A: I’ve resurfaced a number of decks and yes, you do need to remove the old wood before synthetic lumber goes down. Structurally speaking, applying composite decking on top of non-rotten wood will work for a while, but it will also promote rot of the boards underneath because of the trapped water between the two layers. When the wood underneath rots, you’ll have a mushy mess supporting your synthetic deck boards. Not good.
Looking at the stairs in the photo you sent, it looks like the span of the treads is 2 1/2 or 3 feet wide. This is too far for synthetic lumber to span on its own, so you’ll need to add supports underneath. Lengths of 2×4 on their edge like little floor joists extending from one stair stringer to the other will provide the kind of support needed. Also, use the thickest synthetic lumber you can for the stair treads. Material 1 1/2” thick is the thickest I’ve seen.
There are two tools that will help you remove your existing deck boards more easily. A nail puller is an effective tool for gripping and pulling nails that are pounded in all the way. A reciprocating saw with a hacksaw blade is my favourite power tool for careful disassembly of the kind you’re planning. Simply saw through the nails and screws holding the deck boards down and you can remove them without damage or struggle.
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