Technical Procedures Disclaimer
Prior to inclusion in GSA’s library of procedures, documents are reviewed by one or more qualified preservation specialists for general consistency with the Secretary of Interior Standards for rehabilitating historic buildings as understood at the time the procedure is added to the library. All specifications require project-specific editing and professional judgement regarding the applicability of a procedure to a particular building, project or location. References to products and suppliers are to serve as a general guideline and do not constitute a federal endorsement or determination that a product or method is the best or most current alternative, remains available, or is compliant with current environmental regulations and safety standards. The library of procedures is intended to serve as a resource, not a substitute, for specification development by a qualified preservation professional.
We’ve reviewed these procedures for general consistency with federal standards for rehabilitating historic buildings and provide them only as a reference. Specifications should only be applied under the guidance of a qualified preservation professional who can assess the applicability of a procedure to a particular building, project or location. References to products and suppliers serve as general guidelines and do not constitute a federal endorsement nor a determination that a product or method is the best alternative or compliant with current environmental regulations and safety standards.
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.
- Fatigue Failure
- 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.
- 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.
- Slipping Failure
- Fasteners that have corroded or broken, or too few fasteners, will cause individual sheets to slip or buckle.
- Sheets that have simply slipped out of place can be pushed back into place and refastened.
- 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.
- Poorly executed previous repairs
- a. Condensation:
- 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.
- An off-white, pink or brown flaky powder on the underside of the sheets is evidence that this is occurring.
- To prevent condensation from occurring, the underside of a lead sheet metal roof must be well ventilated.
- In addition to condensation, certain acids will cause lead to corrode such as:
- Acid rain water run-off on lichen covered roofs (particularly in lead gutters and downspouts).
- Masonry cleaning acids such as hydrochloric and hydrofluoric acids.
- Acetic acid, often emitted by breweries and sawmills.
- Formic acid from ants and insects.
- Nitric Acid
- Acids found in wood rafters and beams can lead to corrosion, especially when coupled with condensation on the underside of the roof.
- Run-off from cedar roofing shingles can cause deterioration of lead flashing and gutters.
- 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.
- a. Condensation:
C. See 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
- Safety Precautions
- Historic Structures Precautions
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- 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).
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
A. Oxyacetylene flame torch
B. Stiff natural bristle brushes
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.
- Remove lead to a distance of 2" beyond the edges of the damaged area.
- 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.
- Clean backside of patch and edges around cut out with a neutral pH soap and a soft cloth or natural bristle brush.
- 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.
- Pigeon droppings will cause lead to corrode and should be removed, along with any dirt that accumulates.
- Wash with a neutral Ph soap in warm water, and a natural bristle brush.
- Thoroughly rinse with clean, clear water and allow to dry.
- Proprietary lead cleaning gels are also available from lead manufacturers.