Roof Structure and Cladding – Comprehensive Notes (Video 1)
Roof Structure and Cladding – Comprehensive Notes
Learning outcomes (from transcript)
Discuss the different elements of the roofing system.
Identify roof cladding used in New Zealand buildings.
Real-world context
In New Zealand residential areas, you’ll see many different roof designs and claddings; no two houses have the same roof design.
Roofs include protrusions such as chimneys; these affect detailing and flashing requirements.
Where trusses come from and how they are delivered
Trusses are assembled with nail-plate connectors and are typically factory-made in jigs.
Large trusses may be transported in two sections and joined on-site.
Follow manufacturer recommendations for joining on-site; poor assembly impairs high performance.
On-site erection should maintain verticality; bending of trusses can loosen nail plates and joints.
Erection uses a high crane to lift trusses into position vertically; trusses are designed to remain upright once placed.
Truss design considerations
Primary design goal: carry weight safely. This includes:
Self-weight of the truss and timber, and weight of roof coverings (e.g., concrete roof tiles, slate, corrugated iron, etc.).
Weight of the ceiling attached to the truss, lighting fittings, and workers on the roof.
Wind loads and other dead loads such as snow or hail.
Roof structure can be prefabricated (trusses) or framed on-site from members.
Traditional practice involved firming up members on-site; modern practice favours prefabricated trusses for quality and speed.
Key terminology (roof trusses and framing)
Ridge beam: supports rafters at the ridge.
Rafters: framing timber parallel to the slope of the roof; provide support for purlins or roof coverings.
Purlin: members spanning across rafters or trusses to which roof cladding is attached.
Reach: the apex/top section of the truss (ridge area).
Rafter: common rafters running from the ridge to the top plate.
Top plate: the horizontal timber at the top of the wall framing that the rafters sit on.
Bird’s mouth: cut shown where a rafter sits on the top plate; consists of a seat cut (horizontal) and a heel cut (vertical).
Hip rafter: rafter located at the hip of a hip roof.
Jack rafter: shorter rafters that do not run from ridge to wall plate.
Bottom coat: bottom member of the truss (rafters are the top members; tie beams form the bottom of the framed system).
Tie beam: bottom tie member connecting rafters; helps form the triangular truss shape.
Web members: internal members between the top and bottom chords of a truss (in some designs).
Ribbon board: board running along the wall under the roof, providing a mounting surface for rafters.
Fascia board: board at the outer edge of the roof eaves, often supporting gutters.
Sprocket edges: referenced in the transcript as edges where rafters or joists terminate; related term in some framing systems.
Truss types and connections
Gusset plates: joints that connect truss members; can be metal or plywood.
Pre-fabricated vs on-site joinery: factory-truss joints rely on gusset plates; on-site cutting or drilling of trusses is generally discouraged and may void warranties.
Tension vs compression members: some trusses have members designed primarily for tension and others for compression; improper handling can distort the tension/compression balance and lead to failure.
Proper erection practice preserves alignment; trusses must be placed in one plane and not bent or rotated during installation.
On-site installation guidelines (summary)
Trusses must be erected vertically, plumb, and without bow.
Cantilever/trusses with asymmetric shapes must be installed correctly (the right orientation).
Do not cut or drill trusses after manufacture unless approved by the manufacturer.
Wall plates must be firmly bedded on supported walls.
Use hardwood shims or wedges to level/tru-truss; wedge on both sides and nail into the top plate.
After loading, verify vertical alignment and plumb line; gaps between the line and timber indicate lean.
Before final fix, inspect trusses for plumbness and alignment; ensure no bending or twisting remains.
Timber treatment and codes
Roofing timber is treated to a minimum standard: Boron treatment H1.2 (minimum).
This treatment provides termite and decay protection for timber components in roof assemblies.
Roof fixings, straps, and connections
Straps (tie-downs) secure trusses to the wall frame; dog legs (left/right) are used in true connection details.
Connections at the top plate are secured with straps and occasionally dog legs on either side.
All strap positions should be tight against the timber; avoid extending straps away from the member they are securing.
Bird’s mouth, heel cuts, and seat cuts must be properly formed for correct load transfer.
Important design and performance concepts
The design aims to prevent ringing (unwanted oscillation) by ensuring forces in truss members meet at nodes in a way that prevents instability.
On-site orientation and joint integrity (gusset plates, fasteners) are crucial to prevent joint loosening and potential failure.
Wind loads and roof gradient influence uplift forces; flat or low-pitch roofs experience higher uplift in gusty conditions.
Wind loads and roof gradients: practical implications
Flat roofs tend to generate large negative suction zones when wind passes over them, increasing uplift risk.
Increasing roof pitch reduces uplift (negative suction) and improves cladding performance under wind loads.
