I have a 2nd floor patio that I would like to tile. The patio is covered by a overhang roof and sreened from floor to ceiling. It has a plywood subfloor (exterior carpet currently).

Although the patio is covered, it does get some rain (not direct pours) if it storms. I want to use backer board (maybe 'Wonderboard' brand) over the plywood. I am leaning towards also using 'Redgaurd' waterproofing over the backerboard. Is this overkill?

I live in Northern Virginia/across the river from DC where we usually have mild winters with snow so freezing is a factor also. I am hoping Redguard will help with cracks associated to freezing moisure. I am not loyal to any brand because I am a weekend DIY... the brands I plan on using are in stock at my local harware store.

Any advice or recommendations will be most appreciated. Thanks...

You did not mention a critical item how thick is the ply, is it tongue and grove and what is the spacing of the joists. You need to make this floor stiff and a layer of Hardiebacher or Wonderboard might not do it for you. 3/4" t&g ply on 12" centers should be the minimum.

Subfloor is either 1/2 or 3/4 inch plywoord on 12" center. I bought the house three years ago and it was built in 1971.

The "Wonderboard" I plan on using is 1/2" because it is exterior. Again, not toatlly exposed to rain, but wind driven rain/moisture does sometimes make it.

This is the reason I am asking about the Redguard watersealant.

Drill a hole somewhere and find out which you have, 3/4" minimum is required. Considering the age I'd go thicker before adding the backer. The thickness of the backer has little influence to the structural integrity.

Thanks for the advice... I drilled and it is 3/4.

OK so you are at the minimum and are you willing to stick with minimum? Its up to you but I'd also check for deflection. Lay a steel straight edge or piece of conduit perpendicular to the joists. Have someone put their eye right near the floor as you walk around the straight edge. See if they spot any additional light coming under the straigh edge as you walk on the floor. If none, screw and glue your backer down per James Hardie's instructions. HARDIBACKER INSTALLATION GUIDE Hardibacker Installation Instructions also available in (http://www.holdenhumphrey.com/jh/hardibackerinstall.htm)

Need to add a little here...

Did you pull up that ext. carpet? Make sure that ply is sound and not already damaged by previous water infilteration. What kind of tile

Also, you will need to ensure that this an sturdy area. This is NOT an interior application. You will have to use all materials, including thin set, that are rated for exterior applications. Covered area or not.

You need to think of this area as you would any other project being outside.
I am not big on tiling over ply to an ext. room.

I checked on "ditra" underlayment. This can be used on the exterior, do a search and read up on that product. As long as you have a very solid surface (you may have to add another layer), this may do the trick. Just be sure you have a way for the water to drain out of your covered patio/area after you install your tile.

Just be sure you have a way for the water to drain out of your covered patio/area after you install your tile.

In my opinion that's the most important part of this entire task.