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Historic Preservation - Technical Procedures

Spectitle:

Three Methods Of Preventing Icedams On Slate Tile Roofs

Procedure code:

0731504R

Source:

Developed For Hspg (Nps - Sero)

Division:

Thermal And Moisture Protection

Section:

Slate Shingles

Last Modified:

02/24/2012

Details:

Three Methods Of Preventing Icedams On Slate Tile Roofs



THREE METHODS OF PREVENTING ICEDAMS ON SLATE TILE ROOFS


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure describes three alternatives for
         preventing icedams on slate tile roofs.

    B.   Ice-damming can occur on roof eaves when ice and snow
         back up along gutters.  

         1.   Ice can build up at the eaves from the intermittent
              melting of snow.  As the snow above melts and runs
              down the roof, it eventually reaches the cold roof
              eaves where it may refreeze creating a "dam" and
              enabling additional snow to accumulate.

         2.   Accumulated water may then seep behind roof
              shingles causing damaged to interior building
              components such as insulation, plaster and framing
              members.

    C.   See 01100-07-S 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)

         These guidelines should be reviewed prior to performing
         this procedure and should be followed, when applicable,
         along with recommendations from the Regional Historic
         Preservation Officer (RHPO).


PART 2---PRODUCTS

2.01 MANUFACTURERS

    A.   W.R. Grace & Co.
         62 Whitmore Ave.
         Cambridge, MA  02140
         800/242-4476
         "Ice & Water Shield"

    B.   Windy Hill Forge
         3824 Schroeder Ave.
         Perry Hall, MD  21118
         301/256-5890
         "Snowguards"

2.02 MATERIALS

    NOTE:  Chemical products are sometimes sold under a common
    name.  This usually means that the substance is not as pure as
    the same chemical sold under its chemical name.  The grade of
    purity of common name substances, however, is usually adequate
    for stain removal work, and these products should be purchased
    when available, as they tend to be less expensive.  Common
    names are indicated below by an asterisk (*).

    A.   For Metal Ice Edge:

         1.   Galvanized Steel:  Available in 4 ft. x 8 ft.
              sheets that permit a 3 ft. exposure of the metal
              with a 1 ft. overlap of shingles.  For more steeply
              pitched roofs, less exposure is needed and 3 ft. x
              8 ft. sheets should be adequate.

         2.   Metal primer and paint for exterior use

         3.   Mineral Spirits:

              a.   A petroleum distillate that is used especially
                   as a paint or varnish thinner.

              b.   Other chemical or common names include
                   Benzine* (not Benzene); Naphtha*; Petroleum
                   spirits*; Solvent naphtha*.

              c.   Potential Hazards:  TOXIC AND FLAMMABLE.

              d.   Safety Precautions:

                   1)   AVOID REPEATED OR PROLONGED SKIN CONTACT.

                   2)   ALWAYS wear rubber gloves when handling
                        mineral spirits.

                   3)   If any chemical is splashed onto the
                        skin, wash immediately with soap and
                        water.

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

    B.   For Ice & Water Shield:  

         1.   A waterproofing sheet made of flexible, rubberized
              asphalt laminated to a polyethylene film.  

         2.   Used as an underlayment along the edges of wood
              shingle, slate, tile, cedar shake or metal roofs.


PART 3---EXECUTION

3.01 ERECTION, INSTALLATION, APPLICATION

    A.   Method 1 -  Ensure There is Adequate Insulation and
         Ventilation in the Attic.

    B.   Method 2 - Install a Metal Ice Edge:  A metal edge
         absorbs heat faster than shingles and, thus, tends to
         shed ice from its slick surface during thaw.

         NOTE:  GALVANIZED STEEL IS PREFERRED OVER ALUMINUM.  IT
         IS NEARLY AS INEXPENSIVE, MORE DURABLE AND DOES NOT
         EXPAND AND CONTRACT AS MUCH.  HOWEVER, IT MUST BE
         PAINTED.

         1.   Paint the galvanized steel.

              a.   Wash the metal with a solvent such as paint
                   thinner or mineral spirits to remove any dirt
                   or oil.

              b.   Etch it with a zinc primer and apply two coats
                   of exterior paint.

         2.   Firmly bed the metal in roofing cement and join in
              sections with a crimped standing seam.

              NOTE:  It is preferable to cut the sheets in half,
              providing seams every 4 feet, though it is possible
              to join the full 8 foot length sections.  HAVING
              MORE JOINTS GIVES GREATER ALLOWANCE FOR EXPANSION
              AND CONTRACTION.

    C.   Method 3 - Install an Ice & Water Shield (in combination
         with a snowguard):  

         NOTE:  THE BEST TIME TO INSTALL THIS SYSTEM IS DURING A
         REROOFING JOB.

         1.   Apply to a smooth and continuous sheathing.

         2.   The space under the membrane should be well vented,
              as the shield acts as a vapor barrier.

         3.   If the shield is not installed during a reroofing
              job, the lower 3 feet of the exposed roof will need
              to be removed.

                         END OF SECTION