That looks like Trex or a similar composite plastic lumber. It lasts a long time, but if it's going to support any weight the structure under it can't be the usual 16" on center. It should really be closer to 12" on center lying on flat ground or you'll get the kind of flexing that you seem to be experiencing. Just a guess/suggestion.
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City of Cambridge (Verified Official)
Robert Winters (Registered User)
Robert Winters (Registered User)