Replacing Broken Glass In Wood And Metal Windows

Procedure code:
Hspg Prepared For Nps - Sero
Doors And Windows
Glass & Glazing
Last Modified:


Replacing Broken Glass In Wood And Metal Windows

Procedure code: 0880001R



    A.   This procedure includes guidance on replacing cracked, broken or missing panes of glass, replacing cracked or missing window putty and cleaning glazing in standard windows and doors. Repairs that involve art glass, stained glass, leaded glass, beveled glass or plate glass require special skills, and should only be attempted by or under the guidance of a trained conservator.

    B.   Broken or cracked glass panes and missing or cracked window putty may be the result of weather, neglect, or vandalism.  In any case, it is a matter that requires immediate attention.

    C.   For temporary repairs to broken glass until a permanent replacement can be performed, see 0880002R "Temporary Patching of Chips and Cracks in Window Glazing".

    D.   See 0110007S for general project guidelines to be reviewed along with this procedure.  These guidelines cover the following sections:

         1.   Safety Precautions

         2.   Historic Structures Precautions

         3.   Submittals

         4.   Quality Assurance

         5.   Delivery, Storage and Handling

         6.   Project/Site Conditions

         7.   Sequencing and Scheduling

         8.   General Protection (Surface and Surrounding)



    A.   A window glass is in proper condition when it is set securely and tightly into the window frame, is properly caulked, and is not scratched, cracked, or broken.


    A.   Coordination of Work:  The coordination of glass repair or replacement with other proposed projects must be considered.  For example, if work on other window elements is anticipated (involving but not limited to the frame, sash, trim, lintel, sill, or hardware,  paint removal, cleaning, or repairing), it best to postpone any glazing work until last.



    A.   For new glass you will find numerous North American manufacturers.  Most repairs will be undertaken utilizing the products are distributed in your area, and because of industry standardization there will be little difference between them.  What will have the most important impact is the specification of the proper type of glass to be used in a particular application, something that is usually an engineering, architectural, building code or safety regulation compliance issue.  Some manufacturers include:

         1.   Advanced Coating Technology;  AFG Industries, Inc.; Cardinal IG; Environmental Glass Products; Falconer Glass Industries;  Ford Glass Division; Guardian Industries Corp.;  Hordis Brothers, Inc.;  Libby-Owens-Ford (LOF) Glass, Inc.; Pilkington Sales (North America) Limited; PPG Industries, Inc.; Saint-Gobain/Euroglass;  Spectrum Glass Prod. Div., H. H. Robertson Co; Viracon, Inc.


    A.   Linseed oil putty (for wood windows)

    B.   Glazing compound or elastomeric sealant (for metal windows):

             1.   One-component advanced urethane sealant such as

a. "Bostik 2000" (Bostik Construction Products Division)

b.  "Pecora DynaTrol��� I-XL" (Pecora Corp.)

- or approved equal.

2.   One-part non-acid-curing silicone glazing sealant such as

a. "Chem-Calk 1250" (Bostik Construction Products Div.)

b.  ���Dow Corning 790" (Dow Corning Corp.)

c. "864 NST" (Pecora Corp.)

-or approved equal.

             3.   These glazing sealants should comply with the following requirements:

a.   Must be compatible with other materials with which they will come into contact.

b.   Must be suitable for applications indicated and conditions at time of installation.

c.   Colors:  Provide color of exposed sealants as selected by the RHPO from manufacturer's standard colors.

d.   Hardness & Flexibility:  Consult the manufacturer to determine if the sealant meets the actual hardness or flexibility parameters needed for this particular installation and use. 

              e.   Sealants and materials used with laminated glass to be 100% solids, containing no solvents.

    C.   Materials for Removing Glazing Compound:

            1.   Mineral Spirits:

                        a.   A petroleum distillate that is used especially as a paint or varnish thinner.  It was developed as an inexpensive replacement for the vegetable-based turpentine, and is a light version of kerosene. It comes in three grades, and cost rises as refining quality increases.

