I live in an 18 year old house which is sided with 5/8″ T1-11 fir siding with

redwood or cedar 1/4 (rough surface) corner board and window trim. 

All the wood is stained with a semi-transparent stain. The

siding is installed over 3/4″ foam insulation and is cut to withing 3/8″ of the

windows. The windows are aluminum clad wood with no built in moulding –

just a plastic strip to nail it to the wall. The 1/4 trim is nailed to the siding,

butting it up against the aluminum window frame. My main point of concern is the

way the siding and the trim are installed around the windows.

Earlier this spring, I was washing the outside windows when I noticed that the

plywood was rotting under the trim on the bottom of the window. When the

trim was removed, I found the siding rotted both on the top and the bottom

the entire width of the window. There was no caulk in the seam between the

plywood the window or the plywood and the trim. When I purchased the house

several years ago, I had caulked all the trim at the top of the windows to prevent

water going down behind the top trim pieces.

Question: Is this an acceptable way of intalling t1-11 around a window? I find

that hard to believe.

I haven’t noticed any other decay around any of the other windows. This window

is on the north face of the house, near a big oak tree. I’ve notice that it doesn’t

get much sun there and stays damp long after it rains. I1 have also trimmed the tree to allow more sun in.

Question: Is it a combination of poor contruction and the shade that is causing

the problem or should I expect the plywood to rot around all my windows soon?

I’ve been doing some research at the lumber yard and home centers and have

discovered that I could trim the window with a wood or urethane brick mould and

caulk between it and the window frame. Then I could install a brick mould flashing

on the top edge of the moulding which would extend under the siding. Finally, I

could replace the T1-11 with Hardipanel which looks like T1-11.

Question: If I put a brick mould and flashing in place, How do I finish off the

edge of the siding panel where it meets the flashing? This window is 8′ wide

which means if the edge is exposed, at least two (probably 3) sheets of

siding will have to be cut very accurately to look nice. And won’t the exposed

edges just suffer the same fate as when covered with the cedar trim?

Right now, I’m tempted to put it back the way it was and just caulk every

possible seam as tight as possible. However, If anyone has any suggestions,

I’d be most grateful.

John