Can I install new decking directly over old decking?
No; this is not a recommended practice. In many decking replacement jobs, joists end up being replaced, repaired or capped. This is due to the common discovery of decay beneath the decking, usually where the fastener penetrated the joist. When updating a deck with new decking, it’s crucial that the old decking be removed so the joists and beams can be assessed and repaired if necessary.
Installing decking over old decking will drastically reduce the lifespan of the material. The joints between two materials will readily trap water in wet environments. This contact area should be limited only to the top of the joists, leaving most of the decking exposed on both sides. If placed on top of other decking, there will be little drying potential beneath the new decking and more rapid decay of both the new and old can be expected. For manufactured products, installations over old decking or solid surfaces are prohibited and will likely void the warranty.