Roofs

Roof

A roof is the uppermost part of a building, supported on walls or columns, which covers the building and protects the occupants and contents from rain, snow, sun, wind and heat.

Types of roof

Roofs are classified as:

1. Flat roofs: nearly horizontal (slope up to 10 degrees, about 1 in 40 to 1 in 60 for drainage); RCC or stone slab, jack-arch and terrace roofs.
2. Pitched (sloping) roofs: slope more than 10 degrees; covered with tiles, slates, sheets or thatch over timber or steel frames. Types: lean-to, gable (couple), hip, gambrel, mansard, hip-and-valley, north-light, butterfly, conical.
3. Curved (shell) roofs: shell, dome, vault, barrel and folded plate roofs for large halls.
4. Pitched-and-flat combinations: e.g. mansard and hipped-gable roofs.

 Lean-to       Gable (couple)    Hip
    /|             /\            /--\
   / |            /  \          /    \
  /__|           /____\        /______\

 Mansard           North light
   ____             /|  /|  /|
  /    \           / | / | / |
 /      \         /__|/__|/__|
 |      |

Functional requirements of a roof

1. Protection from weather: must keep out rain, snow and sun and shed water quickly (proper slope, no leakage).
2. Structural stability: must safely carry dead load, live load, wind load and snow load (and earthquake forces) and transfer them to the walls or columns.
3. Thermal insulation: should keep the interior cool in summer and warm in winter.
4. Sound insulation: should reduce rain noise and outside noise.
5. Durability and low maintenance: resistant to weathering, decay, termites and corrosion.
6. Fire resistance: material and joints should not support fire spread.
7. Drainage: rainwater must drain freely through gutters and rainwater pipes.
8. Light weight and economy: the roof should be light so the supporting walls and foundation are economical.
9. Good appearance: pleasing shape and finish, suited to the building.
10. Lightning and wind safety: firmly anchored against uplift; lightning conductor where needed.

Single timber roof

A single roof has only rafters (no purlins), with or without a tie. It is used for small spans (up to about 5 m). Its types are:

1. Lean-to roof: rafters rest on a high wall at the top and a low wall/post at the bottom; span about 2.5 m (up to 4 m); used for sheds and verandahs.
2. Couple roof: pairs of rafters meet at the ridge and rest on wall plates; no tie, so they push the walls outward; span up to about 3.6 m, used only where walls are strong.
3. Couple-close roof: same as couple roof but the feet of each pair of rafters are joined by a tie beam, which removes the outward thrust; span up to about 4.5-5 m.
4. Collar beam roof: a couple roof in which a collar beam joins each pair of rafters at about one-third to one-half of the height from the wall plate; this reduces sagging of rafters; span up to about 5 m.

 Couple          Couple close        Collar beam
   /\               /\                 /\
  /  \             /  \               /--\ <-collar
 /    \           /____\             /    \
                   tie beam

Double timber roof

A double (purlin) roof is used when the span is more than about 5 m (up to 8-9 m). Along with rafters it has purlins running parallel to the ridge, supported on the cross walls or on binders (beams) laid across the span between the gable walls. The binders are supported on the walls or on king/queen posts. The common rafters are fixed on the purlins, and the whole roof rests on wall plates. Roof battens are fixed over the rafters. Typical members are wall plate, rafters, purlins, binders, struts and (for long spans) tie beams.

        ridge
          /\
         /  \ common rafter
   purlin o--o purlin
       /      \
      /  binder \
   __/___________\__
  wall           wall

A pitched roof (sloping roof) is a roof with one or more sloping surfaces, the slope being more than 10 degrees (usually 25-45 degrees), so rain and snow drain quickly. It is formed of timber or steel rafters/trusses covered with tiles, slates, sheets or thatch.

Types of pitched roof

 Lean-to       Gable (couple)    Hip
    /|             /\            /--\
   / |            /  \          /    \
  /__|           /____\        /______\

 Mansard           North light
   ____             /|  /|  /|
  /    \           / | / | / |
 /      \         /__|/__|/__|
 |      |

Advantages

• Quick drainage of rainwater, less chance of leakage.
• Cheaper for large spans with light coverings; space under roof for attic.
• Good in rainy and snowy regions like Nepal.

Disadvantages

• Requires more maintenance (repairs), is less fire-resistant, and gives no usable terrace.

