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Historic Preservation - Technical Procedures

Spectitle:

Repair Of Star Cracks In Copper Roofs

Procedure code:

0761012R

Source:

Hspg Prepared For Nps - Sero

Division:

Thermal And Moisture Protection

Section:

Sheet Metal Roofing

Last Modified:

02/24/2012

Details:

Repair Of Star Cracks In Copper Roofs



REPAIR OF STAR CRACKS IN COPPER ROOFS


THIS PROCEDURE HAS THE POTENTIAL TO ALTER THE HISTORIC APPEARANCE
OR CHARACTER OF A BUILDING.  IT SHOULD ONLY BE PERFORMED BY AN
EXPERIENCED PROFESSIONAL AND ONLY UPON APPROVAL FROM THE REGIONAL
HISTORIC PRESERVATION OFFICER (RHPO) OR DESIGNATED REPRESENTATIVE.

THIS PROCEDURE SHOULD BE PERFORMED UNDER THE DIRECTION OF AN
HISTORICAL ARCHITECT OR ENGINEER TO DECIDE THE MOST EFFICIENT AND
LEAST DESTRUCTIVE MANNER FOR EXECUTING THE WORK.


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on repairing copper
         sheetmetal roofing damaged with star cracks.  GENERALLY,
         THIS WORK SHOULD BE ACCOMPLISHED BY AN EXPERIENCED
         ROOFING CONTRACTOR.

         1.   Star cracks are star shaped cracks which occur
              because of repeated flexing or bending of the
              sheetmetal due to wind loads or thermal stresses.
              As the sheets are flexed up they tend to fold in a
              diagonal pattern.  Stresses are created where these
              lines cross and "star cracks" develop.
   
         2.   If left unrepaired, star cracks will get larger and
              eventually allow rain and wind to penetrate the
              covering.  The combined wind loading above and
              below the sheet can cause large areas of the
              covering to become detached from the decking, and
              torn from the roof by a high gust of wind.

    B.   Bay size in copper roofing is extremely important, as it
         can limit the potential for damage to bays by windlift.
         Copper roofing is more susceptible to damage by windlift
         than other materials for several reasons:

         1.   Copper roofing is relatively light weight and,
              therefore, lacks some of the inherent stability of
              heavier materials whose shear weight allow them to
              resist negative wind loading.  

         2.   Individual copper bays have a soft temper in order
              to facilitate hand fabrication.

         3.   Individual copper bays are not profiled, fixed, or
              bonded in the areas between joints to allow for
              thermal movement of the sheetmetal.

    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).

1.02 DEFINITIONS

    A.   anneal--the operation of heating and cooling the metal to
         soften it and make it less brittle.

    B.   bay--a unit of sheet covering as laid between rolls or
         standing seams.

    C.   cleats or clips--metal strips cut to lengths to suit roll
         or seam, placed at intervals and securely fixed to the
         roof base, the ends being welted in with the edges of the
         sheets to hold the sheetmetal roofing in position.

1.03 DELIVERY, STORAGE AND HANDLING

    A.   Packing and Shipping:

         1.   Hoisting equipment and procedures will depend on
              the design of the panels, weight, and length.
              Lengths of approximately 35 feet are generally the
              maximum for rail or truck shipment, but expansion-
              type truck trailers are used to handle lengths up
              to 60 feet.

         2.   Manufacturer's delivery or job markings on metal,
              and adhesives for manufacturer's labels shall be a
              neutral material.  In no case shall such material
              be alkaline; any staining of the metal by alkaline
              materials will cause for the rejection of the
              piece.

    B.   Storage and Protection:

         1.   Material storage:  Keep uninstalled roof materials
              under cover, dry, free from scratches,
              condensation, and distortion during delivery,
              storage, and handling.

         2.   Salvage storage:  Historic material to be used as
              example of original construction shall be stored as
              directed by the RHPO.

         3.   Heavy bundles of nested panels require suitable
              mechanical equipment and reasonable care must be
              taken to prevent damage to corners and edges.

1.04 PROJECT/SITE CONDITIONS

    A.   Environmental Requirements:  Do not begin repair in misty
         or rainy weather.  Do not apply metal roofing to wet roof
         sheathing.

