A Sill Replacement


Editorial & Photography By: Dr. Christopher Cooper


Image 1

Many early domestic and commercial buildings built of brick and random rubble stone have wooden sills, possibly because the stone or brick mason on the job site was not well versed in the subtleties of fine-cut-stone sills or because the owner of the home could not afford such a luxury.  Many Georgian through Queen Anne Revival brick homes in Canada have wooden sills.  The Blenheim Farms House is no exception.  The house is a sturdy triple-brick house built in 1837; however the odd take on a common brick bond denotes a style probably executed by someone who was not in the masons’ trade very long, or, possibly, by the home owner himself.  The rather crude lay-up of the bricks could have been the reason for the wooden sills: the mason was not experienced in the art of refined stone work.

The problem with wooden sills is that the copious amounts of water held in any porous masonry during rainy and damp seasons tend to transfer to the wooden sill unabridged by a non-penetrable barrier, which was not available 169 years ago, except for tin and lead.  Most of these wooden sills were cut and shaped to size and a bed of mortar was laid with the sill simply mortared into place; thus the wooden sills are subject to the ravages of cuboidal wet rot.

I have also noted that the sub-sill on the main window frame, if not adequately caulked, will allow rain water falling on the window and sub-sill to migrate under and between the sub-sill and the main sill, the catalyst for the rot process.  In our “Project Sill” at Blenheim Farms House, all the sills not having been caulked properly, are doomed.

Image 2

Caulking the exterior of a window is important in stopping air infiltration between the brick and the exterior wooden architraves (casing).  But one must remember to caulk only three sides of the window – top, left and right sides (you must also caulk the joint between the sub-sill and main sill).  Never be tempted to caulk the area below the main sill, since doing so will trap water and rot the sill.  Lime mortar should be used in this area, allowing water and moisture to pass through. 

We chose one of the worst sills to replace: the west sill below one of the original 6/6 wooden windows on the first floor (see image 1).  We could simply purchase a piece of timber with the dimensions of the original.  However, larger pieces of timber today are boxed-heart (the centre of the tree is in the centre of the raw dressed timber), making the timber subject to tangential shrinkage, which will crack if the wood is not perfectly dry.  Moreover, finding a good supplier of larger timbers is a job in itself, especially quarter-sawn timber that would be the timber of choice, eliminating distortion and splitting.  We have taken the high-tech approach to building a laminated wood sill out of kiln-dried 2 x 12 Douglas fir, select structural timber, laminated together with waterproof glue.

The sill will have three lengths of Douglas fir glued together, available at your local lumber yard; this will create a very strong sill once it has been sealed with a liquid epoxy penetrant on all six sides before priming and painting.  The good thing about using three individual pieces is that you will not require large heavy-duty band saws to cut the angled profile required on the face of the sill.  Each of the three pieces can be passed through a small table saw to achieve the desired angle.  Also the dip edge for the bottom of the sill can be easily cut using the table saw as well.

Image 3

A word of caution.  The wooden sills, frames, and sub-sills, present in wooden structures and some masonry structures, are part of the structural system of the building, effectively supporting the level above (floor, siding or masonry), especially in a balloon-framed house.  A structural engineer must be consulted prior to executing any of the works noted below.  Always work safely and cautiously and use eye protection, ear protection, and CSA approved work boots.  Always read and understand all safety precautions when using power tools.  Never remove safety features and fences from power tools.

The first step is to document the sill in place prior to its removal, using a digital camera.  A small card on the face will denote location and date (see Image 2).  Take pictures from both sides, and the top and the bottom, as well as an overall image of the window.

Carefully remove any mortar around the sill that is, in most cases, the only material device holding the sill in place.  Take great care to not damage any of the bricks, note the sloppy Portland cement patches (see Image 3).  The sub-sill may have some spikes driven through and down into the main sill.  These can be cut off easily with a hacksaw blade slid between the sub-sill and the main sill.  In the case of our “Project Sill” if there were any spikes used, they have long since corroded away. 

Image 4

By grasping both sides of the sill you can now extract it with small horizontal side-to-side movements while pulling the sill towards you, away from the window (see image 4).

You can see that this sill is completely rotted out to the brick, the failure being due in part to the failure to caulk the union of the sub-sill and sill (see image 5 & 6).

Image 5

Image 6

Removal of the sub-sill can be easily done by cutting any spikes nailed from the sides of the frame.  If dadoed in place, cut the sub-sill in half and pull down and towards the centre effectively pulling the halved sub-sill away from the frame and any spikes.  Cut the spikes flush with the inside of the dado. 

At this time inspect to see if there are signs of insect infestation and if present, contact your local pest control firm to give the best advice on the eradication of insects prior to your proceeding any further with this project.  Note: the carpenter ant damage on our project sill (see image 7).  With a shop vacuum clean up the area of the sill.  You will soon see if the bottom of the frame is rotted.  The lower area of the frame at the junction of the sub-sill is rotted on this window, and requires an in-situ epoxy repair (see image 8).

