When faced with the prospect of old deck covering, the knee-jerk reaction of many is that the entire decking needs to be replaced. However, in many cases, the old decking can continue to serve as a base for a new layer, provided it is in good condition and the proper installation methods are used.
Cleaning and Sealing
If you are working with a composite decking material, wash the decking with a pressure washer to remove any mold, dirt and debris that can get in the way of the new installation. With wood decking, first wash the deck clean with a pressure washer, and give the deck at least 48 hours to dry. Once it has dried, apply a layer of sealer to ensure the wood is properly protected against moisture and insects before installing a new layer on top.
Any old, loose planks or sections must first be fastened down or completely repaired. In the case of composite decking, often times, the screws and mounts come loose with age and traffic, so check to ensure these are tightly fastened. Replace old fasteners if required. For wood, also check that the wood is in good condition. Any rotted or insect-damaged pieces of wood decking first must be replaced with a similar piece of material and installed in a similar fashion with the surrounding pieces to provide a suitable mount for the new decking.
You have several choices when installing new decking material. Follow the same direction as the previous installation for continuity, or install the new decking in the opposite direction, provided the previous layer is soundly fastened. When working in the same direction as the first time, either directly overlay the new planks on top of the old ones, or offset them so they are spanning the slight gap between the planks from the first installation. Optionally, run things at a diagonal if you are proficient with carpentry tools and have the extra time for this more complex layout.
Remember to provide spacing between planks per the manufacturer's instructions for expansion and contraction. Failure to do so results in deck planks buckling under pressure in warmer and wetter months. When installing on top of an existing layer of decking, make sure the fasteners (nails or screws, as directed by the manufacturer) are twice as long as recommended since you are going through double layers of deck planking before sinking into the deck joists. This allows for superior mounting strength.