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Reglazing a Historic Double-Hung Window Sash

Before you replace your historic home's original wood windows, consider the historic, economic, sustainainability, and technical benefits of repairing the ones you already have. Despite what the salespeople tell you, replacement is NOT necessarily the best option... The original wood windows in your house may have been there for 100 years already - with proper repair and maintenance, they may be able to serve you for 100 more!

Repairing and sealing wood windows is not rocket science. In fact, it is within the technical grasp of most people. This article describes how to reglaze one type of historic double-hung wood window - yours may look a little different or be in worse condition. If you have any questions, please consult a professional.

The photos below were taken at a Des Moines Rehabbers Club meeting where contractor Angela Thorne demonstrated the reglazing process for the group.

This article covers reglazing only, not window repair.

Step 1 - Planning

Plan for the reglazing process to take between two to three hours for the first sash. This does not include time for repairing a damaged sash - only replacing the glass (if necessary) and reglazing. Subsequent sashes will take less time as you become better acquainted with the process. Your first one should be a sash that is in relatively good condition - tackle the serious repairs only after you are comfortable with the reglazing process.

    Note about lead based paint: most historic windows were painted with lead-based paint. Lead is toxic to humans, particularly children. Before undertaking any home repair where paint will be disturbed, it is important to understand and follow lead safe work practices.

Assemble your tools and arrange a work area where you will be able to lay the window flat. It is advisable to work at a table, either outside or in an area that is easy to clean. Old glazing is often brittle and may fly long distances when you are chipping or scraping it from the sash. Lay out thick plastic sheeting under your work area to catch debris.

Step 2 - Tools and Supplies

Tools for removing the sash from the window frame:

  • Utility knife
  • Flat prybar
  • 5" putty knife (2)
  • Hammer
  • Vice grips (2)
  • 10d or 12d Nails (2)
  • Needle nose pliers

Tools for reglazing once the sash has been removed from the window frame:

  • Scrapers (various sizes)
  • Utility knife and straight razor holder (not shown)
  • Glazing knife (essentially a putty knife with an angle bend)
  • Needle nose pliers

Supplies - available at most local hardware stores:

  • Glazing points
  • Glazing compound (DAP33 or similar)
  • Replacement glass if required
  • Sash rope or chain if required
  • Boiled linseed oil

Also needed: elbow grease and time.

Step 3 - Remove the Window Sash

Historic double-hung windows were typically designed to be removed for repair. The interior sash is held in place with a wood stop that runs vertically along the jambs. Sometimes this stop has visible screws. Other times the interior stop is tacked in with nails.

First, push the window into the down (closed position). Attach vice grips to the sash rope or put a long nail through the sash chain close to the pulleys on each side. Use a nail with a head that is too big to pass through the chain link if you choose that route. This will prevent the weights from falling down into the weight pockets when you detach the window sash.

To remove the interior stop, run a utility knife along the visible joint to break the paint seal (if it is painted). Remove any screws holding it in place. Using a thin pry bar, gently work it away from the jamb. You only may only need to remove one stop in order to ease the interior sash out of the frame. Once the sash is out of the frame, you can detach the sash ropes or chains. There are many different means of securing them, so you will have to determine the best method for yourself.

The exterior sash is held in place with thin strips of wood called "parting stops". The parting stops are usually nailed into a groove in the jamb. If you can locate where the nails are, the stop can be gently pulled out with a pair of pliers. Because the parting stop is a thin and sometimes brittle piece of wood, it is always possible that you will break it as you try to remove it. However, as with the interior stop, you may only need to remove one in order to angle the sash out.

Secure the ropes and disconnect them the same way you did with the interior sash.

You have now removed your sashes and are ready to tackle the next step!

Step 4 - Remove the Old Glazing Compound and Glass

Lay the window on your covered table with the exterior side of the window facing up. As you remove the damaged and cracked glazing compound, the glass pane will continue to be supported by the wood sash

Remove enough of the hardened glazing compound to carefully take out the glass pane. This job can go very quickly or painfully slow depending on the sash. Work slowly and carefully enough so as not to break the glass!

Chip out the "easy" glazing by inserting utility knife or razor blade (in a holder) in a crack and prying gently. Use a small scraper with a little more elbow power on the more difficult sections as shown below.

Buried underneath the glazing compound are tiny metal triangles called "glazing points". These points are actually what hold the glass pane in place. The glazing compound seals the window from wind and water. Points are typically placed every 6-8 inches. Remove them with needle nose pliers, being careful not to press against the glass. If one is particularly well secured, try bending it up (to allow removal of the glass) and wait to take it out until after the glass is out.

Test your progress periodically by GENTLY lifting up on the glass from underneath the sash. You will feel resistance from areas where the glazing compound or points still hold the glass in place. When you have removed enough of the glazing compound and points, you will be able to lift the glass pane from the sash. Set it carefully to the side and move on to the next step.

Step 5 - Prepare the Sash

If you were unable to remove any of the old glazing points and had to bend them up to remove the glass pane, now is the time to remove them. Since the glass is out of the sash, you can put some real muscle into pulling them out with a pair of needle nose pliers. Glazing points can have very sharp edges, so wearing leather gloves is particularly recommended.

Use your scraper to remove any remaining hardened glazing compound around the rabbetted (inset) edge where the glass sits.

Step 6 - Repair the Sash

This article covers the reglazing process only - repairing damaged, out-of-square, and rotted window sashes is beyond the scope. There are a variety of epoxy products for rebuilding damaged sashes and consolidating rotted wood. A carpenter (or other generally handy person) can probably repair or rebuild all but the most damaged sashes. In a worst case scenario, a mill shop can replicate a sash that is beyond repair.

Whether or not you are doing repairs to the sash, before you reinstall the glass, apply boiled linseed oil to the exposed wood rabbet where the glass is seated. It will probably soak into the wood quickly. Boiled linseed oil helps treat the wood and cure the glazing.

Step 7 - Clean the Glass

Before reinstalling the glass, scrape any paint remnants from it and clean it.

Step 8 - Reglaze the Sash

Grab a handful of glazing compound and work it in your hands until it becomes pliable and warm.

Roll a small portion into a small "snake" and place it around the rabbet where the glass will go. Seat the glass into the glazing putty. It is okay if some squeezes out - you can clean up the excess later. Install glazing points by pressing them along the surface of the glass and into the wood sash with a putty knife (2 or three per side).

Roll out larger "snake" shape the same length as a side rail, and gently place it into location. Use the 45 degree glazing knife to firmly press the glazing compound into position - the corners of the sash will serve the guide as you pull the knife towards your body. The excess glazing compound will be pressed against the corner of the wood - use your other hand to pull off this extra material.

Proceed down the entire length pressing the glazing compound into place. It doesn't have to look neat and tidy. The goal at this point is to remove the excess material and to provide the general shape. There will be ridges where you shift position.

Remember, the glazing compound is just the weather barrier. The glazing points hold the glass secure.

Next, position the glazing knife parallel with the glass, creating approximately a 30 degree angle. Run the knife in a smooth motion from one corner to the other to provide the final shaping to the glazing compound. This step may take some practice to get comfortable with, but remember: you can always remove the glazing compound and try again!

Do the same for each side and the bottom rail of each sash. Depending on the type of sash you have, the top rail may traditionally not be glazed. The rabbet (where the glass slides into a notch) provides the weather seal.

Step 9 - Replace the Sash

Replacing the sash should follow approximately the reverse procedure for removing it. Reattach the sash rope, reinstall the stops, and remove the vice grips that hold the sash rope from falling. You may also need to touch up paint on the trim and interior stops.

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