How to prolong rot in a decking sub-base

I found some rot in my decking which I laid about 15 years ago but when I took a couple of the lengths up the frame underneath was in quite a poor state. Rotten in places and heavily black stained and damp in others.

I've decide to completely renew but wonder how to protect the subframe from the same fate.

I'm using pretreated timber but the surfaces that trap water like under the joint between deck board and joists is a problem also cut ends on the timber frame.

My initial thoughts turned to painting the top surface of the frame with Blackjack to seal the interface between the deck and the joist and also paint over the cut ends of any joist which need cutting.

Is this a good way forward or are there better solutions bearing in mind that once down the base cannot get any further protection like the decking boards.

Richard

Have you considered using plastic for the parts in contact with the ground? Won't rot and you won't have to look at it. More expensive, though.
https://www.envirobuild.com/products/solid-anti-rot-joist

Yes, cost is an issue as it is a fairly large deck and I have just taken delivery of the timber.

Richard

I used walther strong deck tape last year to good effect. Put it over all the joists and beams and am only a year in but it seems to be keeping everything nice and dry.

That looks good but I've just had 150M of 47mm x 95mm delivered so 3 rolls plus extra for noggin ends, so likely to be about £80. Think I will stick with Black Jack.

At least manufacturers have recognised the problem with decking joints.

Richard

Yeah it's certainly not the cheapest, but they do give you a free packet of biscuits...