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.
Be sure you live your life, because you are a long time dead.-Scottish Proverb