Sill Weather Checks

Removing & installing sash, temporary boardup, frame & sill repairs, wall sealing, frame & trim painting.
johnleeke
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Sill Weather Checks

Postby johnleeke » Mon Aug 29, 2016 7:04 pm

Fill Sill Weather Checks

References: the book Save America’s Windows, p. 24, 2009

Title of Treatment: Fill Weather Checks
Class of Treatment: [ ] Maintain, [ ] Stabilize, [X] Repair, [ ] Upgrade, [ ] Exception
Type of Treatment: [ ] Traditional, [X] Contemporary

Condition to be Treated: Weather checks in the top weathering surface of wood sills. This method is also effective for weather checks in casing boards and other exterior woodwork.
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Description: Deep cracks, called weather checks, often occur in sills on the south and west side of the building. The purpose of filling weather checks is to provide a continuous surface that is smooth enough to hold paint and to drain water down the slope of the sill. Minimize damage to the outer surface of the sill when removing paint, cleaning out the checks and trimming the cured filler. The checks must be filled completely down to their narrow bottoms. There must be bands of bare wood at the top surface between the checks that have no epoxy treatment at all so moisture can escape from within the sill. It is seldom necessary to make the surface perfectly flat and true. A flat true surface carried across the whole sill might look odd next to weather beaten exterior casing and clapboards. Usually the somewhat uneven eroded contours and “corduroy texture” of the sill can be accepted as a sign of age as long as the checks are filled and the surface can drain.

Typical Procedure:
1. Remove Paint from sill. Minimize damage to the wood. In some cases paint can be left in place when the paint film is in good condition. With the paint in place epoxy materials may be less likely to soak into the wood surface between the checks.
2. Clean out checks, removing all dust and debris down to the narrow bottom. Scrape inner sides of checks to expose bare bright wood. Minimize damage to the wood.
3. Dry the wood of the sill.
4. Prime checks with epoxy primer or consolidant. Apply with a narrow spouted bottle and artist's pallet knife to keep the primer in the checks and limit the spread of the primer onto the surface of the sill. Apply a few or several times as it soaks in to assure complete penetration.
5. Fill checks with epoxy paste filler and allow to cure.
6. Trim filler flush with surrounding wood. Minimize damage to the wood.
7. Sand surface of sill.
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Materials:
Long-term side-by-side comparative field testing has shown that epoxy fillers provide a longer-lasting repair than other types of fillers. Epoxy materials formulated especially for wood-epoxy repairs should be used. The epoxies adhere tenaciously to the sides of the checks, flex with seasonal wood movement and provide a good base for paint. Other materials could be used with this method, such as, traditional glazing putty made of linseed oil and whiting or contemporary sealants made of acrylic or other flexible compounds. These other materials may have a much shorter maintenance cycle than epoxies. But, they may provide other advantages, such as reversibility, which may needed in a conservation context.

· Epoxy liquid resins, 2 part, to use as a primer and consolidant, formulated especially for wood-epoxy repairs
· Epoxy paste filler , 2 part, made of epoxy resins and powdery fillers, formulated especially for wood-epoxy repairs
· Solvent for clean up
· Rags for clean up

Quality of Results:

Best Work: Strips of bare untreated wood are exposed between the filled weather checks, allowing moisture to migrate out of the sill. Epoxy filling is flush with surrounding wood and fills the entire check to the very bottom. The sill surface forms a contour that is flat enough to hold a paint system well and to drain well.
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Inadequate Work: Epoxy forms a thin skim or thick plate that covers large sections of the surface of the sill, which can trap moisture leading to decay of the sill. Some weather checks are not filled.
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For more details of this work also see p. 20-27 of the book "Save America's Windows":
http://saveamericaswindows.com/get-the-book/
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johnleeke
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Drying the Sill

Postby johnleeke » Tue Jul 04, 2017 7:04 pm

If possible schedule this work in the middle or at the end of the dry season, whenever that is for your region. For example, here in Maine that is April and May. Down South that might be in the Fall. In Tucson that might anytime other than the Monsoons.

For wood-epoxy repairs in general the wood should be 15 to 20% EMC (Equilibrium Moisture Content) or dryer, as measured by a moisture meter. Here in Northern New England most painted exterior softwood is about 15% during service, so after I have removed the paint from the sill and removed the debris from the weather checks, I try to dry the wood of the sill down to 12 or 13%. To do this I cover it with clear 6-mil poly sheeting. See the beginning and end of the video below, and you will notice how I have tapped or stapled the edge of the sheeting under the lower edge of the sill. During drying I put 3 or 4 stones or chunks of wood scraps on the top of the sill, flip the sheeting up so it loosely covers the sill, then close down the sash on the sheeting, which holds the sheeting in place. I leave the ends of the sheeting open so air can breeze through there and carry away the moisture as it rises out of the wood. Sun shining on the sill through the sheeting warms the wood, helping to dry it out. In a temperate climate during the spring, summer or fall it might take 2 to 3 weeks to dry out a 2 to 3 inch thick sill so it is ready for check filling.

During epoxy work and painting, the sheeting is flipped down as you see in the video below, and protects the wall from dripping epoxy and paint, and can be easily flip up to protect the epoxy and paint from rain and damp weather over night, then flipped down and back up for successive coats of paint.

johnleeke
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Filler Materials

Postby johnleeke » Tue Jul 04, 2017 7:08 pm

Filler Materials

The materials used have far less effect on the durability than the repair design and the methods of preparation and application. For example, drying the full thickness of the sill down to 2 or 3 percent moisture content below the equilibrium moisture content that the sill will attain during the later service life, will result in the wood swelling slightly after the repair is completed and painted, putting the filler in slight compression, meaning the bond of the filler and the sides of the checks will not be in tension that could result in failure, allowing water to get into the sill.

Epoxy: 38+ year proven life (+ means it hasn't failed yet) in formal field tests using the method shown Window Preservation Standards, pages 41-42 and Save America's Windows, pages 21-27, using epoxy materials.

Putty: 8+ year proven life in formal field tests using the methods shown in the video below.
https://youtu.be/TvW4GF26vyo

Typically I expect putty filler to have an 8-15 year service life, and epoxy to have a 30-50 year life. If you are encountering failures in less time than this I would expect that the cause of the failure has more to with the design of the repair or the preparation and application methods, than the filler material. This is based on the many failures I have investigated over the past 40 years. To see a common failure type look up page 23 in Save America's Windows.

We have also done one field test with high-performance sealants, which can work quite well, because they can have greater adhesion and greater elongation resilience that either putty or epoxies. Products like Sonelastic's NP1, a one-part moisture cure urethane, came out ahead in the test with a service life of 23+ years, although this is just one test. Your mileage may vary.


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