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Historic Preservation - Technical Procedures

Spectitle:

Repairing A Bowing Sheetmetal Roof

Procedure code:

0761003R

Source:

Hspg Prepared For Nps - Sero

Division:

Thermal And Moisture Protection

Section:

Sheet Metal Roofing

Last Modified:

02/24/2012

Details:

Repairing A Bowing Sheetmetal Roof



REPAIRING A BOWING SHEETMETAL ROOF


THIS PROCEDURE 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 refastening a bowing
         sheetmetal roof.  GENERALLY, THIS WORK SHOULD BE
         ACCOMPLISHED BY AN EXPERIENCED ROOFING CONTRACTOR.

    B.   Where cross welt clips have been omitted between pans of
         a sheet metal roof, the central zones of entire pans can
         be pulled upwards, eventually by as much as four to 6
         inches.  At this point upstands to standing seams and
         roll joints are pulled apart and the sheets of metal will
         be irreparably damaged.  The amount of bowing varies
         depending on the metal used, and its weight.

    C.   Safety Precautions:

         1.   Wear rubber-soled shoes that have nonslip or grid
              type tread (preferably sneakers with a high top for
              good ankle support).  Avoid wearing loose clothing.

         2.   Wear a safety belt or harness and secure it to a
              substantial chimney or to a window on the opposite
              side of the house.  Leave only enough slack so you
              can work comfortably in one area, and adjust the
              slack as you work on other sections of the roof.

         3.   Be sure the roof is clear of debris and water.

         4.   Do not work on wet snow covered roofs.  Work on
              cleated walkboards.

         5.   Steep roofs:  On roof slopes greater than 4 inches
              rise per foot, special consideration must be given
              to both footing and materials handling.

              a.   Secure chicken ladders or cleats at the top
                   for adequate footing.

              b.   Hang and secure approved safety lines with
                   manila rope.

              c.   Carry a limited number of materials so that
                   balance and footing are not impaired.

              d.   Use scaffolding, ladders, and working
                   platforms as required to execute the work.
                   Ladders shall not be supported on hanging
                   gutters.  These gutters may be distorted which
                   can affect the slope to drain.

    D.   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 before performing
         this procedure and should be followed, when applicable,
         along with recommendations from the 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.

    D.   flat seam--a seam between adjacent metal sheets, formed
         by turning up both edges, folding them over, and then
         flattening.  A flat seam joint is usually soldered.

    E.   standing seam--a seam between adjacent metal sheets,
         formed by turning up edges of two adjacent sheets, and
         then folding them over, but leaving them standing.

    F.   batten seam--a seam in metal roofing that is formed
         around a wood strip.

    G.   cross seam--cross seams are intended to provide the
         correct length for sheet metal pans, they should be
         staggered from bay to bay to make the seams stronger.

    H.   pan--a formed metal sheet, usually about 21 inches wide
         by 28 inches long (maximum allowable length depends on
         the type of metal used).  Includes both the flat sections
         and any upturns or folds required for the seams.

    I.   welting--joining metal sheets at their edges by folding
         together.  Welting may be single or double folds, called
         single and double welts respectively.

    J.   dummy welt--folding of a long length of sheet metal
         without actually cutting the metal.  The resulting
         appearance mimics a true welt.

1.03 SYSTEM DESCRIPTION

    A.   Metal sheet pans should show no evidence of bowing or
         deformation in the central zone of the panel.  All metal
         pans, clips or cleats, and all fasteners are of the same
         or compatible metal.

1.04 DELIVERY, STORAGE, AND HANDLING

    A.   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.  Often original roof metal
              scrap pieces with exposed weather can be found in
              attic spaces.

         3.   Heavy bundles of nested panels require suitable
              mechanical equipment.  Take care to prevent damage
              to corners and edges during handling or storage of
              metal roofing.  

         4.   Manufacturers' delivery or job markings on metal,
              and adhesives for manufacturers' labels shall be
              either a neutral or slightly acidic material.
              Never shall such material be alkaline; any staining
              of the metal by alkaline materials will be cause
              the rejection of the piece.

         5.   Hoisting equipment and procedures will depend on
              the design of the panels, weight, and length.

1.05 MAINTENANCE

    A.   Clean the roof of dirt build-up annually by rinsing with
         clean, clear water.  

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

    C.   Inspect for and eliminate ant hills and/or bird droppings
         that can corrode sheet metals.

         1.   Bird droppings can cause localized corrosion on
              sheetmetal because of the acids found in the
              droppings.

         2.   Remove droppings using a wooden spatula; wash
              surface with a neutral detergent.

         3.   Rinse with distilled water and wipe dry with a
              clean soft cloth, to prevent water spots and
              streaks.

