Indoor/Outdoor carpet over deck boards - HELP!

[westbrooklawn]

I covered my existing deck and turned it into a screened porch. I want to put indoor/outdoor carpeting down, but know it will show the cracks between the deck boards if I don't put something down as an underlayment.

What is the least expensive (but still satisfactory) way to handle this situation, also with the least additional weight? Would a roll roofing material work or do I have to go with a wood underlayment? If so can I get by with luan with felt paper over it or do I have to go with a marine or treated ply? If I do have to use a plywood or luan what is the best fastener to use. I have both roofing and utility coil nailer.

Thanks in advance for any advice!!

[westbrooklawn]

Forgot to add....this surface will get wet from time to time due to blowing rain, etc. coming through the screen.

[westbrooklawn]

I feel sure someone has the answer to this question?? Any help would be appreciated.

[Dragon]

Anything that will keep moisture next to wood is a bad idea.

[Animal AKA Will]

Well it's a year later & I wonder how the screen house has worked out. My suggestion would have been to put a layer of the thin astroturf below the indoor/outdoor. Better still would be to replace the deck with that composite deck material that is impervious to water then proceed with the astroturf with carpet above.