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Historic Preservation - Technical Procedures

Spectitle:

Minor Repairs To Lead Roofing And Accessories

Procedure code:

0761008R

Source:

Hspg Prepared For Nps - Sero

Division:

Thermal And Moisture Protection

Section:

Sheet Metal Roofing

Last Modified:

10/22/2014

Details:

Minor Repairs To Lead Roofing And Accessories



MINOR REPAIRS TO LEAD ROOFING AND ACCESSORIES


PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on making minor repairs
         to lead sheet metal roofing and lead roof accessories.
         If the damage is such that it cannot be repaired as
         directed below, than large areas of the roof must be
         replaced.  The size of the individual sheets should be
         limited to that which will allow thermal movement, as
         recommended by the manufacturer.

    B.   Lead sheet metal roofing and lead roof accessories fail
         for a number of reasons:

         1.   Fatigue Failure:

              a.   Caused by the use of sheets that are too large
                   for their thickness (most common type of
                   failure) or the use of too many fixings which
                   does not allow for movement due to normal
                   thermal expansion and contraction.

              b.   Oversized sheets and the use of too many
                   fasteners usually does not allow adequate
                   movement of the individual sheets and can
                   place excessive stresses on the metal at the
                   ridge and cause cracks in the sheet metal.

         2.   Slipping Failure:

              a.   Fasteners that have corroded or broken, or too
                   few fasteners, will cause individual sheets to
                   slip or buckle.

              b.   Sheets that have simply slipped out of place
                   can be pushed back into place and refastened.

         3.   Creep:  The stretching of sheet lead over time due
              to its own weight is called creep.  Sheets which
              have buckled and formed ridges may have been
              deformed due to creep.  This is also caused by
              using oversized sheets, but failure of the lead
              sheet due to fatigue will usually occur long before
              it is deformed by creep.


         4.   Poorly executed previous repairs

         5.   Corrosion:

              a.   Condensation:
                   
1)   The condensation of moisture on the
                        underside of lead sheet roofing is one of
                        the most serious problems as the
                        corrosion thus formed will eventually eat
                        through the lead causing pin holes at
                        first and finally significant loss of
                        material.  

                   
2)   An off-white, pink or brown flaky powder
                        on the underside of the sheets is
                        evidence that this is occurring.

                   
3)   To prevent condensation from occurring,
                        the underside of a lead sheet metal roof
                        must be well ventilated.  

             
b.   In addition to condensation, certain acids
                   will cause lead to corrode such as:

                   1)   Acid rain water run-off on lichen covered
                        roofs (particularly in lead gutters and
                        downspouts).

                   2)   Masonry cleaning acids such as
                        hydrochloric and hydrofluoric acids.

                   3)   Acetic acid, often emitted by breweries
                        and sawmills.

                   
4)   Formic acid from ants and insects.

                   5)   Nitric Acid

              c.   Acids found in wood rafters and beams can lead
                   to corrosion, especially when coupled with
                   condensation on the underside of the roof.

              d.   Run-off from cedar roofing shingles can cause
                   deterioration of lead flashing and gutters.

              e.   Galvanic corrosion will not occur between lead
                   and copper, zinc, aluminum, nor painted iron.
                   In a marine environment, however, lead to
                   aluminum contact should be avoided.

    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 MATERIALS

    A.   Lead sheet metal which matches the original in color and
         appearance, and is of proper thickness for its size

    B.   Copper fasteners as appropriate and required

    C.   Neutral pH soap such as "Joy" (Procter & Gamble).

    D.   Clean, soft cloths

   
E.   Clean, potable water

2.02 EQUIPMENT

    A.   Oxyacetylene flame torch

    B.   Stiff natural bristle brushes


PART 3---EXECUTION

3.01 EXAMINATION

    A.   Determine the cause of the failure and eliminate the
         cause before making repairs.  Otherwise the problem will
         only reoccur.

3.02 ERECTION, INSTALLATION, APPLICATION

    A.   Areas which have developed ridges or are corroded will
         require patches rather than welds due to the change in
         the molecular structure of the damaged lead.

         
1.   Remove lead to a distance of 2" beyond the edges of
              the damaged area.

         2.   Cut a patch from sound lead material which matches
              the original in color and appearance, and which
              overlaps the cut out area by one (1) inch on all sides.

         3.   Clean backside of patch and edges around cut out
              with a neutral pH soap and a soft cloth or natural
              bristle brush.

       
 4.   Weld or lead-burn patch into place.  Protect
              historic materials to prevent damage by fire.

              NOTE:  DO NOT ATTEMPT TO SOLDER THE PATCH IN PLACE.
              THE DIFFERENCE IN THE COEFFICIENTS OF EXPANSION
              BETWEEN LEAD AND SOLDER WILL CAUSE A SOLDERED PATCH
              TO FAIL.

   
B.   Cleaning:

         1.   Pigeon droppings will cause lead to corrode and
              should be removed, along with any dirt that
              accumulates.  

              a.   Wash with a neutral Ph soap in warm water, and
                   a natural bristle brush.

              b.   Thoroughly rinse with clean, clear water and
                   allow to dry.

         2.   Proprietary lead cleaning gels are also available
              from lead manufacturers.


                           
 END OF SECTION
 


lead roofing, repair of lead roofing, lead roof accessories, lead creep, lead roof creep