Minor Repairs To Lead Roofing And Accessories

Procedure code:
761008S
Source:
Hspg Prepared For Nps - Sero
Division:
Thermal and Moisture Protection
Section:
Sheet Metal Roofing
Last Modified:
10/22/2014


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.



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