Roofing Structural Requirements

Outline: The Importance of a Roof in Building Construction

Introduction

  • A roof is the final layer of a building that protects occupants from weather conditions.

  • Roof covering prevents unwanted elements from entering the building.

Classification of Pitched Roofs

  • Pitched roofs have a pitch over 10 degrees.

  • Roofs with a pitch over 70 degrees are classified as walls.

Basic Functional Requirements

  • Primary function is to exclude weather elements like rain, wind, snow, and dust.

  • Keep the interior cool in summer.

  • Design to handle stresses and transfer them safely to external walls.

  • Provide airtightness, resistance to lateral loading, and movement due to temperature and moisture changes.

  • Offer lateral restraint to prevent wall bowing under excessive loading.

  • Prevent excessive heat loss in winter.

Structural Considerations

  • Factors Influencing Roof Design

    • Appearance/aesthetics impact

    • Height of the building

    • Cost considerations

    • Performance requirements

    • Thermal ambience expectations

    • Location-specific considerations (wind loading)

  • Dimensional Stability

    • Importance of long-term robustness, strength, and resilience

    • Effects of misalignment of ridge and purlins on roof strength

    • Weakening of roof members due to misalignment

    • Measures to prevent loss of stability and endurance (e.g., wall restraint ties)

  • Aesthetics

    • Overlooked function of roofs

    • Roof's role in defining the visual appearance and beauty of a building

    • Significance of the roof in enhancing the overall aesthetics of a house


General Roof Terminology Outline

  • Span

    • Clear horizontal distance between internal faces of supporting walls

  • Rise

    • Vertical height from lowest mid position to highest point of a roof truss

  • Pitch

    • Slope or inclination to the horizontal

    • Pitch = Rise / 1/2(Span)

    • Efficient pitch: 35 to 60 degrees, influenced by roof covering

  • Ridge

    • Spine of a roof; where Rafters are nailed

    • Ridge can be a single beam section or shorter pieces called ridge boards

    • Depth of Ridge board determined by roof pitch

  • Wall Plate

    • Beam distributing roof load evenly along internal wall leaf

  • Internal Leaf

    • Load-bearing skin of the wall

  • External Leaf

    • Protective cladding or facing on the wall

Outline: Different Rafters

  • Spars or Common Rafter

    • Forms the diagonal leg of a right triangle

    • Vertical leg equals the rise, horizontal leg equals the run

  • Valley Jack Rafter

    • Runs from valley rafter to roof ridge at 90° angle

  • Hip Rafter

    • Runs diagonally from wall plate to roof ridge for outside corner

  • Hip Jack Rafter

    • Runs from wall plate to hip rafter at 90° angle

  • Common Rafters

    • Main loadbearing members spanning apex to wall plate at eaves level

  • Hip Rafter

    • Spans between ridge and wall plate for external angle of roof

  • Ends of Rafters

    • Trimmed to 90 degrees for fascia board and eaves fixing

  • Birds Mouth Joint

    • Allows improved restraint to rotation and better fixing contact than truss clip

  • Wall Plate

    • Provides bearing and fixing mechanisms for roof parts

Outline: Different Roof Shapes and Types

Trussed Rafters

  • Factory-made prefabricated trusses

  • Assembled by specialist roofing firms registered with the Trussed Roofs Association

Fixing of Trusses

  • Roof should transfer lateral loads to supporting walls safely

  • Anchored using standard installations and fixings methods

  • Ensure trussed members are properly aligned, plum, and anchored

  • Effective restraint to wind loading and imposed loads

Truss Positioning

  • Tie beam should not project more than 50mm beyond inner leaf blockwork

  • Avoid cantilever effect

Truss Clips

  • Ideal anchor point between wall-plate and tie-beam/rafter junction

Lateral Restraint to Trussed Rafters

  • Restraint needed between trusses, on gable ends, and along load-bearing walls

  • Provides strength, stability, and dimensional coordination for the roof

  • Protects roof coverings and other finishes

  • Minimizes lateral movements for water-tightness and efficiency

  • Effective restraint against wind loading


Principles of Roof Bracing

  • All roofs require permanent bracing:

    • Stability bracing: maintains truss rafters vertical, straight, and parallel at correct spacing for a stable structure.

    • Wind bracing: ensures roof acts as a rigid component to transmit horizontal forces.

      • Diagonal

      • Longitudinal

      • Chevron (for trussed rafter spans > 8m)

  • Diagonal and longitudinal bracing:

    • Necessary in all trussed rafter roofs.

  • Longitudinal bracing:

    • Longitudinal braces at each node (except wall plate) extending the length of the roof.

    • 5 longitudinal braces in a Fink truss.

Outline for Lateral Restraint to Roofs

Approved Document A Requirements

  • Roof should transfer lateral loads to supporting walls safely

  • Roof members tied to walls using tension straps

  • Tension straps high strength for corrosion resistance

  • Straps made of galvanized steel or austenitic stainless steel

Strapping for Eaves Truss/Wall Connection

  • Roofs face uplift forces due to wind loadings

  • Wallplate secured to external wall using galvanized holding down straps

  • Wall plate straps: 30 x 2.5mm galvanized steel, nailed to plate, >1m long, screwed to blockwork or bent into bed joint, @ 2m centres

Rafter to Wall Strap

  • Galvanized steel tension straps (30x5mm) for lateral restraint at ceiling level

  • Straps nailed along ceiling binders at 2m centres using 3.35 x 65mm corrosion resistant nails


Outline: Lofts: Allowing for Open Plans within a Roof

  • Introduction to Lofts

    • A room in the roof

    • Adds value and space to a house

    • Importance of ventilation and thermal assembly

  • Traditional Classification of Roofs

    • Close Couple Roof

      • Spans around 5m

    • Lean-To Roof

      • Monopitch, suitable for terraced housing extensions

      • Spans around 3.5m

    • Couple Roof

      • 100x50mm rafters @ 400c/c

      • Rafters pitched against each other, not suitable for larger spans

    • Collar Roof

      • 100x50mm collar joists placed higher up from ceiling level

      • Collar fixed at 1/3 of R, provides restraint to walls spreading out

    • Purlin Roof

      • Horizontal timbers supported by end walls or struts

      • Supported on masonry corbels or metal joist hangers

    • Double Roof

      • Hanger in tension prevents ceiling sagging

      • Strut in compression prevents purlin sagging