         CAUTION:  DO NOT USE BLEACH TO REMOVE BIRD EXCREMENT.
         BIRD DROPPINGS CONTAIN AMMONIA AND IF MIXED WITH BLEACH
         CAN FORM TOXIC GASES.

    D.   Inspect the secureness of cleats and fasteners and the
         condition of the sheet metal after particularly heavy
         storms.

    E.   Never use any black goop (asphaltic roofing compound) or
         caulk to seal joints on a metal roof.  Asphalt attacks
         metal roofing, and no caulk lasts long enough for this
         application.


PART 2---PRODUCTS

2.01 MATERIALS

    A.   Nails and/or screws, metal to be compatible with roofing
         material

    B.   Cleats or clips, metal to be compatible with roofing
         material

    C.   Sheetmetal, to match type, weight/thickness, and size of
         original

    D.   Rosin Paper

    E.   Lumber for batten seams as appropriate

2.02 EQUIPMENT

    A.   Chicken ladder, safety belt or harness.

    B.   Snips, as required, for cutting.

    C.   Hammer or screw driver.


PART 3---EXECUTION

3.01 EXAMINATION

    A.   Whenever possible, make inspection from ground, or from
         above if possible.

    B.   Indications of a bowed panel:  Inspect for panels which
         do not lie flat, and which make a drumming noise in the
         wind.  If the panel is bowed in the middle, the seams and
         fasteners will be under stress, and some may be undone or
         damaged.

    C.   Inspect the underside of the roof deck from the attic to
         detect leaks.

3.02 PREPARATION

    A.   Protection:  At the end of each work day, provide
         building protection for any exterior roofing element
         removed during repair.

    B.   Surface Preparation:

         1.   Carefully examine, measure, and record existing
              metal shingle or sheetmetal patterns at edges,
              hips, ridges, and other special conditions.

         2.   Be careful not to damage old metal wall and vent
              flashing that may be used as a pattern for cutting
              templates.  If metal cap flashing at the chimney
              and other vertical masonry wall intersections have
              not deteriorated, bend them up out of the way so
              that they may be used again.  Carefully repair
              roofing in these areas to avoid damaging reusable
              base flashing.  

         3.   Inspect the deck to determine whether it is sound.
              Make whatever repairs are necessary to the existing
              roof framing to strengthen it and to level and true
              the deck.  Replace rotted, damaged, or warped
              sheathing or delaminated plywood material.  

         4.   For installation of new material, verify the type,
              thickness, weight/gauge before installation.

         5.   Before installation, remove all oil, dirt, and
              other debris from the surface.  All surfaces shall
              be dry and free from frost.

         6.   For safety of the personnel, keep the roof clear of
              waste material as the work proceeds.

         7.   Only work on a quantity of roofing which may be
              repaired on that same day.

3.03 ERECTION, INSTALLATION, APPLICATION

    A.   If the damage is discovered early, when deformation is no
         more than 1-1/4 inches, several options are available.

         1.   For small areas, 2-3 pans:

              a.   Carefully pry open the cross welts.

              b.   Anneal each opened joint.  Use damp rags to
                   keep the surrounding metal cool where it is in
                   contact or close to the wood roof decking.

              c.   Insert a 2" wide clip into the crosswelt.
                   Make sure the metal used is compatible with
                   the metal of the roof.

              d.   Fasten the clip to the understructure with
                   nails or screws, again they should be
                   compatible with the metal of the roof.

              e.   Reanneal all edges and close all seams as
                   originally finished.

         2.   For a large area:  Each bowed area or bay is cut
              down the center, its entire length, from ridge to
              eaves.  The edges are turned up to form upstands as
              required by either a standing seam or batten seam,
              depending on the seam type of the remainder of the
              roof.

              a.   Standing seam method:

                   1)   Prepare a new narrow strip of appropriate
                        sheetmetal with upstands to complement
                        those of existing sheets.  Lay over new
                        rosin paper or other appropriate
                        underlayment.

                   2)   Space new clips at 15 inches on center
                        along the length of the new seams.

                   3)   Fasten clips to deck with two nails or
                        two screws per clip, using fasteners
                        compatible with the clip metal.  Clips
                        should not be placed at junctions with
                        cross welts to avoid building up an
                        unmanageable thickness of metal in the
                        standing seams.

                   4)   Provide for longitudinal expansion by
                        using dummy welts.

              a.   Batten seam method:

                   1)   Fasten a new timber batten into the
                        decking with countersunk head steel
                        screws (screw must not contact capping
                        material).
   
                   2)   Insert clips under the batten, turn up on
                        each side, and fold in with the new
                        capping strip welt.  Make sure metal of
                        clip is the same metal as the roof.

                   3)   New capping strips to cover the top of
                        each batten are to be formed from a fully
                        annealed metal strip the same thickness
                        as the existing roof covering and the
                        same metal.

                             END OF SECTION