1.05 MAINTENANCE

    A.   Keep the roof clear of debris, and trim all overhanging
         branches that might cause mechanical damage.

    B.   Inspect for and eliminate ant hills and/or bird droppings
         which can corrode sheetmetals.

    C.   Inspect the secureness of cleats and fasteners and the
         condition of the sheetmetal after particularly heavy
         storms.


PART 2---PRODUCTS

2.01 MATERIALS

    A.   Copper nails with large flat heads and barbed shank

    B.   Copper cleats

    C.   Copper sheets, to match weight and temper of existing

    D.   Rosin paper

2.02 EQUIPMENT

    A.   Chicken ladder, safety belt or harness

    B.   Protective gloves and gear

    C.   Straight snips for cutting straight or slightly curved
         lines in sheetmetal 24 gauge or lighter

    D.   Metal seamer

    E.   The application of sheetmetal roofing requires a full
         range of metal-working tools and shop equipment, plus
         special handling, hoisting equipment, and machinery for
         long lengths

    F.   Fire extinguisher


PART 3---EXECUTION

3.01 EXAMINATION

    A.   Detect for early signs of wind displacement:  Listen for
         a drumming noise during windy conditions caused by the
         vibration of the sheets striking the roof deck.
 
    B.   Look for poorly designed roof edges or improperly spaced
         or decayed decking which permits wind penetration.  Check
         the underside of the roof deck from the attic to detect
         leaks.  Make repairs as necessary.

    C.   In addition to scheduled inspections, inspect after each
         exposure to unusually severe weather conditions
         especially strong winds.  Look for visual cracking as
         described in Section 1.01 A. 1. above.

3.02 PREPARATION

    A.   Protection:

         1.   At the end of each work day, provide building
              protection for any exterior roofing element removed
              for repair or replacement.

         2.   Remove only a quantity of roofing which may be
              replaced on that same day.  At the end of the day,
              use 15 pound roofing felt or polyethylene sheeting
              to drape over missing roofing and insert under roof
              unit laps or temporarily secure areas of existing
              roofing and roof as required to make roof
              watertight and windproof.

3.03 ERECTION, INSTALLATION, APPLICATION

    A.   For localized damage such as at the verge, ridge or eaves
         of a roof, replace damaged sheets with narrower and
         possibly shorter verge bays and shorter ridge and eave
         bays.

         NOTE:  AT THE VERGE BAYS, DO NOT EXCEED A NET WIDTH OF
         1'-3" WITH DOUBLE LOCK CROSS WELTS SPACED NOT MORE THAN
         3'-0" ON CENTER.

         1.   Remove and replace the affected section, or shorten
              the original bay by at least a third.

         2.   If cracks are widely distributed across a bay,
              replace the entire bay with two new bays.  

              NOTE:  EACH NEW BAY SECTION SHOULD BE APPROXIMATELY
              HALF THE LENGTH OF THE ORIGINAL.

              a.   Before replacing a bay, cut back worked edges
                   of affected bays to flat unworked metal.

              b.   If worked edges must be retained, flatten
                   edges and anneal before reworking to join
                   replacement bays.

                   NOTE:  WHEN ANNEALING, TAKE PRECAUTIONS
                   AGAINST FIRE.  INSERT FIRE-RESISTANT
                   INSULATION PADS OR SHEETING BETWEEN COPPER AND
                   ANY COMBUSTIBLE MATERIAL DURING ANNEALING AND
                   LEAVE IN PLACE UNTIL THE COPPER HAS COOLED.

                   NOTE:  DO NOT ANNEAL LATER THAN TWO HOURS
                   BEFORE WORK IS FINISHED FOR THE DAY.

                   NOTE:  DO NOT RE-USE FASTENERS OF ANY KIND.
                   COPPER CLIPS, FIXING STRIPS, NAILS AND SCREWS
                   SHOULD BE NEW AND OF THE APPROVED SIZE AND/OR
                   THICKNESS.

    B.   If cracks are widely distributed throughout the entire
         roof, the roof will need to be replaced using narrower
         and shorter bays.  Selective replacement is not
         appropriate, because it is likely that remaining
         undamaged bays will eventually become damaged.

                             END OF SECTION