                         b.   Other chemical or common names include Benzine (not Benzene); Naphtha; Petroleum spirits; White spirit; Varisol; Solvent naphtha; Stoddard solvent.

                        c.   Potential Hazards:  TOXIC AND FLAMMABLE.

                        d.   Safety Precautions:

                                    1.   Work in a well ventilated area.

                                     2.   ALWAYS wear proper PPE such as rubber gloves, safety glasses/goggles and a properly rated respirator when handling any solvent such as mineral spirits.

                         3.   AVOID REPEATED OR PROLONGED SKIN CONTACT.  If any chemical is splashed onto the skin, wash immediately with soap and water.

                                     4.   Available from construction specialties distributors, hardware store, paint store, or printer's supply distributor.


              Linseed oil or thinned primer

    D.   Glass to match existing

1. Glass comes in a variety of thicknesses and types.  For this application we will be discussing standard flat glass, often called ���float glass��� because of the process used to create sheets.  It can be cut with specialty hand-tools (glass cutters with a steel or diamond tip). This material will usually come in thicknesses of :

a.         3/32��� (2.5mm) (formerly referred to as single-strength glass, not to be confused with single-pane glass in windows and doors)

b.         1/8��� (3.2mm), which was formerly referred to as double-strength glass, not to be confused with double-pane or insulated units.  

2. Although you will probably not be using safety glass, it is important to understand its uses, properties, and the corresponding weight of each type. It will most frequently come in one of the following types for windows 

            a.         Tempered glass, which will usually come in 1/8��� (3.2mm) thickness, and can be identified by a small safety rating and manufacturer symbol etched into a corner of the surface, with slightly rounded edges all around the piece.  .  It will weigh the same as a piece of non-tempered glass of the same dimensions.  This type needs to be custom-ordered from a factory and made to exact size.  As it has been heat or chemically treated, it will shatter if you attempt to cut it.  

b.         Laminated glass will most commonly be encountered as two sheets of 3/32��� (2.5mm) glass layered together like a sandwich, with a clear sheet of Polyvinyl or another modern plastic in between. This particular type would be referred to as 5.38mm laminated glass, and weighs over twice the amount of an individual single-thickness sheet of glass (which on a sash window, or other application where balancing weights are installed, will have a significant impact).  It can only be cut to size in a professional glass shop.

            c.         Wired glass is infrequently seen except in areas requiring fire-resistance, such as in fire doors or fire-rated partitions (such as entries to boiler rooms or exit stairwells). It is quite thick (1/4��� or greater), and the weight is correspondingly larger.  It is not as widely used today as safety glass because new methods have supplanted many of its previous uses, and personal contact with it can still cause serious injury. 

    E.   Glazier's points

                        1.         There are several types, the three most common are the triangular point, the diamond point, and the so called ���push��� point.  Of these three, the last is the easiest for the casual user to install.  These are usually formed from the metal zinc.

    F.   Neoprene setting blocks and shims

    G.   Clean, potable water

    H.   Ammonia

    I.   Paper towels or rags



    A.   For Replacing a Window Pane:

         1.   Goggles and gloves for protection when removing broken glass

         2.   Hammer and chisel

         3.   Soldering iron wrapped in foil, to help warm and remove old glazing compound

         4.   Needle-nose pliers, end ���cutters, and chisels for maneuvering glazier's points

         5.   Sandpaper

         6.   Very fine 0000 steel wool

         7.   Paint brush to apply primer

         8.   Glass cutter and straight edge

         9.   Putty knife or glazier's tool for smoothing glazing compound



    A.   Check for cracked, broken, chipped, or otherwise damaged glass. 

    B.   Inspect glazing putty on both sides of pane for cracked, loose, or missing sections which allow water to attack the window components, especially at the joints.

