Trex is engineered for 16" spacing and that is why you had a failure at the
47" space.
The design you show would work.
I would use 2x6 or same size joist hangers.
I avoid sandwiching of boards where possible.
Especially with planters and wet surface.
Trex cut like wax/plastic/sawdust -- in other words it cuts like butter.
I use a 1/4 round on edges.
You may see some bubble air pockets in any rip.
pressure treated material underpinning is best.
Screws work....torque drive 3" stainless best.
They do have special brackets now for trex also.
john

"Bill" wrote in message ...

SWMBO left heavy flower pots on each end of our 48" unsupported step
made out of Trex this summer, and the Trex boards warped (very bad--more
than 2").

The design looks poor, so that's where I'll point the blame. ; )

Here is a drawing of the current design along with my proposed solution:
http://web.newsguy.com/MySite/deckstep.jpg

Questions:
1. I would prefer to use screws (rather than nails) to add new
structural support, so I don't cause damage. 2.5", pre-drilled?

2. In the diagram of my proposed solution, I would have screws being
(not) held by end grain. I know that this is a problem.. What is a
smart, easy and inexpensive way to get around this here? It looks like
I need a brace/bracket of some sort. I would like my repair to last! : )

3. One of the new 5.5" wide Trex boards will have to be "ripped" to
4.25". Is the stuff difficult to cut? Can I use a cheap 40 or 60 tooth
blade (do I need to use a router)? The Trex website suggests a 40-tooth
"carbide tipped" blade and/or router, I believe. I can practice on the
stuff I remove, but as soon as I remove any, I'm going to create a
"safety hazard". I haven't even bought the new Trex yet, I'm just planning.

Here is a copy of the SU file, if anyone would care to use it.
http://web.newsguy.com/MySite/DeckSteps.skp

(Thanks)
Bill