POCKET GUIDE TO CONVENTIONAL ROOF CONSTRUCTION FACTS Structural members 2”x 6”s or larger Strength is derived from dimensional width and depth of lumber Longer spans require larger structural members Larger structural members and larger lumber results in longer times to failure when exposed to fire Substantial construction may allow for additional time to conduct and coordinate vertical ventilation operations IDENTIFICATION EXTERIOR FEATURES Masonry walls Buildings constructed prior to 1960 Arched and/or inset windows King rows Tie rod & rafter tie end plates Arched truss roof Bridge truss roof TOP SIDE FEATURES (Identified from interior or inspection/indicator/plug cut) Multiple layers of composition roofing material COMMON FEATURES AND DANGERS LOAD BEARING WALLS / PILASTERS Trusses/rafters are supported by: • Load bearing walls • Pilasters embedded in exterior walls Exterior walls and conventional trusses yield strongest areas for travel and egress DIAGONAL SHEATHING Head and base cuts made on diagonal sheathing may result in a diagonal spring board Diagonal sheathing offers greater strength than straight sheathing, however… MODIFICATIONS/UPGRADES (Frequently laid on top of original sheathing) Plywood/OSB AND multiple layers of composition roofing material Rain roof (double roof) Styrofoam insulation Makes locating Increases chances of trusses/rafters difficult when inadvertently cutting through sounding a rafter Makes cutting slow and Masks failure of structural difficult members beneath VULNERABILITIES FIRE LOAD / UN-PROTECTED STRUCTURAL MEMBERS Typically house heavy fire load Frequently without a working sprinkler system Often no ceiling, leaving trusses directly exposed to fire Lumber degrades 1/8” every 5 minutes when exposed to fire CONVENTIONAL ROOF TYPES CONVENTIONAL FLAT BEAM or WALL (MAY NOT BE PRESENT) RAFTERS CUT CONVENTIONAL FLAT Longer spans will be supported by an interior wall or beam (girder) perpendicular to the rafters • Rafters laid on bottom edge across exterior walls or may sit in steel hangers • Spans of 15’ to 25’ are typical • 16" to 24" on center spacing Nailing blocks, bridging, or scissor bracing stabilizes rafters CONVENTIONAL FLAT WATCH OUTS Rafters held by hangers or toe-nailing may fail earlier than those laid over a wall or beam 24" on center rafter spacing with plywood or OSB decking… offers less support than… 16" on center rafter spacing with 1"x 6" sheathing CONVENTIONAL FLAT WATCH OUTS • Sheathing and plywood/OSB decking will typically fail ahead of rafters • Plywood/OSB decking can fail beneath intact roof coverings • Rafters spanning longer distances offer less support and fail earlier when exposed to fire • Rafters exposed directly to fire may fail in 10 to 20 minutes depending on dimension, span, and load • Be alert for spongy, sagging or self venting areas CONVENTIONAL FLAT OPERATIONAL CONSIDERATIONS Rafters do not always run parallel to Ventilate as close to the seat of the shorter walls the fire as safely possible Inspection cuts are imperative to determine: • That you are not on a lightweight roof • Rafter size & direction • Type and thickness of • Exterior walls yield strongest areas decking/sheathing for travel and egress • Proximity to dead loads, rafters seated in hangers, areas visibly sagging, and areas where water Nailing blocks, bridging, or scissor bracing pools present weak areas will interfere with parallel cuts and louvers ARCHED TRUSS ROOF TRUSS CUT RAFTERS ARCHED TRUSS ROOF Top and bottom chord composed of 2”x 12” lumber Rafters are 2”x 6”s or larger running perpendicular and between trusses Typically laid on edge atop Steel bolts and plates hold truss together trusses, but may sit in hangers BRIDGE / GABLE / PARALLEL CHORD TRUSS ROOF TRUSS CUT Variations in truss geometry: Bridge Gable RAFTERS Parallel Chord BRIDGE / GABLE / PARALLEL CHORD TRUSS ROOF Rafter direction is not always obvious when top chord is flat and may require an inspection cut Design and features similar to arched truss roofs but with straight edges ARCHED & BRIDGE TRUSS WATCH OUTS Failure of a truss will result in collapse of rafters and sheathing/decking attached to neighboring trusses (ie failure of a large section) Rafters/planks may be laid across bottom chords for storage • Adds to dead load and places fire load within the truss work • Leads to early failure when exposed May also result in failure of to fire neighboring trusses ARCHED & BRIDGE TRUSS WATCH OUTS • Sheathing and plywood/OSB decking will typically fail ahead of trusses/rafters • Plywood/OSB decking can fail beneath intact composition roofing material • Rafters spanning longer distances offer less support and fail earlier when exposed to fire • Rafters exposed directly to fire may fail within 10 to 20 minutes depending on dimension, span, and load • Be alert for spongy, sagging, or self venting areas ARCHED & BRIDGE TRUSS WATCH OUTS ARCHED & BRIDGE TRUSS OPERATIONAL CONSIDERATIONS Ventilate as close to the fire as safely possible • Stout construction may allow additional time to conduct and coordinated vertical ventilation • Rafters typically span 15’ to 25’ between trusses • Exterior walls and trusses yield strongest areas for travel and egress • Proximity to dead loads, rafters seated in hangers, areas visibly sagging, and areas where water pools present weak areas TIED ARCH (TIED TRUSS) ROOF RAFTERS TRUSS CUT TIED ARCH (TIED TRUSS) ROOF Top chord composed of 2”x 12” (or larger) lumber TIED ARCH (TIED TRUSS) WATCH OUTS TIED ARCH (TIED TRUSS) WATCH OUTS • Sheathing and plywood/OSB decking will typically fail ahead of trusses/rafters • Plywood/OSB decking can fail beneath intact composition roofing material • Rafters spanning longer distances offer less support and fail earlier when exposed to fire • Rafters exposed directly to fire may fail within 10 to 20 minutes depending on dimension, span, and load • Be alert for spongy, sagging, or self venting areas TIED ARCH (TIED TRUSS) WATCH OUTS TIED ARCH (TIED TRUSS) OPERATIONAL CONSIDERATIONS • Failure times may be less than arched truss roofs • Rafters typically span 15’ to 25’ between trusses • Exterior walls and trusses yield strongest areas for travel and egress Ventilate as close to the seat of the fire as safely possible • Proximity to dead loads, rafters seated in hangers, areas visibly sagging, and areas where water pools present weak areas Director of Training LACoFD Training Services Battalion Chief Derek Section Alkonis (323) 881-2436 .