Very low-pitch roofs with lightweight coverings are more susceptible to tie-beam buckling under wind-induced uplift; design must account for tension/compression in truss elements.
Roof geometry terms (recap)
Ridge: top apex line where two roof planes meet.
Gable: triangular wall section at the end of a pitched roof; the “gable end” forms a triangle with rafters.
Valley: internal angle where two roof planes meet; requires effective flashing to channel water.
Hip rafter: rafters that terminate at the hip corners, forming the outer edges of the roof.
Rafters: primary sloped members supporting roof coverings.
Top plate: horizontal member at the top of the wall that supports rafters.
Seat cut: horizontal cut on the rafter where it sits on the top plate.
Heel cut: vertical cut at the end of the rafter against the top plate.
Roof cladding, underlays, and weather protection
Roof underlay: a plastic membrane beneath the roof cladding; acts as a moisture barrier, catches condensation, and can improve air movement in the roof space.
Condensation issue: heat moves upward, condenses on colder roof surfaces, and can cause rust and corrosion if condensation is not managed.
E2 external moisture (NZ Building Code context): emphasis on durability and watertight performance; products used must be approved or proven to meet performance requirements before consent.
Location of underlay and overlap: underlay should extend over ridge and be overlapped to direct any penetration away from the interior.
Roof underlay and condensation: roles and mechanics
Absorbs condensation forming in the roof space; reduces rust risk on metal cladding below.
Helps reduce dust entry into the roof cavity and improves overall durability of the roof system.
In the NZ context, underlays are critical for durability clauses and building code compliance.
Roof cladding types and NZ context
Common roof coverings in NZ include:
Metal roofing sheets (pre-coated/coated with protective finishes).
Concrete tiles and clay tiles.
Slate tiles (natural or imitation).
Shakes and shingles (timber-based products).
Rubber and asphalt membranes.
Light-transmitting roofing (e.g., polycarbonate sheets).
Less common but noted: thatch roofs (primarily in Africa; not common in NZ).
Market share and preferences in New Zealand
Market share (roughly): red segment = sheet metal dominates; followed by metal-and-concrete roof tiles; other materials (plastic/membrane) are less common.
Typical NZ housing uses sheet metal cladding and metal/concrete tiles.
Cladding classifications and weight considerations
Cladding classifications for roofs follow a light vs heavy framework similar to walls.
Light vs heavy roof cladding examples:
Light: timber shingles, plastic or metal tiles, lightweight membranes, shakes.
Heavy: clay tiles, concrete tiles, slate, single shingles (in some classifications).
A practical threshold for heavy cladding by mass per area is approximately .
Coatings and finishes for metal roofing
Finishes: galvanised, zinc-aluminium alloy, aluminized coatings, pre-painted coatings, PVF (polyvinyl fluoride) laminates.
PVF coatings (e.g., Stedlar): laminated coatings designed for climatic conditions; may be hard to distinguish from pre-painted finishes in appearance.
Coil coating process (long runs): rolling metal through rollers to create continuous coated sheets; enables longer runs and consistent coatings.
Short-run production is used for shorter pieces; long-run runs were popular starting in the 1970s due to better technology.
Fixings: roof screws with appropriate washers (synthetic rubber washers) are essential to seal penetrations; wrong placement or missing washers can cause leaks and corrosion.
Galvanized vs chip-coated finishes: chip-coated finishes have stone-chip top layers; potential for surface wear and micro-cracking if damaged.
Metal roof profiles and installation details
Common profiles: corrugated troughs, trapezoidal, and other pressed metal tile shapes.
Fixing pattern: screws are typically installed along ridges and purlins; avoid penetrating through critical cladding surfaces.
Roof battens and underlay sit beneath the metal roof sheets; batten spacing and underlay overlaps are important for weatherproofing.
Coating durability: scratches or coating damage exposes steel to environmental conditions and accelerates corrosion.
Concrete and slate roof tiles
Concrete tiles: strong but require proper framing; most are pre-drilled at factory; avoid drilling on-site whenever possible to prevent weakening.
Slate: natural product; durable and aesthetically pleasing but sensitive to UV exposure and copper- or iron-based nails; local suppliers preferred to ensure compatibility and durability.
Imported slate can have higher iron content leading to rust or delamination; local NZ slate is typically recommended for UV conditions.
Roof tile underlay remains important for durability and weatherproofing; predrilled holes should be used where possible to reduce on-site drilling.
Timber shingles and shakes
Overlaps must be carefully arranged to avoid water penetration and ensure durability.
Fiber cement imitation slate is brittle and can crack when installed on traditional building paper.
Light-transmitting roofing and membrane options
Light-transmitting roofing provides natural light but can be prone to long-term yellowing or reduced translucency; failure to maintain can compromise visibility and performance.
