Patching and restoration of the sandstone facade of this classic San Francisco building was extensive and involved the removal of over 90 tons of deteriorated stone.
The Flood Building sits at the intersection of Powell and Market Streets, above the cable car turnaround and the very busy adjacent sidewalks. Working in such a tightly populated environment created special problems for our crews.
Colusa sandstone is a typcial siliceous and sedimentary material. Its primary resemblance to other sandstone's is its lack of resistance to moisture. Some sandstones can be superb building materials, but many are similar to this material in that they do not like water, which is not a good general characteristic for an exterior cladding stone. Here you can see the delamination (the tendency to peel like the layers of an onion) of the stone due to the presence of moisture (rain).
Note here how even the delamination is on each of these balusters. Also evident is the owner's concern regarding the stability of the balusters and top rail of each balustrade.
Standard preparation of an historic surface for patching begins with the cutting and removal of all unsound material. Each patch area must be cut with diamond blades, either wet or dry, to remove not only the unsound stone, but to define the perimeter of each patch area.
Here we are making the demo cuts in the repair area with water using a hydraulic-powered cutting machine to avoid electrical hazards to the operator. Water-cutting was required in this area close to the street level to avoid any environmental hazards with free silica dust in the air. The water forms a slurry mixed with the stone dust, and we could collect the runoff and dispose of it in a safe manner.
Since the patching procedure specified that the repair mortar should be installed in maximum 3/8" lifts, we marked each blade around the perimeter to help the operator always cut to a depth of 3/4". That way we knew that the material removed would average 3/4" in depth, and we could assume two applications of repair mortar, each 3/8" deep. This allowed for the proper shrinkage of each application of cementitious mortar.
After the deteriorated areas are identified and marked, the perimeter cuts are made, and then the interior area of each patch is cut into 1" squares. They are then easily removed with small pneumatic chipping hammers.
We prefer Italian stone-carving hammers for this type of work because they have a relatively light impact compared to heavier demolition hammers. At 300 blows per minute, these hammers give the precise control needed to remove deteriorated stone without damaging sound material.
In many locations, we were forced to remove large areas of unsound material to prepare the facade for patching. We kept track of the material removed and counted about 90 tons of rotten stone removed in 5 gallon buckets.
Since the specifications for repair called for the use of a cementitious patching mortar, a system of mechanical attachment to the facade was also part of the repair scheme. Here the holes for anchors are being drilled on 9" centers, on average.
The stainless steel threaded rod anchors will be attached to the substrate with epoxy adhesive.
In this image, all the unsound stone has been removed and the stainless steel pin and anchor system installed, providing an extra mechanical anchor for the patching mortar. Note also the chisel marks on the face of the patch areas; this roughened surface will provide additional mechanical adhesion for the patching mortar.
Here an entire decorative pedestal cap has been removed and prepared for patching.
Here the patch area has been fully prepared for the installation of new patching mortar. Note how the anchor wires do not cross over the grout joint between two different stones. This 'break' in the pattern allows for the movement of each stone over time, and reduces the chance of through-cracks between masonry units.
The same sort of repair across a grout joint, but in this case the decay pattern tends more toward the vertical, corresponding with the water drainage pattern.
The installation of the repair mortar was designed to be completed using a standard Portland cement-based formula. Because Portland cement has a relatively high shrinkage rate, the patches were built up in 3/8" maximum lifts, averaging 2 applications per patch area.
The scratch coat is applied to the pre-moistened surface and built up to approximately half the depth of the prepared patch area. Note the scored surface to promote adhesion with the finish coat.
The finish coat is applied and the surface treated with special tooling made to match the regular chisel marks of the original stoneworker's tools.
Great care was taken to reproduce and match the adjacent original profile and surface.
In some cases, special finishing tools were fabricated just for a specific detail and location on the facade.
In the case of this large round element at the corner of the building, a special technique known as "run-in-place" mortar installation allowed the exact radius of the curve to be recreated.
In all areas of mortar repair, the newly installed repair material was kept moist by periodic soaking with portable pressure sprayers to retard the cure and shrinkage of the cement patching material.
In addition to being sound preservation theory, the retention of all areas of sound stone provided a benchmark and reference point for the recreation of the surfaces.
The decision to paint the restored sandstone surfaces was a difficult one to make, but it was supported by two factors: 1) The stone was entirely too susceptible to decay by exposure to moisture [An all too likely occurrence in San Francisco]. 2) The building had been previously painted in 1908 on the Powell St. elevation. where fire damage from the 1906 earthquake was most pronounced. This old cementitious paint was firmly adhered to the stone - removing it would have damaged the original stone surface.
Several areas of deteriorated linear decoration such as this horizontal egg-and-dart were more efficiently replace with cast stone, in this case Graphite Fiber Reinforced Concrete.
Significant mold work was required to recreate the original carved design for production.
After installation, the entire area was coated with the same exterior latex coating used on the rest of the facade.
Another interesting aspect of the project was the wholesale reproduction of the bottom 24 feet of the original facade (removed in the 60's) using Graphite Fiber Reinforced Polymer, basically fiberglass, but done with skilled color and texture control.
The bottom portion of the building shown in this image is GFRP, and the upper half is the original sandstone. Even today, you have to walk up the building and tap the surface to tell the difference.
The Flood Building is one of San Francisco's great landmark structures, and the work shown here insured that this building will be around to enjoy 100 years from now.
