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rotting T & G fir decking/finished undersides

I need to replace 2nd floor painted T&G fir decking that is on a plywood sub-floor covered with rubber matting. Wood began rotting after 3 years. The underside has a finished ceiling. Third floor drips on second floor porch, and the drop is about 2" for the 8ft. depth. Can you recommend a way to waterproof the deck and prevent the boards from rotting. House in midwest, so have snow/ice in winter and heat/humidity in summer. I have columns and metal rails on each porch as well. Thanks for any ideas!:confused:

Re: rotting T & G fir decking/finished undersides

Based on your description a few things come to mind as to what's contributing to your problem.

I would have to guess the t&g planks may be a softwood like pine or perhaps a medium harder wood something like fir. These woods on their own will rot when they are subjected to constant moisture.

Painting decking materials isn't a very durable finish since paint doesn't hold up very well under traffic. Besides .... unless all 6 sides of each plank were painted there isn't a total protection given to the wood.

Being they are t&g they are relatively tightly joined together which doesn't allow water and snow to drain off very well even though there is a slope to the deck.

Depending on how much spacing there is between the underside of the decking to the membrane covered plywood base will trap moisture and not breath well because of the t&g planking. This will allow the t&g to be constantly moist underneath which will eventually rot the wood.

It might be worth considering replacing the decking with either cedar , pressure treated or composite materials. when doing this leave small gaps between each plank to allow water and snow to drain off as well the gaps will allow air circulation for the underside to dry. Also you can build the decking in sections to allow you to lift them for cleaning debris like leaves that will accumulate as well as for maintenance of the membrane if needed.

Just a thought.:)

Re: rotting T & G fir decking/finished undersides

The fir decking was coated on all sides,extra on the outward tips, too. It seems they sit on the barrier and just stay wet..no ventilation. Right now the planks run outward toward the edge of the porch with a 2" fall for the 8ft. length. I wonder if I would be better putting support strips outward, then attaching the boards lengthwise, thus allowing the underside to possibly drain and get air, and if this would be enough ventilation or just place them on the barrier again. I just don't know what to do about the outside edge finish, as the underside/bulkhead is dryvit with an under ceiling of vinyl beadboard and lights. I will have to replace the fiberglass column bases, and this design will also change the height a tiny bit. I have also thought about the lock-dry aluminum decking system, but I really like the wood look/feel if I can get a system that would last........so you think cedar would work better?

Re: rotting T & G fir decking/finished undersides

Your decking is absolutely installed wrong. It should not be installed directly on the rubber matting, this traps water under and indirect contact with the decking boards. You should install nailing spacers perpendicular to the decking, the spacers should be cut or lifted to allow water to flow under them also, this allows water to run down the matting and off the roof. T&G although tightly joined are not water proof and water will seep through the joints. A proper path for the water to escape and good air circulation is key. Because the spacers are installed through the rubber matting it is important the the nail or screw holes be sealed also.

Re: rotting T & G fir decking/finished undersides

Thank you both for your responses. I will go with the spacers perpendicular to the house,using boards lengthwise. I will figure out what kind of wood to use for the decking and how to finish off the outward edge with an extended drip ledge or small gutter. The air circulation problem solved will get me on the right track for this upcoming project. Thanks once again.

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