    C.   Examine the condition of the window components for rot, corrosion, loose connections, etc.

         1.   Note if the glass rattles or moves in the glazing system.

         2.   Note if the glass stops are intact.

    D.   Inspect all surfaces which are to receive glass and/or glazing sealant.  Take special note of any defects or condition which will interfere with, or prevent a satisfactory installation.  Correct all defects prior to installation of new glass.

    E.   Verify the glass type in each window type prior to the installation of new glass.


    A.   Surface Preparation:

         1.   Prior to reglazing, remove all oil, dirt, rust and other materials from the glass and the metal framing members using solvents such as toluol or xylol.

Afetr removing loose material on a steel window frame, neutralize rust with one of the commonly available products  on the market such as Loctite ���Extend Rust Neutralizer���   or CRC ���Rust Converter���

         2.   Clean and prime all glazing rabbets prior to glazing. Reference 0861001R and 085002R.

         3.   Maintain glass than has been removed in a safe place and in reasonably clean condition during construction, so that it will not be damaged or need to be recleaned of corrosive contaminants.



    A.   After the intended pieces of glass are removed, remove the glazing compound / putty left behind using any or all of the following  methods:

         1.   A hammer and glazing chisel (at the risk of damage to adjacent glazing) or a triangular / tear-drop scraper.         

2.   A soldering iron wrapped in aluminum foil can be used to soften the putty to ease removal.

         3.   Chemicals such as a mineral spirits or linseed oil (if the originally putty is linseed oil based, which most legacy putties are).  CAUTION:  Consult the MSDS for handling cautions and to understand the PPE requirements on these materials.

    B.   Remove glazier's points using needle-nose pliers and end-cutters.  Discard.

    C.   Special Procedures For Wood Windows:

         1.   Thoroughly clean the sash of any remaining compound and sand rabbets smooth. Be alert for leftover glazing brads in the rabbets of the wooden glazing bars.

         2.   Apply linseed oil or thinned oil based primer to rabbets to prevent wood from absorbing oil from new putty.  If primer is used it should be applied in two coats, 24 hours apart. Make sure you verify your new putty is compatible with linseed oil before attempting this step.

    D.   Special Procedures For Metal Windows:

         1.   While the glass is out, clean/repair/replace, prime and paint the metal frame, the mullions, muntins, sash, and other window components prior to glass reinstallation.

         2.   Apply glazing compound to the rabbets of the window sash.

         3.   Metal windows use special, sometimes proprietary fasters.  Therefore, the existing glazing clips, glazing beads, and other fasteners should be cleaned and reused whenever possible.  Where existing metal glazing clips are missing you will need to supply and install new wire (metal) glazing clips to match existing, even if it means having them specially fabricated.  Architectural salvage operations are a good source for these items.

    E.   To replace damaged panes, new glass will need to be cut 1/8" smaller in length and width than the existing opening.  There are several online videos readily accessible that explain this process.  PLEASE NOTE: Proper PPE, especially including eye and hand protection, is absolutely necessary when cutting glass.

         1.   Practice cutting on an unusable piece of glass first.

         2.   Make sure the working surface is perfectly clean and do not press too hard with the glass cutter.

                        a.   Old window glass can be quite thin, and may also contain impurities and irregular internal tensions.  Pressure from the wheel cutter on even a tiny piece of dirt can cause the glass to split or "run" in an unintended direction.

         3.   When cutting straight pieces, use a straight edge as a guide.

                        a.   Score the piece with one firm, even stroke of a sharp glass cutter.  If it is a carbide steel cutter, dipping it in kerosene or mineral spirits will improve the cutting action, while lengthening the life of the cutting wheel.  If using a diamond cutter lubricant should not be used.

                        b.   Once glass is scored, there are several methods of breaking it along the score line. Here are three: 

1.         The first method involves placing the glass with the score up and lined up with the edge of a workbench. 

a.   Holding the ���waste��� piece beyond the score with a gloved hand, lift the overhanging edge about 1��� above the workbench (the other end will remain on the workbench). 

 b.   Next, bring the glass down sharply against the table edge, and the glass should snap along the score.  When trying this for the first time, do so gently to start, and then each time increase the speed of the drop and the downward pressure until you become practiced at gauging the right balance for a clean break along the score.

