A few years ago New York subway riders were startled to read "car card" ads which urged them to "Keep New York Plastered". Many tried, in more than one sense, but their effort failed. The reference was to the fact that the city was about to embrace a new building code which would permit the use of gypsum board or "drywall" for interior finish. Prior to this, wet plastering had been required. The all plaster, essentially continuous sheath of the wood lathed "New Law" tenement provided (until the plaster deteriorated with age) an almost complete and usually adequate barrier to the extension of fire to the combustible inner structure of the building.

Today, almost universally, interior finish is of "drywall" or gypsum board. Gypsum board consists of a layer of gypsum, sandwiched between two sheets of paper.

It is offered commercially in sheets from 3/8 to 5/8 inch thickness, 4 feet in width and 7 to 10 feet long. By including vermiculite or glass fibers in the gypsum, the time a given thickness of gypsum will resist fire can be increased. If thicknesses greater than 5/8 inch are desired, successive layers of gypsum board are used.

Gypsum itself provides negative Btu yields of as much as -580 Btu per pound, measured by the "bomb" calorimeter method.

Underwriters' Laboratories "Fire Resistance Index" (see fig. 1) contains a number of fire resistive (combustible) wall and "floor and ceiling" assemblies, tested and rated according to the standard fire test, ASTM E119. In most cases the essential protective element is gypsum board.

The test assemblies are simple structures. Walls are of vertical wood studs, without any offsets, and are fire stopped'. Floor and ceiling assemblies are usually on 2 x 10 inch wood joists fire stopped.

Building codes contain various requirements for fire resistive (combustible) construction.

The "as built" wall and floor ceiling assemblies are far more complex than the test assemblies. It appears that builders, building officials, and inspectors are quite unfamiliar with all the details of the assemblies which passed the tests and were rated.

One of the most persistent and dangerous errors is the assignment of the fire resistance rating to the gypsum board itself rather than to the assembly as erected in the test facility. Another error is the failure to understand the meaning of "combustible" when added to a fire resistance rating.

Gypsum board walls and ceilings competently installed under the watchful eye of inspectors who understand the full installation requirements can and have performed remarkably well. An excellent example can be found in the Harbour House project in Annapolis. There, at least 11 fires have made substantial attacks on the gypsum sheathing, but in no case was the sheath penetrated. (An aluminum soffit permitted an "end around" extension.)

Up to this point the author has, artfully he supposes, led the reader to believe that there is such a concept as a continuous fire resistive sheath protecting the combustible structure from a fire in the contents. In fact, there is apparently no such conception on the part of those who build, regulate, supply materials or test. The building is simply never looked at as a whole. This is another example of component testing as opposed to systems safety.

While the chief problem appears to be the failure of continuity of what could be an adequate protective sheath, there are some other areas of design construction and layout features which contribute to the extension of a typical confinable fire, to a building conflagration (see fig. 2).

Figure 2. Sketch of garden apartment house showing all pinholes in the sheathing. "Any lack of continuity in the sheathing permits fire to enter the interior voids of the building where it can spread unchecked." (Courtesy of Montgomery College, Learning Resources)

The various modes of failure are interrelated, but it is possible to break down the discussion into a few headings for convenience.

1. Ways in which fire penetrates the gypsum sheath and enters the inner structure of the building.

2.

Fire spreading substantially in combustible voids, within the inner structure of the building, usually interconnected with the huge combustible void of the attic.

3. Fire extension through or around fire walls and barriers between buildings.

While loss of life was not very common in the fires studied, a few observations of life safety, both of occupants and firefighters are warranted.

4. FIRE PENETRATION MODES

Inadequate nailing, particularly of ceilings is common. At times, entire 4' x 8' panels of gypsum board fell early in the fire, before the board should have failed by fire exposure. Examination showed that nailing was short, as little as 12 inches on centers versus the 6 inches usually required.

2.

The only U.L. listed standard for floor and ceiling assemblies using trusses rather than joists, requires two 1/2 inch layers of gypsum board. Ceilings nailed to gusset plate trusses are nailed only on the 24 inch spacing of the trusses rather than 16 inch spacing. Single layer 1/2 inch or 5/8 inch ceiling constructions permitted under trussed roofs appear to be unwarranted extrapolations from the tested constructions.

3.

Gypsum board was found of less than the required thickness, 3/8 inch in some cases.

4. Two (gypsum board) layer construction in which the underlayer was made up of incomplete "scraps" with large areas uncovered, was noted.

5. Plywood panels are used to cover openings. The commonest example is the omnipresent wooden panel which gives access to the bathtub plumbing. Many access panels to attics are of plywood. Most of the buildings involved were heated by hot air heat. In one case hot or chilled water was delivered to con

vectors. rooms.

One convector in an interior wall between two bedrooms served the two A 3' x 4' plywood panel gave access to the convector for maintenance. A typical "child set" fire in the bedroom entered the wall space through the plywood, extended upward through the walls along the water pipe openings to the next floor level, broke through the corresponding plywood panel. extended fire did more damage to the upper apartment, than to the apartment of origin.

6.

The

An attractive pass through between the kitchen and dining area could be closed off by sliding louvered doors. The doors slid back into a space provided by doubling the wall, much like old fashioned sliding doors. The fire entered the wall space through the opening, and extended to other apartments by interconnected voids. A picture taken at the time the fire department was "turning out" shows that the smoke was already coming from two other apartments.

