Roofs Flashcards
Gable Roof Conventional
2x6” rafters or larger, 16-24” on center, 1x6” ridge boards or rafters butted up together, ceiling joist/collar beams typically 2x4”
-Steep pitched roofs up to 36” rafter spacing
-Commonly utilize 1x4 or 1x6 spaced sheathing
-Convention gable construction will last longer. Strong areas of the roof are the ridge and the area where the rafters cross the outside walls
Gable Lightweight
2x3” or 2x4” wood trusses held together by metal gusset plates spaced 24” on center.
-Trusses have top/bottom cords in compression/tension and webbing. Bottom cord acts as the ceiling joist in lightweight construction which may not rest on interior walls.
-Metal gusset plates are 18 gauge steel plates with prongs that produce 3/8” penetration. Connector plates will pull out of wood under fire causing collapse
-Truss members may only be supported at their outside edges unless used as a cantilever truss. Partition walls may not support the truss unless 18 gauge roof truss clips (found at every 3-5 trusses) are used. This can contribute to total collapse.
-Commonly use 3/8” or 1/2” plywood decking which burns faster
Hip Roof
-Ridge board and rafters are usually 2x6 in older hip roofs. “Rough cut” 2x3 or 2x4” rafters 36” on center for steep pitch roofs
-Hip rafters extend from ridge board down and across the corners at the outside walls.
-Valley rafters are utilized where two roof lines join together
-Jack rafters are shorter rafters
Strengths:
-Ridge board, hip and valley rafters, and area where rafters cross outside walls are all strong points
Hazards:
-Lightweight hip roofs (2x3 or 2x4 rafters)
-Steep pitch roofs and tile roofs can cause a slipping hazard. Tile will need to be removed.
Bridge Truss
Construction:
- 2x12” wood truss members, 2x6” rafters, covered by 1x6” sheathing and comp. Identified by sloping ends.
-Sheathing was straight prior to 1933 and diagonal post 1933 due to increased earthquake resistance. May also have plywood decking installed over the sheathing.
-Vertical metal tie rods may be used for additional support
Strengths
-Strong roof, early collapse not a concern. However, depending on the fire the roof will collapse in sections
Hazards:
-Underside of the roof is usually exposed, can also be modified to allow storage in attic area, and have sheathing/plywood attached to bottom cord contributing to collapse
-Ventilation personnel must be aware of plywood, metal straps, and supports.
Bowstring Arch
Construction:
-Similar to bridge truss and found on small/large commercials
-2x12” or 2x14” trusses, 2x6” rafters or large covered by 1x6” sheathing (pre-33 straight or post-33 diagonal) and comp. May also have plywood decking installed over the sheathing.
-Generally for all arch roofs, trusses run shorter end of the building
Strengths/Hazards:
-Same as bridge truss roof. Different design but will have members in compression/tension as seen in the bridge truss roof.
Lamella Arch
Construction:
-Diamond patterned roof, common in gymnasiums/large buildings such as supermarkets, Frank Hodgkin’s Memorial Training Center is lamella arch
-2x12” wood framing w/ steel plates and bolts at junctions of frame, covered by 1x6” sheathing/comp
-Supported by exterior buttresses/internal tie rods w/ turnbuckles
-Identifiable by lack of parapet wall, “egg” shape
Strengths
-Solidly built w/ good construction techniques
Hazards:
-Total roof collapse can occur if fire removes more than 20% or more of the roof structure (domino effect)
Tied Truss Arch
Construction:
-Similar in appearance to bowstring/lamella roofs, but use metal tie rods to offer lateral support for walls of the building
-Tie rods usually 5/8” diameter, w/ turnbuckles used below each arch member to ensure arches don’t push exterior walls outward. Tie rods may be placed through the outside walls into tie plates which can be used for identification of this roof
-2x12” or larger arch members, 2x10” rafters covered by 1x6” sheathing/comp (pre-33 straight or post-33 diagonal). May also have plywood decking installed over the sheathing.
Strengths:
-Large lumber and sheathing
Hazards:
-Early failure of tie rods and turnbuckles which provide lateral support for the exterior walls
-Failure may occur in sections or total failure depending on the fire
Sawtooth
Construction:
-Used on commercial buildings to yield light/ventilation for manufacturing type occupancies
-2x8” or larger rafters w/ wood or metal supports for bracing; sloping portion covered w/ 1x6” sheathing/comp or 1/2” plywood in newer roofs.
-Windows will typically face NW or SW to block direct sunlight coming in but still allow some light in
Strengths:
-Well constructed. Early collapse of fire not a concern.