Rubber or EPDM membranes: durable, color options, variable thicknesses; ensure subfloor ventilation to prevent bubbling.
Thatch roofs (context and considerations)
Not common in New Zealand; more prevalent in parts of Africa for bush environments.
Thatch requires specific grass types, proper thickness (minimum around ), and engineered timber posts.
Maintenance and regulatory considerations apply to thatch structures (grass collection, fire risk, etc.).
Flashings: purpose, materials, and configurations
Flashings are rigid or flexible waterproof materials that drain or deflect water away from joints or penetrations in the cladding system.
Common flashing materials: lead, soft zinc, soft aluminum, copper, galvanized sheet, zinc-aluminium steel, stainless steel, synthetic rubber, concrete, and clay cap tiles.
Key flashing locations: valleys, roof-wall junctions, roof penetrations (chimneys and vents), changes in roof line, gables, and ridge lines.
Valley flashing is essential where two roof planes meet and channel water into gutters.
Chimneys require step flashing around the chimney and cover/apron flashings; proper flashing prevents leaks.
For flat concrete roofs, counter flashing and precast coping with appropriate flashing are typical details.
Chimneys and penetrations: flashing details
Step flashing around chimneys with under-flashing and soakers beneath roof tiles.
Cover flashing (vertical against the chimney) and apron flashing (downwards below) are used depending on position.
When installed properly, flashing prevents water ingress and extends roof life.
Valleys and gutters: drainage considerations
Valleys channel water from two roof planes; improper flashing or leakage can lead to leaks along the valley.
Gutters collect and channel water away from the building; seamless gutters reduce leak points at joints and connections.
Special notes on installation quality and code compliance
NZ Building Code E2 requires acceptable performance for external moisture; products not approved must be demonstrated as equivalent in performance to approved products.
Durability is a critical consideration; the aim is a watertight roof with durable materials and proper flashing details.
The choice of cladding and underlay should reflect local conditions (wind, precipitation, UV exposure) and desired aesthetics.
Condensation and ventilation considerations
Heat transfer moves upward from the interior to the roof cavity; condensation can form on cooler roof surfaces and cause rust or damp issues if not properly managed.
Underlay plays a role in moisture management, dust prevention, and condensation control; proper ventilation improves overall durability.
Practical implications and connections to broader principles
Structural design connects to general load paths: самe principles apply to walls and floors (e.g., load transfer, joints, and fastenings).
Ethical/practical implication: use only approved products; if using non-approved products, you must provide evidence to the building consent authority that performance is equivalent.
Real-world example: improper flashing around a chimney or valley increases risk of roof leaks in heavy rain events; proper step flashing, counter flashing, and sealants are essential.
The importance of material choices (e.g., slate durability, copper nails in slate, local sourcing) highlights long-term maintenance and lifecycle costs.
Quick recap: concepts to remember for exams
Truss components and terms: ridge beam, rafters, purlin, tie beam, seat cut, heel cut, bird’s mouth, jack rafter, top plate, wall plates, gusset plates.
Key design drivers: weight, wind loads, snow, and ceiling/occupant considerations.
On-site erection rules: keep trusses vertical/plumb, do not cut/truss post-manufacture without manufacturer guidance, level with wedges on both sides, secure to top plates with nails.
Flashing and valley details: step flashing around chimneys, apron/cover flashing, and counter flashing for flat roof coping.
Cladding and underlay choices: light vs heavy cladding; typical NZ materials; importance of underlay for moisture control.
Maintenance and compliance: E2 external moisture; use of approved products or demonstrable equivalence; durability and weatherproofing.
Tutorial preview (from transcript)
A short tutorial will follow to review exercises related to on-site truss erection, flashing details, and cladding choices.
References to the original content for study purposes
Terminology illustrations (ridge beam, rafters, purlins, bird’s mouth, heel/seat cuts, top plate, wall plates, etc.).
Visuals showing straps, dog legs, and connections at the top plate and wall frame.
Diagrams of wind uplift effects on flat vs pitched roofs and associated implications for tie beam buckling.
Key LaTeX recap of numbers and technical values mentioned
Purlin spacing example:
Heavy cladding mass threshold:
Timber treatment:
Lead-free and Galvalume coating composition (Zn-Al alloy): ext{Zn} igr= 45 ext{ ext{ a0%}}, ext{Al} igr= 55 ext{ ext{ a0%}}
Localized thickness/overlap terms (e.g., 370 mm centers for purlins; 150 mm minimum for thatch thickness when applicable): these are referenced values in the context of the materials described.
Final takeaway
The roof system is a complex integration of structural members, cladding, underlays, flashings, and drainage details.
Correct design choices, precise installation, and adherence to approved materials and codes are essential for a long-lasting, watertight roof.
The next tutorial will focus on practical exercises to reinforce these concepts.