7.

In a number of cases fire penetrated the drywall sheath around hot air heat register openings, and light fixtures.

8.

9.

Penetrations were noted around door frames.

The basement areas of garden apartments are used for utility meter rooms, rubbish rooms, coin laundry rooms, and storage rooms. The typical storage room has a series of wire cages, one per tenant, individually locked.

The storage area particularly can represent a substantial fire load. These areas are arranged to be usually locked with a key provided to each tenant, but are often found open. They are a common point of origin of incendiary

fires.

10.

Because of many utility openings, the integrity of the gypsum sheath on the basement ceiling is particularly dubious.

11. Utility meters are commonly grouped for convenience. In one case the gypsum ceiling was cut short 2 inches from the wall. The gap was made up by a piece of wood 1" x 2". The electric cables passed upward through holes drilled in the wood. The fire extended upward by burning out the wood.

12. The difficulty of fitting gypsum around the gas pipes caused the area around the gas meters to be left unprotected. The fire extended upward through the unprotected area.

13. Oversized holes for plumbing pipes, or unclosed openings in the ceiling for repairs also permitted extension.

The

14. Fire came into a building following the main electrical service. fire department sized the fire up as an electrical fire. Following cables, which went up through an opening in the ceiling, the fire extended upward through the building and involved a dozen or more apartments.

15. Access plates are sometimes provided in basement ceilings for access to plumbing. In one incendiary fire it was believed that an access plate had been deliberately opened to permit extension of the fire.

16. The built-in metal bathtub "substitutes" for the drywall sheath. In one fire, studied before this investigation, it was evident that fire extended into the apartment from the wall by conduction through the bathtub. At least one building code requires that drywall be complete behind a plastic bathtub, but I have not seen sheathing behind a steel or cast iron tub.

17. A wooden floor joist with plywood nailed on top and gypsum nailed on the bottom (the ceiling) serves as a stop to the spread of fire at right angles to the joist. In earlier construction practices, a double joist was placed under each partition wall to carry the weight of the plaster. This practice is not necessarily followed in current construction. Floor joists are laid on 16 inch centers and the partitions of the floor above are laid out as directed by the architect. As a result a partition wall between apartments may rest on the plywood floor, between two joists. In two cases a fire starting in one apartment close to the partition wall burned down through the floor to the joist void, which was common to the next apartment. The fire then burned up into the next apartment. In one case this was how the fire was discovered.

18. Plywood surfaces are installed for decorative effect. Often the plywood is the only wall sheath, and thus, the void space is quickly penetrated. Walls of hardboard decorated to look like plywood have also been noted.

19. More expensive detailing does not necessarily produce greater fire safety. In low cost work the walls are finished and then kitchen cabinets are hung on the wall. In more costly work, soffits are built down from the ceiling

The entire testing concept of "fire-resistive combustible" floor and ceiling assemblies is open to question. The test fire attack is made only against the gypsum board ceiling. No assemblies are tested by a fire attacking the wooden floor. Fires in fact burn downward through floors quite readily. Those constructions which provide huge voids under the floor, such as trussed floors, provide a potential for disaster when the floor is penetrated from

above.

Figure 3.

Kitchen cabinets. These "built-in" cabinets are attractive in appearance, but the only barrier between a kitchen fire and the inner voids of the building may be 1/8 inch of hardboard forming the top of the cabinet.

line. Gypsum board is applied to the ceiling and the vertical surface of the soffit. The bottom of the soffit is left open. The cabinets are then installed. The only barrier between a kitchen fire and the void spaces is the thin top of the kitchen cabinet, often 1/8 inch hardboard (see fig. 3). The determining factor in the confinement of a kitchen fire to the kitchen is often, whether the cabinet doors are open or closed. Where kitchens are back-to-back these soffits are common voids.

Soffits are also provided to conceal heat ducts. Along interior apartment partition walls they are often back-to-back creating another void common to two or more apartments. These voids are often interconnected to vertical voids discussed further on.

20.

Fire extended through the narrow vertical void formed by gypsum board furred out from a masonry wall on 1" x 2' strips.

21. While no fire extension has been noted, some apartments are built with "sunken living rooms". The offset between the rest of the apartment floor and the living room floor is covered by a vertical board nailed in place. It appears likely that this would pass fire sooner than a floor all in one plane. There is no listed construction of this type.

1. In a few cases nominally flat roofs are used. Even such a roof has interconnected void spaces because a pitch must be provided to run the rain off the roof. The difference between the pitched roof and the horizontal ceiling joists creates openings so that the roof joists do not serve as fire stops. In one case there was no void (see fig. 4). The roof was of plank on beam construction and the interior followed the room pitch. It did not fail structurally, but fire charred the entire surface, possibly due to the use of a high flame spread finish.

2. The usual attic is a high pitched gable roof, of sheet plywood on lightweight gusset plate trusses. When the attic is involved the fire is spectacular (fig. 5), and often is stopped only by a true fire wall or by the end of the building.

The attics are provided only for heat relief, architectural effect and sometimes utility services. They are not used for storage. Access is usually by a scuttle hole in the top floor stairway ceiling.

Fire can reach the attic in several ways.