Terms used in a pitched roof

• Ridge: the horizontal line at the top where two sloping surfaces meet.
• Hip: the inclined line (external angle) where two adjacent sloping surfaces meet; hip rafter runs along it.
• Valley: the inclined line (internal angle) where two slopes meet to form a depression, drains water.
• Eaves: the lower edge of the roof that projects beyond the wall face to throw water clear of the wall.
• Purlin: horizontal member running parallel to the ridge across the principal rafters/trusses, supporting common rafters.
• Rafter: inclined member running from eaves to ridge on which battens or boarding is fixed (common rafter, principal rafter, hip rafter, jack rafter).
• Batten: thin wooden strip fixed over rafters to which tiles or slates are hung or sheets are fixed.
• Rise: vertical height from the wall plate (eaves) to the ridge.
• Span: clear horizontal distance between the supports (inner faces of walls).
• Pitch (slope): ratio rise/span, or the angle of the slope with the horizontal.
• Gable: triangular portion of end wall between the sloping edges.
• Wall plate: timber/RCC member laid on top of the wall to spread the load of rafters or trusses.
• Tie beam: horizontal member joining the feet of rafters to resist outward thrust.
• Verge: the edge of the roof at the gable end.
• Barge board / fascia: board fixed along the verge/eaves for appearance and protection.
• Cleat: small block fixed to the principal rafter to prevent the purlin from sliding.

Figure

              ridge
               /\
              /  \    <- rafter
     rise ->  /    \
             /      \
   eaves ->_/________\_<- eaves
         wall plate  wall
         |<-- span -->|

          ridge
          /\ \
 rafter  /  \ \ purlin(o)
        /  o \ \
       /     \   hip (in a hip roof the sloping
 eaves ======= \  external angle)

The hip, valley, ridge and eaves are the four lines on the roof surface; rafters, purlins and battens are the frame members that carry the covering.

Roof covering is the outer weather-proof layer laid over the rafters/battens of a pitched roof. The choice depends on climate, slope, cost and availability.

Brief notes

• Thatch: bundles of straw or reeds tied on bamboo/timber battens at 45-60 degree slope; cheap and cool but burns easily.
• Tiles: clay tiles (Mangalore, country tile) hung on battens; good thermal comfort and durable.
• Slate: thin sheets fixed with nails; very durable, fire proof, expensive.
• AC and CGI sheets: large light sheets fixed by hook bolts to purlins; quick and economical, CGI most common in Nepal.
• Stone slate (patthar): used in hilly regions of Nepal, very heavy.

A flat roof is nearly horizontal (slope under 10 degrees, only enough for drainage); a pitched roof has a slope steeper than 10 degrees.

Advantages and disadvantages

• Flat roof: advantages are usable terrace, easy future extension, fire safety and strength against earthquake when RCC; disadvantages are leakage, cracks due to temperature changes, and high cost.
• Pitched roof: advantages are quick drainage, lower cost for larger spans, good insulation; disadvantages are fire risk, high maintenance, no terrace.

Couple closed roof

A couple-close roof is a single roof in which each pair of common rafters meeting at the ridge has its feet connected by a horizontal tie beam. The tie takes the horizontal thrust, so only vertical loads reach the walls. Span about 4.5-5 m.

          ridge
           /\
          /  \ rafters
         /    \
        /      \
     __/________\__
    wall plate  tie beam  (wall plate)

Features:

• Rafters about 50 x 100 mm to 50 x 125 mm at 400-600 mm c/c, ridge piece 40 mm thick; tie beam 50 x 100 mm or more.
• Rafters notched/spiked to wall plate and tie.
• Advantage: no outward thrust, simple. Limitation: rafters sag if span exceeds 5 m, so a collar roof or double roof is used.
• Used for small houses, sheds and verandahs.

Collar roof (collar over couple roof)

A collar roof is a couple (or couple-close) roof in which a collar beam (horizontal tie) joins each pair of rafters at about one-third to one-half of the rise above the wall plate. The collar stops the rafters sagging and spreading, and allows slightly larger span (about 5 m) than couple-close roofs.

            ridge
             /\
            /  \
     collar/----\ collar beam
          /      \
         /        \
      __/__________\__
     wall plate   wall plate

Members: wall plate (75 x 100 mm), common rafters (50 x 100 mm at 450 mm c/c), collar beam (40 x 100 mm, bolted), ridge board (40 mm thick), battens and covering.

