Low-level deck joist trade-off - thickness vs ventilation

TaraL

Hello workshop community,

I have a question about low-level decks and the trade-off between joist thickness and ventilation.

I have been reading through the many wonderful posts about low level decking, and I understand that 90x45 joists (/framing as it will be one layer) are seen as standard that can, in a pinch be replaced with 70x45 with more pedestals/supports (at 700mm instead of 1000mm).

We are looking to install a low level deck on top of existing pavers (these are quite uneven in places already). The height difference is around 1200mm between the internal floor and the existing pavers. We could go a bit higher as the sliding door frame comes up about 20mm, so we would not be disrupting an otherwise even transition. I'm leaning toward the minifoot pedestals (https://www.bunnings.com.au/builders-edge-25-40mm-minifoot-pedestal-feet_p2450073) for ease (plus I'm not too sure about dynabolting L brackets to the pavers as each is only 300x300mm).

The consideration I have is that we could use 90x45 joists with a 25mm ventilation gap (90mm joist + 25 mm pedestal + 23 mm for decking board = 138mm), or we could use 70x45 with a 45mm ventilation gap. Which one would be best for durability? We are planning to use joist tape and possibly other treatment on top. I would prefer to use H3 as it is a lot cheaper and more available.

Another consideration I have is that I would like to place weed matting underneath as we are already having issues with weeds between the pavers, which were installed with very little sand or weed protection by the previous owners. I am thinking that his matting would likely impact airflow as well.

I have attached a photo (the air-con unit is being moved) as well as my blueprint for the deck (I used SmartDraw which only has cms for metric measurements).

JacobZ

Hi @TaraL,

It is great to see you've been doing your research and have reached out in the later stages of planning. I'm excited to help out wherever I can so we can make your deck a reality. 

If you have not already seen it, the advice I've given in Deck over a tiled alfresco area is very relevant to your question.

The following articles will also be worthwhile -

• How to build a deck | Bunnings Workshop community
   
• How to build a low-level deck
   
• How to understand deck components
   
• How to understand deck span tables.

Adding a weed mat over your pavers is worthwhile and it shouldn't have any great effect on the airflow or ability for water to drain through it and the pavers. This is definitely worth adding.

Ultimately, either timber size will be low to the ground so there is an increased risk of rot because of the lack of airflow to dry out moisture beneath the deck. This risk can be reduced by using Woodhouse TerraFrame structural pine, which is treated for use in close proximity and direct contact with the ground. The higher treatment grade will increase the overall cost of the deck, but it will also significantly increase its lifetime, so it is worth considering.

If you were to use 70x45, the increased clearance would help to prolong the lifetime of the deck and reduce the material cost of the timber, but you would need to use an increased number of pedestal feet and there are no joist hangers designed for use with this size of timber, so the strength of your timber connections would be greatly reduced.

If you use 90x45 timber, the clearance to the ground would be reduced, meaning the potential for issues relating to moisture would increase, but your deck would require fewer pedestals, there are joist hangers for this size of timber and there is more information to go off in terms of timber spans.

Either option can be used, but you should weigh up the pros and cons of each when deciding. 70x45 will increase airflow at the cost of strength and 90x45 will increase strength at the cost of airflow. I suppose your decision will come down to what you value more.

Ultimately, the choice of materials is yours, but you should weigh up the pros and cons of each when deciding your materials. 

I am more than happy to advise further if you have any further questions or would like assistance with designing your deck.

Jacob