How to seal gaps between decking boards?

[Gaj2020]

We are looking for a solution to a problem that has occurred through extensive additions. We extended an existing deck (timber decking, each hardwood 100mmW by 12mm thick with. Spacing of 1.5mm. All decking is undercover however still in the elements and susceptible to rain falling on the decking at times. We live in FNQ with high rainfall so this can also be a big problem on occasion. We have established the guttering is insufficient in one area, and this will be dealt with separately. Underneath the decking we have created a separate entertaining area which is also effected by water ingress due to decking. While we are not talking flooding, it is an issue we need to rectify long term. Succinctly the water ingress is in the majority from the gutter issue which is being dealt with, and secondly rain which is sprayed onto the decking during very heavy falls.

we don’t want to install a ceiling downstairs as it detracts from the original rustic design, plus we will still have a water management issue to contend with. Ideally we would like to tackle this at the decking level, and seal the timber joins with a product that can meet expansion if required, and be durable.

[JacobZ]

Hi @Gaj2020,

Welcome to the Bunnings Workshop community, it is wonderful to have you with us.

Unfortunately, it wouldn't be advisable to seal the gaps between the decking boards for a few reasons. These gaps allow the decking boards to expand and contract. If you were to seal them, even if you used a flexible sealer, sealing the gaps between every board would restrict this movement, leading to buckling, cracking, or boards pushing against each other. Water would also pool on the deck, where it would seep into the timber and fixings, causing rot and rust, which would decrease the lifespan of your deck considerably.

Unfortunately, waterproofing a deck is not something that would be simple in post, and simply sealing the gaps between the boards would just move the problem elsewhere. Instead of this, I would suggest looking at ways to keep the water from getting on the deck in the first place.

My suggestion would be to install bistro blinds around the perimeter of your deck. They can be let down when the rain is coming and put up when it is dry, so that you can still enjoy your deck without obstructing the view or the breeze.

Allow me to tag @Nailbag, @R4addZ and @DIYGnome to see if they have any ideas.

Let me know what you think.

Jacob

[Gaj2020]

Many thanks Jacob for your quick reply. Take on board your advice re addressing the point of entry of water. That in itself can only be achieved partially due to the design of our deck and flyovers but we should be able to minimise the issue.

[Nailbag]

Hi @Gaj2020 

Weather/waterproofing under a deck is both very complicated and costly. My last reno I needed to open a top level deck that had a room underneath I intended to make a living space. So, this needed to be 100% waterproof. I had a mate of mine who is a specialist in roofing and custom guttering make trays in-between the joist under new decking boards. The trays fell to a concealed gutter and to downpipes. 

You could consider lifting the top deck boards and doing the same. However you would still need to consider how your going preteen forward facing water entry as @JacobZ mentioned with something like bistro blinds

Nailbag