How to add airflow to low-level deck covering a pergola?

[jinugu]

Hello experts

I got a deck done - attached images. The deck is flushed to the concrete. How do I ensure that there is enough space for the air to pass through. Used treated timber for everything including Fascia. If i were to dig around the deck, it will look odd aesthetically. What i was thinking was, dig around the and fill it with pebbles. Will that work? Your expert opinion pls.

[MitchellMc]

Hi @jinugu,

I presume your deck frame is built with H3-treated timber, which is only rated for above-ground use and cannot be in contact with soil. Even if the timber is treated, placing soil or pebbles directly against the joists will trap moisture and drastically reduce the lifespan of the deck. A flush deck with the surrounding soil is not ideal because it prevents airflow underneath, leading to a very humid environment and accelerated timber decay.

Using pebbles around the base would be slightly better than soil because they won’t retain as much moisture, but they still restrict airflow. With your current setup, the joists are likely to last far less than the typical 15 to 20 years you might expect from a properly elevated deck and possibly as little as 5 years before rot becomes an issue. If longevity is important, you would need to either excavate to create a clear gap beneath the deck for ventilation or not place material up against the joists.

Let me tag @Nailbag to see if he has any thoughts.

Please let me know if you have any questions.

Mitchell

[Nailbag]

Hi @jinugu 

The first question which @MitchellMc brought up is what level of treated pine was used for the sub-floor? As he stated in needs to be a minimum of H3.

I think from memory there should be a min 50mm air-gap from the underside of the joists to the ground. Do you have any photos of the sub-floor before laying the deck? At the moment most of what I can see is at or sub-ground level.

And unfortunately it appears the decking boards are upside-down. They should be laid with the grooves facing down to allow for airflow but mainly moisture drainage underneath. This prevents water from being trapped, which reduces the risk of rotting boards which is very common with TP deck boards. My advice would be to reverse them and take the opportunity to rectify any clearance issues with the subfloor. You should be able to use the same screw holes again.

Nailbag

[jinugu]

Hi Mitchell/Nailbag

Hope you are well. Thanks for your advice.

Yes, treated pine (H3) was used for sub-floor.

I will remove soil around the perimeter to ensure air flow. I am not too worried about the longevity. 4-5 yrs will suffice for me.

Thanks again for your help.

You have good rest of the day.

Best regards

Ravi