Historic Preservation - Technical Procedures
- Repairing A Bowing Sheetmetal Roof
- Procedure code:
- Hspg Prepared For Nps - Sero
- Thermal And Moisture Protection
- Sheet Metal Roofing
- Last Modified:
- Repairing A Bowing Sheetmetal Roof
- Last Modified:
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.
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 building. 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 or snow covered roofs. Work on
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
rope of sufficient strength.
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
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.
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
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
3. Heavy bundles of nested panels require suitable
mechanical equipment. Take care to prevent damage
to corners and edges during handling or storage of
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.
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 bird droppings and any other debris
that can corrode sheet metals.
1. Bird droppings can cause localized corrosion on
sheetmetal because of the acids found in the
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
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
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
A. Nails and/or screws, metal to be compatible with roofing
B. Cleats or clips, metal to be compatible with roofing
C. Sheetmetal, to match type, weight/thickness, and size of
D. Rosin Paper
E. Lumber for batten seams as appropriate
A. Chicken ladder, safety belt or harness.
B. Snips, as required, for cutting.
C. Hammer or screw driver.
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
C. Inspect the underside of the roof deck from the attic to
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
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
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
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
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
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
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
END OF SECTION