2.         The second method involves scoring the glass as above, but afterward the piece is turned over so the score faces downward and that mark is placed about an inch off the edge of the workbench. 

On the top (or un-scored site), carefully tap along the score with the ball end of the glass cutter to ���run��� the break from one end of the pane to the other, while supporting the scrap part of the pane to be cut off (which is off the table) with your gloved hand. 

Note that glass cutting can be done more safely if you place a large plastic trash can under the edge of the workbench to contain the small shards that are being ejected by the tapping.

3.         The third method utilizes one of several types of plastic glass-cutter's pliers which may be used to snap the glass along the score, with a bit of practice. 

         4.   When cutting  curved pieces:  Practice is highly recommended. 

a.   Make a template out of thick cardboard or masonite board to guide the glass cutter.  The template should be slightly smaller than the desired piece to allow for half the width of the glasscutter, usually about 1/8��� in each dimension.

b.   Practice on a scrap piece of glass- Score the piece with a sharp glass cutter following the edge of the template.  A diamond cutter is preferred for complex shapes. Do not try to score the piece freehand.

c.  Score lines from the initial curved score line off at a tangient to facilitate the removal of extra scrap glass.

d.   Turn the piece of glass over and place it on the workbench with some of the newly scored marks on the bottom, overhanging the edge of the workbench by about an inch.

While supporting the overhanging piece with your free hand, use the ball end of the cutter and tap the top of the glass along the main score, starting in the middle and working toward both ends gradually.  Next, tap the tangential lines made for the scrap parts.

Note that glass cutting can be done more safely if you place a large plastic trash can under the edge of the workbench to contain the small shards that are being ejected by the tapping.

The score-line should fracture along the curve, but even practiced professionals can find this difficult.  Gradual curves may be broken off in one piece, but it is easier when attempting extreme curves to cut and remove one small section of glass at a time.

5.   For pieces with complex cuts, consult a trained conservator or stained glass craftsperson.

    F.   Apply a small bead of glazing compound on the glazing bar rabbet as bedding putty,  to cushion the new glass,  and then install the glass with spacing evenly distributed on all sides. Be sure the compound is properly chosen to work with the window material (wood, steel, etc).

    G.   Replace glazier's points 4" to 6" apart around the perimeter, and tap them halfway in.  If using a glazing point driver (which is similar to a stapler), be sure not to unduly stress the glass with too much downward pressure.

    H.   Form glazing compound into a 3/8" diameter rope and press around perimeter of new glass.  Using a putty knife, triangulate the surface of the compound.  Hold the knife         at a 45 degree angle and align compound with the muntin on the interior.

    I.   Allow the compound to dry for a week, then paint accordingly with 1/16" of paint extending onto the surface of the glass to act as a moisture seal.


    A.   After the installation of the frame is complete, remove all non-permanent labels from the glass.

    B.   After glazing compound has cured (and been sealed with paint in the case of traditional putties), wash the glass on both sides with a mild solution of soapy water.         NOTE:  ALKALINE OR ABRASIVE AGENTS ARE NEVER TO BE USED TO CLEAN GLASS.  CARE SHALL BE TAKEN DURING CLEANING TO AVOID SCRATCHING OF GLASS SURFACES THAT WOULD OCCUR IF USING GRITTY MATERIALS OR DRY CLOTHS.

    C.   Thoroughly rinse away any soap residue with clean, clear water.  If desired, at this point a glass cleaning solution containing alcohol or ammonia may be used on the glass (unless the frame has been finished with shellac).    See 0880001P for details on cleaning glass.

    D.   Dry both sides of glass with a soft cotton dry cloth.  The use of newspapers is an effective traditional method for buffing the surface of glass after work is complete.

    E.   Clean any excess glazing compound or spills from frames, sash and adjacent surfaces promptly after installation.

                             END OF SECTION


Last Reviewed 2015-02-02