Window sill problem

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Forum topic by jklingel posted 02-07-2016 09:29 PM 673 views 0 times favorited 10 replies Add to Favorites Watch
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98 posts in 1126 days

02-07-2016 09:29 PM

I have made 13” wide window sills (inside sills; the outer sills are Azec) and am concerned about how to accommodate the inevitable width change w/ the seasons. The sills are an 8” wide and a 5” wide x 3/4” thick white oak board glued together. When the boards extend out toward the room, they have “ears” that wrap around the rounded corners of the window wells. The wood sills extend into the room 1 3/8”. The ears are where the problem is: If the boards change dimension, those ears will either be tight to the sheetrock or swell away from it. I considered cutting out the sheetrock where the ears are and imbedding the ears into the ‘rock. That way the wood can change dimension and the ears will always be inside the 5/8” sheetrock. At one time I considered rabbeting the two boards along the joining edges and overlapping them, but canned that idea. Now I am thinking about splitting them lengthwise down the middle and inserting each half into a plastic or steel H-beam. I would then secure the outer board near the window and the inner board near the inside (room) edge and let them float in the middle, sliding in the H-beam as they change dimension. I hope I am making myself clear. Questions: (1) How much dimension change is there going to be? We probably have a 30-40% humidity change during the year. (2) Is there a better way to let the boards change dimension w/out being noticed? I am open to ideas, including splitting the boards down the middle, adding a spline to one board and cutting a groove in the other to let the boards move in and out that way. In the winter I would have a small bit of the spline showing, and in the summer the boards would expand and be (fairly) tight. Ideas? Azec instead? Thanks. john

10 replies so far

View TiggerWood's profile


271 posts in 1576 days

#1 posted 02-08-2016 03:16 AM

I’m having a hard time picturing what you are saying. If possible I would put up a couple of pics. With windows that I have made or fixed, the stool was too small to worry about wood movement. It just never concerned me. I also have made windows so that there is space for wood movement to go toward the outside, the “ears” wouldn’t move. Your idea of cutting the sheetrock to fit the ears sounded okay but, I think it would be challenging to make it look nice and I would rather deal with a ~1mm gap there.

View David Taylor's profile

David Taylor

326 posts in 1056 days

#2 posted 02-08-2016 03:22 AM

“How much dimension change is there going to be? We probably have a 30-40% humidity change during the year.”

You don’t say whether you are using quarter sawn or flat sawn material. According to, if flat sawn, oak, white: will swell 0.009 inches in thickness and 0.328 inches in width. Final dimensions will be 0.759×13.328 if quarter sawn, oak, white: will swell 0.019 inches in thickness and 0.162 inches in width. Final dimensions will be 0.769×13.162 For these calculations, I used 70 degrees, with RH going from 30% to 70%. 3/4×13” pieces.

I like the bury it in sheet rock idea, then whatever movement you have won’t show.

-- Learn Relentlessly

View Nubsnstubs's profile


1282 posts in 1699 days

#3 posted 02-08-2016 03:30 AM

After you glue up your wood, fit it to the window, then remove it, paint all raw wood. Install it into it’s opening. Caulk the ears, and enjoy the windows until they rot out in 50+ years…....... Jerry (in Tucson)

-- Jerry (in Tucson)

View JBrow's profile


1350 posts in 889 days

#4 posted 02-08-2016 04:11 AM


Dimensional change in the White Oak is, for me, difficult to calculate. But if the oak is sanded equally of all 6 sides and several layers of a film finish like spar polyurethane are applied to all 6 sides, a significant reduction in wood movement should occur. If you really want to figure out shrinkage, refer to the “Wood Database” for White Oak, where they provide percent shrinkage for white oak from its naturally wet state to an oven dried state – probably not all that helpful. Click on “Shrinkage” on the web page for an explanation of the shrinkage values provided.

Some ideas:

1. Attach the seal and allow for the ears to move as they will. Trim the remaining horizontal and vertical room side window corners with case work where the vertical case work will conceal the movement of the ears.

2) If the white oak sills are attached toward the front (room side) of the sill, then expansion and contraction will occur at the window edge of the sill. As a result the ears would remain tight to the wall while the opposite edge of the oak is free to expand and contract.

Following this method of fastening, an expansion gap would be created at the window. This gap could be covered with a piece of trim affixed to the widow frame and NOT to the sill. Expansion would occur under the trim.

3) Alternatively, if the oak sill could slip under the window, the window itself would hide any expansion and contraction.

4) Another thought is to resurrect the ship lap idea you discarded. However, rather than leaving the ship lap joint square, celebrate the joint with a chamfer or bead detail. Perhaps with the 8” wide stock, create two ship laps with a milling detail. This would avoid an uneven window seal and give the window sill a custom look, when compared to the I-beam idea.

5) The last, somewhat unsettling, option would be to fasten the oak as described above, but leave a ¼” gap from the oak to the window. Then apply a bead of silicone latex caulk to conceal the gap.

View jklingel's profile


98 posts in 1126 days

#5 posted 02-08-2016 04:45 AM

Thanks for all the ideas, links, and time to do those calcs! Sorry that I did not think to mention the cut of the wood; it is all flat sawn. I will have to chew on JBrow’s #4, but I’ll get it figgered out. I am a beginner woodworker, so I don’t want to screw up a bunch of spendy oak by getting too far over my head. Plus, I’ve been working on this f^&%$#*@ house for a couple of years and would not mind getting it behind me. Window sills are the second last project to do, and I’d like to be “done” before the house is so old routine maintenance overwhelms me. I have posted a pic of a sill template, as it just occurred to me what “img” meant above this text box. Again, many thanks. I was just discussing the trim-against-the-window-frame idea w/ my son and will look into using quarter round. I don’t know if the fiberglass frames will take 18 gauge brads, so I may have to rely on goo to hold the 1/4 round to them. cheers. john

View TheFridge's profile


9253 posts in 1455 days

#6 posted 02-08-2016 05:03 AM

Gotcha. I think the grain is usually ran with the sill.

-- Shooting down the walls of heartache. Bang bang. I am. The warrior.

View jklingel's profile


98 posts in 1126 days

#7 posted 02-08-2016 05:11 AM

Fridge: Yes, it will be. This is just a scrap so I can get the ears right. Nothing is perfectly perpendicular to anything else, so getting the ears to fit the inner wall whilst the edge fits the sill wall (return, I think it is called) will be a matter of customizing each side of each sill. Tedious, but certainly doable.

View rwe2156's profile


2886 posts in 1450 days

#8 posted 02-08-2016 12:18 PM

I don’t think it will be an issue.
You can always mount a piece of trim to the wall but not the board.

-- Everything is a prototype thats why its one of a kind!!

View dhazelton's profile


2756 posts in 2266 days

#9 posted 02-08-2016 02:08 PM

As you won’t have any moulding around the window with your rounded corner I would make the piece out of oak veneered plywood with an edge band. It’ll never move.

View jklingel's profile


98 posts in 1126 days

#10 posted 02-08-2016 05:59 PM

Funny you should mention plywood. My wife asked about that last night, so I am going to see who has, or can order, a sheet. So that I don’t waste all the sills I’ve already cut, I think I’ll make the inner edge, including ears, out of solid wood glued to the plywood. Thanks for the suggestions.

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