"Z Flashing" And T1-11 Siding Question ?


Old 07-11-05, 06:38 AM
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"Z Flashing" And T1-11 Siding Question ?


Moved into a house that has T1-11 vertical siding.

At the bottom, the siding is "finished off" with a horizontal board that runs the width of the house.
Typical pine board, perhaps 6" X 3/4 x ...

The bottom edges of some of the siding (rests on top of the 3/4 board edge) has wicked up water over the years, and looks as if
I'll have to replace some of it. Also, a new horizontal board, as it does improve the looks.

It has been suggested that I make sure to have the bottom edge of the siding where it meets the horizontal board
be flashed with a "Z Flashing". Certainly sounds like a good idea.

Found a picture of Z flashing on the web, and my question relates to the top most vertical edge of the flashing:
Obviously there can't be a perfect seal here between the edge of the AL flashing and the face of the siding board.

When it rains, due to small gaps between, or just capillary action, I would imagine water seeps in and probably migrates to the bottom siding edge (again).

Is this anything to worry about ?

Is the paint that goes over all of this supposed to seal it adequately, or should one run a small
bead of RTV here, or... ?

Any thoughts on all of this would be most appreciated.

BTW: T1-11 still used and available ?

Bob (PS: Is this the most appropriate forum for this ?)
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Old 07-11-05, 11:59 AM
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T1-11 siding is known for the problems you mention. Unfortunately, there's no way to prevent it from happening again. It's possible that you could cut the rotton parts off, install the z-flashing, and just put a wider trim board along the bottom. (if you didn't want to replace a lot of siding.) caulking the edge where the siding meets the z-flashing might help keep that bottom edge from getting punk again, but the problem with caulking it is that the flashing is supposed to let any water behind the siding to get out. If no water ever gets behind the siding from above, then it's not a problem. But that's one reason why it's not usually caulked.

If you put the z-flashing in and then caulk it, it's not much different than skipping the z-flashing and caulking it to the trim board. The difference would be that you have an actual mechanical flashing over your trim, rather than just relying on caulk, since caulk doesn't last forever.

One problem with caulking it would be that if the caulking comes loose from the siding in a spot or two, it would actually trap water behind the caulking and it would never get a chance to dry out. That's the other reason the z-flashing is not usually caulked- so that the bottom edge of the siding can dry out after it gets wet. Unfortunately, this is a situation where you can't have your cake and eat it too. You have to decide which would be the better option in your situation.

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