Comparison

The following factors should be considered while selecting a roof covering:

1. Climate and rainfall: heavy rain needs steep slope and impervious covering; strong wind areas need firmly fixed covering.
2. Slope of roof: thatch needs steep slope, sheets can be flatter.
3. Weight and supporting structure: light covering (CGI) needs light, cheaper trusses; tiles and slates need strong trusses.
4. Cost and availability: local material (slate, stone, clay tiles) lowers transport cost.
5. Durability and maintenance: life of covering and need for repair.
6. Fire resistance: slates, tiles and metal are better than thatch and wood.
7. Thermal and sound insulation: thatch, tiles insulate better than metal sheets.
8. Appearance: colour and texture to suit the building.
9. Ease of fixing and repair, and resistance to earthquake damage (light coverings are preferred in seismic zones like Nepal).

For Nepal, light, fire-resistant and earthquake-safe coverings (CGI sheets, light tiles) are generally preferred; heavy stone or clay tiles need strong supporting structure.

A queen post truss is used for spans of about 8-12 m. It has two vertical queen posts instead of one king post, joined at the top by a horizontal straining beam and at the bottom by a straining sill.

                  ridge
                   /\
                  /  \
      princ.     /    \
      rafter    / ____ \
               /_|____|_\ <- straining beam
              /  |    |  \
       brace /   |QP  |QP \ brace
            /    |    |    \
     wall  +-------tie beam---+  wall

Span and use: spans of 8-12 m for halls, godowns and workshops. It is stronger than a king post truss because the load of purlins is carried by two posts, giving a clear central space between the posts for use.

Single, double and multiple timber roofs

• Single roof: has only common rafters (with or without a tie or collar), and no purlins; used for spans up to about 5 m. Types: lean-to, couple, couple-close and collar beam roofs.
• Double roof: has purlins and binders in addition to common rafters; the purlins reduce the span of rafters. Used for spans of about 5-9 m.
• Multiple (triple / framed) roof: has trusses or principal rafters carrying purlins, which carry common rafters; used for spans over about 9 m. Types of trusses: king post, queen post, mansard and combined trusses.

King post truss

A king post truss is a triangular truss for spans of about 5-8 m, in which a single vertical king post is the central tension member suspended from the ridge and holding the tie beam.

                   ridge
                    /\
        principal  /| \  principal
          rafter  / |  \ rafter
                 /  |K  \
          strut / \ |P  / \ strut
               /   \| |/   \
        wall  +------tie beam----+  wall
        plate

The king post truss is suitable for spans of 5-8 m.

A couple-close roof is a single roof in which each pair of common rafters meeting at the ridge has its feet connected by a horizontal tie beam. The tie takes the horizontal thrust, so only vertical loads reach the walls. Span about 4.5-5 m.

          ridge
           /\
          /  \ rafters
         /    \
        /      \
     __/________\__
    wall plate  tie beam  (wall plate)

Features:

• Rafters about 50 x 100 mm to 50 x 125 mm at 400-600 mm c/c, ridge piece 40 mm thick; tie beam 50 x 100 mm or more.
• Rafters notched/spiked to wall plate and tie.
• Advantage: no outward thrust, simple. Limitation: rafters sag if span exceeds 5 m, so a collar roof or double roof is used.
• Used for small houses, sheds and verandahs.

Couple closed roof

A couple-close roof is a single roof in which each pair of common rafters meeting at the ridge has its feet connected by a horizontal tie beam. The tie takes the horizontal thrust, so only vertical loads reach the walls. Span about 4.5-5 m.

          ridge
           /\
          /  \ rafters
         /    \
        /      \
     __/________\__
    wall plate  tie beam  (wall plate)

Features:

• Rafters about 50 x 100 mm to 50 x 125 mm at 400-600 mm c/c, ridge piece 40 mm thick; tie beam 50 x 100 mm or more.
• Rafters notched/spiked to wall plate and tie.
• Advantage: no outward thrust, simple. Limitation: rafters sag if span exceeds 5 m, so a collar roof or double roof is used.
• Used for small houses, sheds and verandahs.

Mansard roof truss

A mansard roof truss has a double slope on each side: the lower slope is steep (about 60-70 degrees) and the upper slope is flat (about 15-30 degrees). The truss is a combination of a queen post and king post type, with a tie beam. It encloses a room in the roof space (attic) and is used for buildings that need extra living space or large spans.