-Easy to ventilate via panes of glass
Hazards:
-Underside is exposed and can be impacted by fire.
-Ventilation personnel should be aware of plywood, metal straps, and supports.
Metal Gusset
2x4” top/bottom cord wood trusses held together by metal gusset plate connectors in spans up to 80’, w/ 1/2” or 3/8” plywood decking/comp
-The most common metal gussets are 18” gauge plates w/ prongs that produce 3/8” penetration
-Roofs can be a variety of shapes, flat, gable, hip, etc.
Strengths:
-Area where trusses cross on the outside bearing walls are considered to be strong
Hazards:
-Under a working fire, metal gusset plates will most likely be the first to fail in addition to the small 2x4 lumber
-Plywood and nailing blocks should be a consideration to ventilation personnel
Conventional Flat Roof
Construction:
- 2x6” or larger rafters laid across outside bearing walls/structural supports w/ 1x6” sheathing or plywood/comp
-Will give you a solid feel with no bounce when you are walking on it
-Important not to mistake lightweight flat w/ conventional flat as they can be similar in design
Strengths:
-Perimeter of building is strong point
Hazards:
-Fire susceptibility is dependent on the rafter size, on-center spacing, presence of metal hangers, and type of decking
-Plywood on newer conventional flat roofs offer little support
-Ventilation personnel should be aware of plywood, metal straps, and supports.
Lightweight Roof Styles - Wood I-Beam
Wooden I Beam
-Top and bottom parallel 2x4” or 2x3” cord connected by 3/8” plywood/OSB stem, 24” on center
-Prefabricated at the factory, when installed into a building, either top or bottom cord can be secured with metal hangers on top of bearing walls, with other chord being unsupported either above or below the wall
Nailing blocks are placed perpendicular to the top chords and spaced 4’ apart to provide additional nailing surface for 4 x 8’ plywood/OSB.
● Diaphragm nailing: plywood sheets are placed so the 8’ dimension crosses the structural members and the 4’ dimensions parallels the roof structural members. The joints of plywood are then staggered every 4’
● There may be unprotected I-Beams due to them running heating / air conditioning ducts and other piping and electrical through the stems, giving a fire horizontal access to other beams and assisting the travel and spread of fire
Strengths:
-Perimeter of the building is strong point
Hazards:
-Plywood stem takes little time to burn causing collapse of the truss chords
-There may be unprotected I-beams due to them running heating/air conditioning ducts and other piping/electrical through the stems, giving horizontal access to other beams and assisting the spread of fire
Lightweight Roof Style Open Web Bar Joist
-Top/bottom cords are made of 1/8” steel angle irons w/ 5/8” steel bar webbing
-May have bar-joists used as girders spaced upto 45 ft.
-Joists are spaced 8’ to accept corrugated metal deck covering covered with tar and tar paper
Strengths:
-Perimeter of the building is a strength
Hazards:
-Steel exposed to 1000F will fail causing quick collapse
Panelized
Panelized roofs consist of 4 components:
1) Main/laminated beams 6x36” spaced 12-40” supported by pilasters, wood, or steel posts, or saddles. May be bolted together to span over 100ft.
2) Perpendicular to the beams are purlins, 4x12” and 8’ on center
3) 2x4” rafters parallel the beams and are 24” on center
4) Covering this is 4x8’ sheets of 1/2” plywood
-4x8’ skylights are common placed on these types of roofs, purlin to purlin
Strengths:
-Strengths of roof are beams, purlins, perimeter
Hazards:
-4” hollow steel pipe can be found to support beams, can weaken and fail.
-Insulation (Kraft paper) can give off flammable gases which can flash and char surrounding wood leaving joist susceptible to fire.
-Lightweight trusses with metal gussets can be used for purlins which fail faster
Concrete Tilt Up
-Building made of concrete slabs that have been “tilted-up” into place to form exterior walls
-Identified by their exterior appearance of large block sections
-Up to 5 stories in height
Hazards:
-Commonly have lightweight construction throughout w/ 2x4s/2x3s in compression and tension with 1/2” plywood
-Facades are common on these buildings
Concrete
-Found in occupancies where extra noise insulation is desired such as near freeways, airports, etc.
-Steel or wood substructure covered w/ corrugated metal (“robertson decking”), then a mixture of sand/cement/concrete is placed on top of it along with 4x4” or 6x6” wire mesh
-Concrete is poured to a thickness of 3-4”
-Roof comp makes up final layer
Strengths:
-Roof is strong and resistant to fire
Hazards:
-Difficult to penetrate. Use rotary saw with diamond blade to cut openings.
