When building a composite deck many people build as if it is lumber not realizing the nature of composite changes how you have to install the deck. Composite which contains plastic, either polyethylene which is recycled and more malleable plastic and wood fiber, or polypropylene which is virgin plastic which is more rigid. When you put plastic and natural fibers together there are certain reactions you should always expect and allow for when building your deck.
First is your deck boards during the rain will absorb some of the moisture like a sponge and expand. Also during extreme hot weather it will expand and in cold it will contract. This is very key to how you apply your deck. When applying in the heat of summer it is best to have an 1/8 inch gap in between each board. You will also want the same gap at the butt joints. This allows for the consistent movement of the boards and will keep your deck looking uniformed and beautiful. Also regarding expansion and contraction if you are not face fastening the boards and instead you happen to be using a hidden fastener system, make sure those clips have teeth in them that bite into the board. This will minimize the movement of the deck boards from season to season. Otherwise if the clips are slick with no teeth you will want to face fasten around the perimeter of your deck.
Next if you are contemplating purchasing a composite products remember one, it is low maintenance, not no maintenance and two it will grow mold. The combination of the wood and plastic is a breeding ground for mold unfortunately. You can however help this by making sure that you clean your composite deck every 4 months or so with bleach based deck cleaner. There are many out there that will help. Also there are mold inhibitors for composites decks now, looking into those would also be wise. The perk is you do not have to paint your deck every year; however you do need to clean it occasionally.
Lastly remember why you purchased the composite product and think seriously before painting your boards. There are composite paints out there that can be used on decking however due to the nature of the product and the plastic that is within the board. Paint will adhere to the surface, however not always forever. A lot of paints will bubble up and peel and you end up having to repaint your deck just as much you would a normal lumber deck.
Composite is a great product for an outside deck, however making sure you know how to apply it correctly and the quirks of how the boards reacts to weather is very key to being ultimately happy with your deck.