          ________
         /        \     upper slope 15-30 deg
        /          \
       /            \
      |              |   lower slope 60-70 deg
      |______________|
        tie beam

Elements: tie beam, principal rafters (lower steep and upper flatter), king/queen posts, struts, collar, purlins at the break line, common rafters and battens. The wide span and the attic room under the truss make it useful for large halls and buildings that need extra accommodation.

A collar roof is a couple (or couple-close) roof in which a collar beam (horizontal tie) joins each pair of rafters at about one-third to one-half of the rise above the wall plate. The collar stops the rafters sagging and spreading, and allows slightly larger span (about 5 m) than couple-close roofs.

            ridge
             /\
            /  \
     collar/----\ collar beam
          /      \
         /        \
      __/__________\__
     wall plate   wall plate

Members: wall plate (75 x 100 mm), common rafters (50 x 100 mm at 450 mm c/c), collar beam (40 x 100 mm, bolted), ridge board (40 mm thick), battens and covering.

The collar beam does not take the full thrust like a tie beam; hence walls should still be strong enough to resist some outward thrust. It is used for small residential spans and cottages.

Components of a collar beam roof (with usual sizes)

A collar roof is a couple (or couple-close) roof in which a collar beam (horizontal tie) joins each pair of rafters at about one-third to one-half of the rise above the wall plate. The collar stops the rafters sagging and spreading, and allows slightly larger span (about 5 m) than couple-close roofs.

            ridge
             /\
            /  \
     collar/----\ collar beam
          /      \
         /        \
      __/__________\__
     wall plate   wall plate

Members: wall plate (75 x 100 mm), common rafters (50 x 100 mm at 450 mm c/c), collar beam (40 x 100 mm, bolted), ridge board (40 mm thick), battens and covering.

Types of roof covering for pitched roof

In the Terai region (hot and humid, heavy monsoon), the commonly used coverings are:

1. CGI (corrugated galvanised iron) sheets

• Pros: light weight, quick and easy fixing, cheap, long span between purlins, good rain protection, fire resistant, suitable in earthquake zones.
• Cons: become very hot in summer and noisy in rain, rust in humid conditions if zinc coating is poor, poor thermal insulation, can be blown off by high wind if not fixed well.

2. Clay tiles (khapra / Mangalore tiles)

• Pros: good thermal insulation, cool inside, durable, fire resistant, locally made, attractive.
• Cons: heavy (needs stronger roof frame), brittle and break easily, need regular maintenance, can leak at joints and are damaged by strong wind and earthquake.

Thatch (khar) is also used in rural Terai; it is cheap and cool but is not fire-safe and needs frequent replacement.

A sloping (pitched) roof has the following key components:

                 ridge
                 /\ \
   common rafter/  \ \ purlin
               /  o \ \
      batten  /      \
   covering  /  wall plate\
   eaves ===/____________\=== eaves
                wall

1. Roof covering (tiles, CGI sheets, slate or thatch) which keeps rain out.
2. Battens / boarding over rafters to which the covering is fixed.
3. Common rafters inclined members from eaves to ridge.
4. Purlins horizontal members under rafters that reduce their span.
5. Principal rafters / trusses and tie beam that carry the purlins.
6. Ridge piece at the top and hip / valley rafters at intersections.
7. Wall plate on the wall that spreads the load.
8. Eaves (projecting lower edge), fascia board and gutter.

Why steel trusses are popular compared to timber

1. Higher strength and larger spans (up to 30-100 m) are possible with lighter members.
2. Timber is scarce and costly; steel is available in standard sections.
3. Durability: not attacked by termites, fungus and rot; long life with painting.
4. Fire resistance is better than timber.
5. Prefabrication: quick erection with bolts or welding, little wastage and can be reused or recycled.
6. Uniform quality and predictable strength, suited to seismic design.
7. Fewer joints and members and slender members make them economical for sheds and halls.

Triple timber roof (types)

A triple (framed) roof is used for spans above about 9 m, typically for halls and sheds. It consists of a framework of trusses placed at 3-4 m spacing:

• Principal rafters (of the trusses) carry purlins laid across them.
• Common rafters rest on the purlins and carry the battens or boarding.
• Trusses are tied by a tie beam and braced with king post, queen posts, struts and straining beams. Common truss forms: king post truss (5-8 m), queen post truss (8-12 m), mansard and combination trusses. Since three sets of members (trusses, purlins, common rafters) are used, it is called a triple roof.

