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ANSWER
You should not have a problem with composite decking shrinking.
Composite boards are made of 50 percent plastic and wood fiber by
products. You shouldn't have any shrinks problem.

You should however, make allowances for expansion and contraction of
composite and plastic boards. It is important to note that all composite and
plastic-based products will experience some degree of expansion and
contraction with changes in temperature. Longer boards expand more than
shorter boards

When screwing the deck you should pre-drill before you screw. Also you
should use screws that is meant for composite decking. One of them is
called TrapEase screw by Master Fastener uses
a built-in secondary thread system to pull excess material down as the
screw burrows in. This prevents mushrooms from forming. Also you can lay
down some marine-grade polyurethane adhesive between the joist and the
deck boards. After the adhesive has dried then you can screw into the deck.
That is what This Old House general contractor Tom Silva does.

Composite Decking

QUESTION
I am building a deck using composite material for the floor
boards. How do I keep them from moving? If I drill them
the composite material will shrink and the deck will warp.

Installing Composite Deck Boards

Composites are not no-maintenance decks; they're low-maintenance
decks.

Composites carry 10-year warranties: better than wood (which has
none), but less than the 50-year offers for solid plastic decking.

The fiber content in composite decking improves plastic’s stiffness,
reduces its tendency to move as temperatures change, and roughens
its texture for improved skid resistance. It also makes the material
easier to cut and install than all-plastic decking. Composites do not
require a finish of any kind, making them easier to maintain than solid
wood.

There are some drawbacks to composite lumber though. Because of
the natural fibers in the material, it absorbs water (although not nearly
as much as solid wood decking), making it vulnerable to decay. It can
stain and discolor from food spills or grill grease. Plus, textures in the
wood tend to hold dirt, which can be a good environment for surface
mold and mildew. As a result, composite decking needs to be cleaned
periodically with a scrub brush and an off-the-shelf deck cleaner or a
detergent that contains bleach.

Plastic vs Composite decking

Composite Stats