Common Decking Materials

• The most common woods for decking boards are redwood and cedar, although exotic hardwoods like ipe and composite decking materials are also becoming increasingly available. Softwoods typically have an open grain that soaks up moisture and releases it readily in dry conditions. This effect is less noticeable in hardwoods although it still occurs. Composite decking materials, even though they are not wood, contain a large proportion of wood fiber and also respond to weather conditions by expanding and contracting. The Trex company, which manufactures composite decking, recommends a 1/4 to 3/8-inch spacing between decking boards.
  
  

Expansion Damage

• If proper spacing isn't provided between decking boards, they can expand to exert enough pressure on each other to curl up at the edges or worse, pull out the fasteners holding them to the joists. This is not only unsightly but also provides a walking hazard that can only be corrected by replacing the damaged boards or removing all the boards and reinstalling them with the correct spacing. Expanding boards also trap moisture between them, promoting the growth of fungus and rot that can force their replacement.
  
  

Drainage

• When water isn't allowed to drain off the surface of a deck, it can collect in pools that destroy the finish and create a slipping hazard. Worse, it can seep through the cracks between the boards and collect underneath where it rots the undersides of the boards and the joists. When the boards are properly spaced, rain water falls through the deck without collecting and sufficient air can circulate and dry the undersides of the boards and the joists when conditions are dry. Proper circulation also inhibits the growth of mold.
  
  

Spacing Procedures

• It usually isn't difficult to provide adequate spacing between decking boards when installing them, and the deck will look its best if the spacing is uniform. Placing a 16d nail between boards before attaching driving screws or nails usually results in a gap of 1/8 inch. If a wider gap is desired, you can prepare a spacing template from a length of plywood or other lumber. Even though decking boards do not expand and contract lengthwise as much as they do across their width, you should still provide a gap of 1/8 inch between the ends of adjacent boards.