Types of triple (framed) roof by the truss used:

• King post truss for spans of 5-8 m.
• Queen post truss for spans of 8-12 m.
• Mansard truss (double-slope) for roofs needing attic room.
• Combined (king-and-queen post) truss for spans above 12 m.

Comparison of king post, queen post and mansard trusses

 King post        Queen post        Mansard
    /\               /\             ____
   /|\              /--\           /    \
  / | \            /|  |\         /      \
 /__|__\          /_|__|_\       |________|

Thatch roof covering

Advantages: cheapest and locally available (straw, khar, reed); good thermal insulation (cool in summer, warm in winter); light weight, so light supporting structure; quick to lay and repair. Disadvantages: high fire risk; short life (3-5 years) and frequent replacement; harbours rats, insects and moulds; needs steep slope (45-60 degrees) and is not easily rain-proof; poor appearance and not suitable for urban areas.

A single roof has only rafters (no purlins), with or without a tie. It is used for small spans (up to about 5 m). Its types are:

1. Lean-to roof: rafters rest on a high wall at the top and a low wall/post at the bottom; span about 2.5 m (up to 4 m); used for sheds and verandahs.
2. Couple roof: pairs of rafters meet at the ridge and rest on wall plates; no tie, so they push the walls outward; span up to about 3.6 m, used only where walls are strong.
3. Couple-close roof: same as couple roof but the feet of each pair of rafters are joined by a tie beam, which removes the outward thrust; span up to about 4.5-5 m.
4. Collar beam roof: a couple roof in which a collar beam joins each pair of rafters at about one-third to one-half of the height from the wall plate; this reduces sagging of rafters; span up to about 5 m.

 Couple          Couple close        Collar beam
   /\               /\                 /\
  /  \             /  \               /--\ <-collar
 /    \           /____\             /    \
                   tie beam

Components shown in the figures

• Rafters: inclined members from eaves to ridge (50 x 100 mm at 450-600 mm c/c).
• Wall plate: timber (75 x 100 mm) on top of wall carrying the rafters.
• Ridge piece / board: at the apex where rafters meet (40 x 150 mm).
• Tie beam: joins feet of rafters in couple-close roof.
• Collar beam: joins rafters at 1/3 to 1/2 height in collar roof.
• Battens and covering over rafters.

Roof

A roof is the uppermost part of a building, supported on walls or columns, which covers the building and protects the occupants and contents from rain, snow, sun, wind and heat.

Requirements of a good roof

1. Protection from weather: must keep out rain, snow and sun and shed water quickly (proper slope, no leakage).
2. Structural stability: must safely carry dead load, live load, wind load and snow load (and earthquake forces) and transfer them to the walls or columns.
3. Thermal insulation: should keep the interior cool in summer and warm in winter.
4. Sound insulation: should reduce rain noise and outside noise.
5. Durability and low maintenance: resistant to weathering, decay, termites and corrosion.
6. Fire resistance: material and joints should not support fire spread.
7. Drainage: rainwater must drain freely through gutters and rainwater pipes.
8. Light weight and economy: the roof should be light so the supporting walls and foundation are economical.
9. Good appearance: pleasing shape and finish, suited to the building.
10. Lightning and wind safety: firmly anchored against uplift; lightning conductor where needed.

Pitched (slope) roof

A pitched roof is a roof with one or more sloping surfaces (slope more than 10 degrees) on timber or steel frames, covered with tiles, slates, sheets or thatch. Rain drains quickly, so it suits rainy and snowy regions.

Types of pitched roof

 Lean-to       Gable (couple)    Hip
    /|             /\            /--\
   / |            /  \          /    \
  /__|           /____\        /______\

 Mansard           North light
   ____             /|  /|  /|
  /    \           / | / | / |
 /      \         /__|/__|/__|
 |      |

Uses of a roof

A roof protects people and property from rain, snow, sun, heat and wind, gives privacy and security, supports services (tanks, solar panels), and provides extra space (terrace, attic).