Image 7

Image 8

The spongy wood is first dried thoroughly using an infrared paint stripper1 (see image 9).  By means of a moisture meter, check the wood fiber for any moisture content (see image 10).   Once an acceptable range of dryness exists, a penetrating liquid epoxy2 can be effectively brushed on and allowed to penetrate the spongy wood fiber, turning it into a structural petrified mass.  After the liquid epoxy has set, a paste epoxy2 can be used to slightly overfill any voids not filled by the liquid epoxy.  Allow the paste epoxy to harden, (usually overnight) sand and/or file the affected areas of epoxy to the desired shape.  The epoxy will hold a screw or nail satisfactorily.  Paint all exposed areas with an oil-based primer and two coats of traditional oil or 100% acrylic latex paint3.

Image 9

Image 10

Once the existing sill and sub-sill are removed from the wall, they require careful documentation; they are the templates for the new sills.  Measure the length, width, and thickness of the sill(s), including the dimensions of the drip edge on the main sill.  Using an adjustable square find the angle to which the face of the existing sill is cut.  Blenheim Farms House has six windows which share the same dimensions of sill and sub-sill; you will use these measurements to transfer onto the sill blanks. 

Rough cut the 2 x 12 timber to the length of the existing sill, adding several inches to the length in case the table saw nicks the ends of the boards when passed through.  Do the same with the sub-sill blank (reclaimed first-growth 6/4 pine).  Set the table saw to the same angle as the face of the existing sill (in this case 8 degrees), and run the three blanks and sub-sill blank through the table saw (see image 11).

Image 11

With the table saw still set to the correct angle, lower the blade and run one of the 2 x 12’s through the saw to cut the drip edge.  Several passes were needed to make the ¼” slot.  Reset the fence for each consecutive pass (see image 12). 

Image 12

Set up a planer and run one of the boards through it to reduce the thickness.  The 2 x 12’s actual sizes are 1 ½” x 11 ½”.  We require a sill 4” thick; therefore, one board (the centre) needs to be reduced by a ½” (see image 13).  Glue up the three 2 x 12’s with the angled faces lined up, a liberal coating of waterproof glue is applied and screwed together using stainless steel screws, make sure not to screw where you need to make cuts, allow the glue to set up overnight, (see image 14).

Image 13

Image 14

In the morning transfer the documented measurements to the top of the main sill and sub-sill blanks, and cut out the waste with a good sharp hand saw (see image 15).  The inside corners of the sill and sub-sill are then squared using a rabbet plane, and the whole of the surface is hand planed and lightly sanded with fine sand paper, giving the sill(s) a hand-wrought appearance (see image 16).

Image 15

Image 16

A liberal coating of liquid epoxy is brushed on to all six sides; paying particular attention to the end grain and drip edge on the main sill (see image 17).  After the liquid epoxy is dry, the sill(s) are lightly hand-sanded and primed with an oil-based primer on all six sides.  Two coats of oil paint were applied over the primer3 in areas which would not be exposed; for example, under the sub-sill.

Image 17

The next task is to install the sub-sill on the window frame, using stainless steel wood screws, fastened from underneath (see image 18). 

Image 18

Image 19

EPDM pond liner is a material which has an unlimited service life and will not be damaged by sharp corners of masonry which would penetrate a regular piece of 6mil poly, rendering it useless for separating the wood from the masonry.  Cut a piece which will effectively wrap the back, top, bottom and sides of the sill where embedded into the masonry (this will also create an effective expansion joint).  Use a staple gun with stainless steel staples along the top and bottom edge to fix the EPDM to the sill (see image 19).  Caulk the inside corners with clear silicone .

Image 20

Image 21

The cavity in the masonry work left by the old rotted sill should now be prepared to receive the new sill.  Remove any loose or damaged mortar and dry-fit the sill into place, with the EPDM pond liner material attached (see image 20).  Check that the sill is sitting in the same position and angle as did the original, by using an adjustable square set to the angle of the face of the original sill (in this case 8 degrees).  Once the sill is effectively dry-fitted, mix a small batch of lime mortar4.  Never use Portland cement!  Moisten the existing bricks with a garden sprayer thoroughly and trowel on a bed of mortar (see image 21).  Set the new sill in place with the EPDM pond liner attached, tap with a rubber mallet to seat the sill in the mortar, checking the angle regularly with the adjustable square.  As added insurance we drove three stainless steel screws vertically through the sub-sill into the main sill and filled the countersunk holes with paste epoxy.

Image 22

Image 23

Repoint the areas around the brick work with the same lime mortar mix4 (see image 22) and trim the EPDM flush with the mortar, being careful not to cut into the wooden sill.  Run a bead of caulking at the joint of the sub-sill and the main sill as well as around the bottom of the frame where it meets the sub-sill.  Apply the final coat of primer and top coat3 to your planned or existing colour scheme.  This project took my apprentices, Joey and David, (this is the first time they have done this type of work) a total of 8 hours in actual time, not including the drying times of the epoxies and paint etc (see image 23, finished prior to final colour top-coat).


NEW WORKSHOPS | MARCH | APRIL | MAY 2020


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