Window RO and an angled framed sill
Tiziano
| Posted in General Questions on
Does framing a rough sill with a 5 degree slant affect the RO needed for the window?
The windows will be outies on a 2×6 wall with 7/16″ ZIP, 2″ CI and a 3/4″ rainscreen gap for a total sill depth of 8 11/16″.
The slant adds 13/16″ to the dimension between the exterior edges of the header and sill. Going in 2″ (a rough estimate of the window’s depth) it’s only 5/8″. From this I assume that I should add 5/8″ to the manufacturer’s RO height.
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Replies
Tiziano,
With outie flanged windows I use a table saw to only slope the portion the sill that will be under the frame. That makes casing the interior easier. If the slope is equal to the height of the shims you are using, the RO doesn’t change at all.
To me the most important elements for protecting against bulk water intrusion there are a sill pan with back-dam, and a drainage path. Sloping the sill I’m not as convinced by.
I’ve removed a lot of old windows with rotten framing where the jambs and sill meet. That was invariably the result of persistent wetting over long periods of time. Much like rain-screens on walls, I have my doubts as to whether the wetting ever amounts to enough volume to actually drain.
Thanks Malcolm.
Seems like we asked the same question within days of one another. I got an answer from Randy: https://www.greenbuildingadvisor.com/question/how-much-height-to-add-to-ro-for-a-sloped-sill
Valahous,
It depends on a couple of variables.
- Whether you are sloping the framed sill, or adding sloped material to it.
- Where the window sits in the RO.
My plan is to add material, like clapboard or a piece of cedar siding to the flat sill, because I'm using a prefab'd SIP panel wall assembly.
My current thinking is to have the windows closer to flush mount and add a picture frame casing. Leaving the rest of the 5.5" width of the wall assembly on the interior flat for the trim inside. I need to double check how thick my flangeless aluminum window units are now that I write that.