Decks, Patios, Porches and Docks - Help! Ant are eating my screen porch!

Just when i think i will have an easy summer i notice a little rot on my screened in porch. As i pull off some boards i notice that the wood was extremly wet, but even worse i noticed that carpenter ants had burrowed deep into the wood. :mad: Eventually i ended up pulling off 16 feet of siding and exposing all of the supporting structure. Almost all of the support below the windows have been infested with ant damage. From the window sill and up the wood is not wet and not infested. I have started to replace the bottom structure with pressure treated wood. My question is how far do i go up? Do i scab in the 2X4s with the beams that go between the windows or do i replace the beams all the way to the roof to have adequate support? I want to make sure i have enough support. I know that i have to find the source of how it is getting wet... but i also want to make sure that the porch is structually sound for the winter snow. Any advice would be very appreciated.
thanks.

1. Find the source of the moisture and eliminate it. Moisture causes Dry-rot.
2. Exterminate the critters.
3. PT lumber is only required within 6" of soil or in contact with concrete.

That said, you can use PT lumber anyplace you want. In Hawaii all framing is done with PT lumber because of the generally moist conditions.

Genarally, it's best to replace each stud from bottom to top. If you scab on the side of an existing stud that's damaged, be sure to cut off the damaged portion and scab another piece in place of it. You want the structural members to be continuous in some way. Nails are not very good for transferring the loads from one member to another. Like I said - just replace each damaged piece of wood completely.

Thanks for the feed back!!
I did cut off all the roted wood so now i have solid wood pressing down on top of the PT wood..... i was hoping this would offer adequate support... but you feel i should still replace it all the way up??.. i was hoping to get around that since it involves almost rebuilding the entire wall... inside and out.. but i still want to do it right and not have to re-do it again next yr...
Can i send you a pic of what i did? maybe that would help?

If you have a 2x4 directly below and in line with the existing stud and have attached it with a piece scabbed on the sid then you are fine.

IOW:

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will work as long as there is at least 18" of overlap of the black piece on both the red and blue pieces with 3 16d nails in each of the pieces.

Thanks Joe for your input..
i just put a 1/8 thickness metal strapping (51/2 inches by 15 inches) used for porches across the 2x4s. its about 15" long so it spans over 7 inches in each direction. Also, since it is going across 2 2x4 nailed together (so width is 3 inches) the metal plate as 5.5inches wide so i bent over 2 inches length wise and now it wraps around the 2x4's and is bolted on both sides. It seems really study and i think that the plywood against it will even make it stronger. Your thougths.
i looked at trying to replace the post all the way up and i would have to take apart the entire wall.

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