invested in properly. 

building the wooden joist

A Simple Step-by-Step Guide to Constructing a Wooden Subframe:

  • Constructing the frame on which you will place your composite decking is an important task and should be completed using high-quality materials. 
  • You should only attempt to construct a subframe yourself if you have experience, as it can be dangerous and should be completed with precaution and safety. If you do not have experience, it is recommended to hire a professional.
  1. Firstly, draw out your plan and mark the location where you will sink the posts that will support your joists.
  1. Once marked, dig into the spots and place the posts deep in them. As a standard rule, it is usually stated to have your posts dug underground at around a third of the height of your post. The posts can be made of wood that has been treated and is strong enough to support the joists. 
  1. Then, concrete should be poured inside the holes and left to dry. Once the concrete is dry and the post is set in place, you can start attaching the joists to the posts.
  1. To build the structure, you have to attach the joists to the pillars with screws or nails. The joists should be laid in such a way that they slope gradually away from the house. This is important so that rainwater will not settle on your deck, but will run off.
  1. When constructing the joists, the space between them should not be more than 300mm. Here is the recommended distance for composite deck installation. This will keep you within your warranty.
  1. If you cut the joists, you will need to paint the top with preservatives to protect it. Also, you have to reinforce the top of the joists with joist tape so that it will not absorb water and rot.
  1. Your joists should be laid in a grid-like formation to provide the decking boards on top with maximum strength. Once it’s all screwed together and secure, you can begin to lay your decking boards on top.
  1.  Since the joists are laid lengthwise from your house, you have to lay the decking boards so that they will be perpendicular to the joists. Laying the boards from the outside inward.
  1. Place the first set of decking boards around the edge with starter clips and the rest with hidden fasteners. Simply screw the clips into the joists below, not through the decking board itself. This will give you a flush surface finish. 
  1. It is crucial to know that there must be sufficient space between the joists where the two edges meet to accommodate the expansion and contraction of the decking and avoid damage.
  1. As stated, your decking surface should slope after you have finished so that there won’t be drainage issues. Pooling water on your surface will cause damage to your decking boards. 
completed decking job

Conclusion

Composite decking can be placed on wooden joists when constructing a deck. You must make sure that the joists that you use are pressure-treated joists and that the foundation of the decking is strong enough to hold the decking boards. A better alternative is to opt for composite decking so that you know that the lifespans will be similar.