Decking Gaps

Hi Guys,

I am in the process of installing some artificial grass over my existing decking. I have bought underlay with the grass as I wanted it as comfortable as possible. I have a problem I was hoping to get some advice on. The gaps in inbetween the decking have opened up over the years. They are roughly around 15mm at the moment! I really want to somehow close these gaps up before installing the underlay as I dont want the grass sagging in the gaps in the future.

The section of decking I am installing the grass on is covered by a porch so I'm not worried about water retention. What would be the best way to close the gaps up? Sisal rope? Some kind of flexible putty? Plywood over the whole section? (Last resort, expensive option!)

Thanks in advance for any replies

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