Drainage Detail for Window Sill Pan
I am replacing some windows and I am looking for advice on how to ensure proper drainage of the window sill pans.
My existing window openings do not have sill pans. The 30+ year old wood house has building paper over the plywood sheathing and cedar clapboard siding over the building paper. No air gap between siding and building paper. (ie no furring strips/rainwall design)
IRC Code 703.4 says: “Pan flashing shall be sealed or sloped in such a manner as to direct water to the surface of the exterior wall finish or to the water-resistive barrier for subsequent drainage.”
I can’t find any details that show how the sloped sill pan will actually drain to the exterior. Dumping the water over the flashing which you adhere to the building paper below window seems like it will cause water to be trapped or blocked by the clapboards (or apron on some of my windows) and not travel down and out or to the exterior of siding.
Ideally I would like to have a good path to the exterior of my siding so I don’t have to worry about the chance of water backing up or being trapped behind clapboards.
You are right this is a detail you do not see as often as you should. The way most windows are flashed they rely heavily on the house wrap to prevent water from entering and it does drain down behind the siding. If using a rain screen the issue is resolved a bit, but house wrap is still getting wet. If you have clapboards butting tight to window best to double flash the sill. So you flash window as recommended by IRC, but install another piece of flashing on top of that. Do not adhere it to the vertical side of wall and have it hang long +/- 12”. When running clapboards this piece of flashing goes over the top of the clapboard just prior to the clapboards that is installed tight to the sill. Trim the flashing just shy of the bottom of the lap by 1” and it will look as nothing is there. The clapboard tight to the sill may rot over time, but all clapboards below are protected. If using trim around windows the detail is a bit different as it usually sits proud of the window and siding.
Thank you, jlyda, for quick reply..
The devil is in the details, so I tried to sketch it up (see attached pics). Is this close to what you were recommending? Looking at blowup detailed sketch, do I create a gap for water to drain behind the tight clapboard or apron using shims (the orange spacer in sketch)?
The red line is the building paper, the green is normal sill pan, and the yellow is the second flashing to kick out water to the outside of clapboards.
What type of window are you installing? New construction with nail fin or replacement windows with no nail fin? From your pics it looks like you have a sill board on the exterior that extends beyond the siding? Are you installing replacement windows on top of existing sill?
RE: From your pics it looks like you have a sill board on the exterior that extends beyond the siding?
YES, this is the detail for only 4 of my windows that are in my 2 walkout bay window bump outs. (see pic) The rest have clapboards under sill.
I have about 30 DH windows that I want to replace. Have not purchased all of the replacements yet, so would like to have a detail or details that work no matter which way I go. For windows with a history of water / sill rot (three) I may go with the Marvin Full Frame Flanged replacement window, but budget is important so if I find a non-flanged full frame replacement that is cheaper, I won't rule it out. For others I would prefer to go with inserts (although people have told me this is not the recommended approach). If I go with inserts for windows with no water problem, I would still like to replace the existing sill with PVC and install the sill pan as best I can.
If keeping the sill you will need replacement windows w/ no nail fin. Simple to do actually if you know how to use a siding brake. Use trim coil and bend your own sill pan covering the exterior sill, all in one piece. The replacement window will sit right on top of that. Need to have a little bend like you see on drip edge for water retention break. Use a thicker gauge trim coil. If you don’t have a lot of experience w/ a siding brake, give a vinyl siding guy a call, they have a ton of experience. Can probably bend all the pieces you need in less than an hour.
Okay, that might be a good approach for some of my windows. Can you comment on my detail with spacing shims to provide drain path....for applications where I will remove the existing window sill and have the pan applied to the rough opening sill?
Thanks for all the support
In my opinion the shimming option isn’t great. I’m making a few assumptions based on how I’m reading your close up detail. I’m assuming you are planning to picture frame around the window? And you are concerned about water making its way into the wall at sill because the picture frame sits proud of the window (which happens often)? While you are protecting siding below the trim with your flashing detail the trim itself isn’t protected from water. It’s all a bit complicated actually I will do my best to explain.
If installing replacement windows, no siding or existing trims come off as you are attaching replacement window to the frame of the original window. You simply wrap the existing sill in trim coil and make that your sill pan, so no flashing of the rough opening sill.
To install new windows in a new work type application I look at it in two ways. If the window is proud of exterior wall finish or inset from exterior wall finish. You will flash the RO sill with both scenarios, but outside they are treated differently.
If your window frame sits proud of the siding or trim you can simply caulk siding or trim at sill and be done. I would still use the method I described earlier to double flash the sill just in case there is caulk failure and water retention at base of window allows for water to make its way behind the siding.
If your window is inset from trim or siding it’s a bit more work and you need to protect the top edge of trim board at sill from standing water. Also, the water isn’t really being diverted away from the outside plane of the wall anymore as the window is basically inset from outside plane of wall. You need to kick the water away using flashing. To do this you will need to drop the exterior trim at sill about 1” lower from bottom of window. You will then bend a custom piece of flashing to go over the trim piece. It’s basically a Z flashing. So you will bend flashing with a 7/8” leg that will go under the window above the trim and sit tight against sheathing/housewrap. This 7/8” leg is the only area that will see nails, use trim coil nails and use them sparingly. You basically make a cap w/ the rest of the bends to go over the trim piece. Wrap around the front of the trim 3/4” to 1” and make a little pressure bend to kick the water away (like a drip edge flashing). Apply caulk over trim coil nails and apply a bead right at base of window where it meets the flashing. Does this make sense? I would sketch something up on paper real quick, but I’m on daddy duty w/ a newborn on my lap at the moment.
Thanks for help on this. To be honest, I am not 100% sure I am following it, but I will study it some more to make sure I get it right. I am very grateful for all the followup.
Congrats on the newborn. My youngest of 4 is 27, so its been a while for me. Enjoy it bc it sure goes by fast.:)