Types of roof with parts

Roofs are classified as:

1. Flat roofs: nearly horizontal (slope up to 10 degrees, about 1 in 40 to 1 in 60 for drainage); RCC or stone slab, jack-arch and terrace roofs.
2. Pitched (sloping) roofs: slope more than 10 degrees; covered with tiles, slates, sheets or thatch over timber or steel frames. Types: lean-to, gable (couple), hip, gambrel, mansard, hip-and-valley, north-light, butterfly, conical.
3. Curved (shell) roofs: shell, dome, vault, barrel and folded plate roofs for large halls.
4. Pitched-and-flat combinations: e.g. mansard and hipped-gable roofs.

 Lean-to       Gable (couple)    Hip
    /|             /\            /--\
   / |            /  \          /    \
  /__|           /____\        /______\

 Mansard           North light
   ____             /|  /|  /|
  /    \           / | / | / |
 /      \         /__|/__|/__|
 |      |

Parts of a pitched roof

              ridge
               /\
              /  \    <- rafter
     rise ->  /    \
             /      \
   eaves ->_/________\_<- eaves
         wall plate  wall
         |<-- span -->|

Main parts: ridge, hips and valleys, eaves, rafters, purlins, battens, tie beam, wall plate and roof covering (see terms: ridge is the top line, eaves the lower overhanging edge, rafters the inclined supports, purlins horizontal supports).

For a flat roof the parts are: RCC slab, screed (brick bat coba or lean concrete) for slope, waterproofing layer, finishing (tiles), parapet wall with coping, and rainwater pipes (RWP).

Requirements of a roof

1. Protection from weather: must keep out rain, snow and sun and shed water quickly (proper slope, no leakage).
2. Structural stability: must safely carry dead load, live load, wind load and snow load (and earthquake forces) and transfer them to the walls or columns.
3. Thermal insulation: should keep the interior cool in summer and warm in winter.
4. Sound insulation: should reduce rain noise and outside noise.
5. Durability and low maintenance: resistant to weathering, decay, termites and corrosion.
6. Fire resistance: material and joints should not support fire spread.
7. Drainage: rainwater must drain freely through gutters and rainwater pipes.
8. Light weight and economy: the roof should be light so the supporting walls and foundation are economical.
9. Good appearance: pleasing shape and finish, suited to the building.
10. Lightning and wind safety: firmly anchored against uplift; lightning conductor where needed.

Coupled roof

A couple roof (simple gable) consists of pairs of common rafters joined at the ridge and resting on wall plates, without a tie beam. Because there is no tie the rafters thrust outward, so it is used only for small spans (about 3.6 m) with strong walls.

        ridge
         /\
        /  \ pair of rafters
       /    \
      /      \
   __/        \__
  wall plate   wall plate

A coupled roof can be of two kinds: couple roof (without tie, span up to 3.6 m) and couple-close roof (with tie beam at the feet of the rafters, span up to 4.5-5 m).

A couple-close roof is a single roof in which each pair of common rafters meeting at the ridge has its feet connected by a horizontal tie beam. The tie takes the horizontal thrust, so only vertical loads reach the walls. Span about 4.5-5 m.

          ridge
           /\
          /  \ rafters
         /    \
        /      \
     __/________\__
    wall plate  tie beam  (wall plate)

Features:

• Rafters about 50 x 100 mm to 50 x 125 mm at 400-600 mm c/c, ridge piece 40 mm thick; tie beam 50 x 100 mm or more.
• Rafters notched/spiked to wall plate and tie.
• Advantage: no outward thrust, simple. Limitation: rafters sag if span exceeds 5 m, so a collar roof or double roof is used.
• Used for small houses, sheds and verandahs.

Roof covering

Roof covering is the outermost weather-resisting layer of a roof (thatch, tiles, slates, sheets) laid on battens or boarding supported by the roof frame.

Difference between double and triple roof

Constructional details

• Double roof: wall plates on both walls; binders spanning between gable walls; purlins on binders (spacing 2-3 m); common rafters on purlins, with ridge board; battens and covering.
• Triple roof: wall plates on the walls; trusses (principal rafter, tie beam, king/queen posts, struts); purlins on principal rafters held by cleats; common rafters on purlins; battens and covering.

 Double roof            Triple roof
     /\                    /\
    o  o purlin         o /  \ o
   /    \             / /|  |\ \
  binder  \          /_/_|__|_\_\
 __________          wall   truss  wall