Growing pressures on land availability and housing affordability are resulting in an increase in demand for attached homes. Zero-lot-line configurations are becoming more popular for detached homes because of more innovative use of small lots. The principal added code consideration for attached and zero-lot-line homes is the requirement for fire barriers.
Requirements of the major model codes are not always clear and are often subject to local interpretation and/or amendment. It is important to understand that all major model codes have an "Alternate Materials and Systems" section and that local code officials have the discretion to approve alternate construction. Appropriate documentation is, of course, usually necessary.
To be acceptable, firewall constructions must be rated by a recognized testing laboratory in accordance with ASTM El 19, Standard Methods of Fire Tests of Building Constructions and Materials. Many different one – and two-hour wall constructions have been approved and are listed in literature available from the Gypsum Association, 1603 Orrington Avenue, Evanston, IL 60201; the National Concrete Masonry Association, P. O. Box 781, Herndon, VA 22070; and other industry associations and product manufacturers.
For zero-lot-line homes, each unit must have an independent one-hour fire-resistive rating. The Standard Code (Southern) Uniform Building Code (ICBO), and the One and Two Family Dwelling Code (CABO), all require one-hour ratings for homes built less than 3 feet from the property line. The Basic/National Code (BOCA) requires a one-hour rating on exterior walls less than 6 feet from the property line. None of the codes permit unprotected openings through a firewall. Normal electrical, plumbing, and ductwork are generally allowed.
The most common one-hour firewall is of wood frame construction with 5/8- inch type X gypsum wallboard or gypsum sheathing attached to each side with 6d coated drywall nails 7 inches on center. Joists are required to be staggered at least 24 inches on center on each side.
For attached homes, a two-hour firewall is required, either as two separate one-hour walls or as a common two-hour wall at the property line. Check your local Code. The two-hour common wall typically built is a single wood frame wall with two layers of 5/8-inch type X gypsum wallboard on each side or concrete block. Two-hour walls typically have restrictions on electrical wiring, plumbing, and ductwork within the wall.
In addition to the firewall, some provision is required to block the spread of fire to the roof of an adjoining unit. For zero-lot-line detached homes, codes are somewhat vague because there is no adjoining roof.
In addition to confusing major model code firewall and roof treatments, some local codes require that firewalls be built of masonry construction.
This requirement is prohibitive for factory-built construction.
As mentioned, three of the four model codes require a firewall if within 3 feet of the property line. The other code, BOCA, requires a firewall if within 6 feet of the line. If a home is built 37 inches from the property line (73 inches under BOCA), no firewall is needed.
If an easement for use of that narrow strip of land is assigned permanently to the house next door, a zero-lot – line effect is obtained without cost of a firewall or roof parapet. It will be worthwhile to check local interpretation of firewall/roof treatment requirements prior to construction.
Since the major model codes are difficult to interpret and have not seriously addressed detached zero-lot – line homes in many cases, a complete review and rewrite of all codes should be undertaken.
Three JVAH sites, (Lacey, WA; Everett, WA; and Santa Fe, NM), all built under UBC, ran into the problem of firewall and roof treatment requirements.
In Santa Fe, the normal city requirement is a masonry firewall between attached garages, including a parapet above the wall. The builder obtained the Fire Resistance Design Manual from the Gypsum Association which shows wood frame firewalls.
In addition, he pointed to the 1,000 square foot per floor exception for roof fire treatment in UBC. These convinced the city that a common two-hour wood-framed firewall with no parapet or roof treatment was adequate.
In Lacey, the city required either a parapet extending 30 inches above the roof or that all framing elements (trusses, wall plates, studs, etc.) within 5 feet of the two-hour separation wall be of one-hour fire resistance construction.
The builder, John Phillips, pointed out that none of the other major model codes had this requirement and that fire-resistive sheathing, installed at least 4 feet from the wall, provides adequate fire safety according to building code experts. The city accepted his documentation which resulted in substantial cost savings.
In Everett, zero-lot-line homes were built with one-hour fire walls. The city accepted the builder’s documentation that type X fire-rated gypsum board under roof sheathing, within 4 feet of firewall was adequate to comply with the intent of the code.
The standard roof truss has become the most common and most cost – effective method of roof framing. Light-weight trusses are the most highly engineered component in new home construction and form the basis of a very efficient roof system. They are easy to install and adapt to many basic designs. Therefore, if cost is the primary consideration, standard roof trusses are recommended.
The "in-line" framing concept discussed in the House and Lot Design section of this manual works very well with roof trusses. That is, the 24- inch on-center roof trusses align with the 24-inch on-center wall studs which in turn align with the 24-inch on- center floor joists. The key to this consistent alignment is to start all layout from the same corner.
Simplification of roof overhang and trim details, consistent with design and function, provides opportumties for cost reduction. For example, the ■ rake overhang is essentially nonfunctional on a gable end roof. A simple fascia board at the siding/roof junction serves to cover the rough edge of the siding and conceal inaccuracies of fit. Several of the nation’s largest builders use this detail on all their production homes.
Roof overhangs are desirable for most designs and provide rain protection for the front and rear of the house. They also can provide summer shading for some windows. When an overhang is used, an inexpensive "open" soffit will eliminate much of the cost of the traditional cornice. All trim details on the underside of the overhang may be
eliminated, leaving the truss or rafter tails exposed. Blocking between trusses or rafters and a 1×6 fascia board are the only finish items needed. If soffit venting is needed, screening between trusses or rafters can be used instead of blocking.
Three-eights-inchplywood roof sheathing with metal plyclips is an acceptable alternative to 1/2-inch plywood.