Roof Structure

The roof structure carries the covering, the plant and services on and under it, wind and snow, and in some buildings ceilings and people. It includes the primary framing (trusses, rafters, portal frames, beams, slabs), secondary members (purlins, battens, ceiling joists), bracing and tie-downs that resist wind uplift and racking, and the connections between them.

What shortens life:

• Water: leaking coverings, gutters and membranes, and condensation in unventilated roofs.
• Alterations: webs and braces cut for ducts and loft conversions, chimney breasts removed below, hatches cut through members.
• Added load: plant, solar arrays, water tanks, heavier replacement coverings, hung services and storage on ceiling joists.
• Termites and borers in timber in Australia, and furniture beetle and death-watch beetle in old British oak and softwood.
• Coastal exposure on purlins, cleats and bolts.

Roof spread

A pitched cut roof pushes outward at the eaves under load. Ceiling joists or collar ties restrain it; where they are missing, too high, poorly fixed or removed, the rafters spread, the ridge drops and the tops of the walls are pushed out. Re-covering a roof designed for slate with heavier concrete tiles is a common trigger.

Truss defects

Nail plates back out where timber shrinks and gets wet; webs cut by trades take a member out of the truss; missing or loose bracing lets trusses lean; tie-downs that do not reach the wall frame fail under uplift. Truss uplift, where bottom chords bow up seasonally with moisture changes, opens a gap between ceilings and internal walls; it is common and usually not structural.

Decay and insect attack

Timber at wall plates, valleys, box gutter lines and under leaks decays; termites attack from the ground up through walls into the roof; borers attack sapwood. Decay concentrates at bearings, where loss of section is most serious.

Steel corrosion

Purlins and cleats corrode at the eaves and along box gutter lines, under translucent sheets and where condensation drips. Portal frame bases corrode where they sit in wet ground or are buried by raised floor levels. Section loss in cold-formed purlins, which are thin to start with, quickly reduces capacity.

Overload and instability

Plant, solar arrays and tanks added without checking the structure overload purlins and slabs. Cold-formed purlins buckle sideways when their bridging or fly bracing is removed or never installed. Ponding from blocked gutters loads light roofs past capacity.

Concrete slab deterioration

Roof slabs deteriorate when their membrane fails: water and chlorides reach the reinforcement, which corrodes and spalls the soffit. Long-term deflection causes ponding, which accelerates membrane failure.

RAAC

Reinforced autoclaved aerated concrete planks are light, weak and permeable. With moisture, cracking, short bearings or corroding reinforcement they can fail suddenly. Where present they are a structural safety matter regardless of appearance.

From the street and from aerial imagery

• Ridgeline dipping or roof plane sagging between trusses or rafters.
• Walls bowing out at eaves level; cracks at the top of external walls near the corners.
• Plant, solar arrays and tanks added on roofs; ponding.

In the roof space

• Nail plates lifted; webs or braces cut or missing; trusses out of plumb.
• Decay at wall plates, valleys and under leaks; termite workings and mud leads; borer holes and frass.
• Rafters spreading at the wall plate; collar ties or ceiling joists detached.
• Corrosion of purlins, cleats and bolts; buckled or twisted purlins.
• Hung services, storage and water tanks on ceiling joists.

Inside the building

• Ceiling cracks along the line of the wall plate; gaps between ceilings and internal walls (truss uplift).
• Spalling and rust staining on concrete roof slab soffits; deflection.
• In industrial buildings, purlins visibly sagging or twisted, rust along gutter lines.

In the records

• Truss design certificates and layouts; structural drawings.
• Engineering for plant, solar and tanks added to the roof.
• Termite inspection and treatment records.
• Records of re-covering, alterations and loft conversions.
• RAAC surveys where the building is of the relevant era and type.

Findings are graded by what they demand: safety (can hurt someone now), failure (failed and damaging the building), condition (distress present and progressing), lifecycle (renewal due on age and exposure), deadline (a regulator or ordinance sets the date), check (investigate before sizing works) and nil (cosmetic or stable).

A structural engineer leads, with a timber pest inspector for timber roofs in termite areas.

• Roof space inspection of all accessible members, connections, bearings and bracing.
• Level survey of ridges and roof planes; plumb of walls at eaves.
• Section measurement on corroded steel; probing of timber for decay.
• Load assessment of existing and proposed plant, solar and tanks against capacity.
• Concrete testing of slab soffits: cover, carbonation, chlorides.
• RAAC identification and assessment to the Institution of Structural Engineers guidance where planks may be present.
• Timber pest inspection to AS 4349.3 in Australia.

• Moisture readings in timber at bearings and under leaks.
• Tie-down inspection in cyclonic regions.
• Deflection measurement under plant against design limits.

Sampling is weighted to bearings, valleys, box gutter lines and areas under plant, where decay, corrosion and overload concentrate.

• Restore ties and bracing: new collar ties or ceiling ties, steel straps, reinstated bracing and webs.
• Strengthen members: sister rafters and purlins, add purlins, steel plates on trusses.
• Replace decayed members and treat for termites with a management system.
• Steel repair: treat and recoat, replace corroded purlins and cleats.
• Support plant on independent frames bearing on primary structure.
• Concrete repair of slab soffits after fixing the membrane.
• RAAC: propping, additional bearing, or replacement of the planks.
• Tie-down upgrade in high wind regions, often at re-roofing.

• Bracing and bridging for cold-formed purlins.
• Propping as an interim measure while strengthening is designed.

Engineering assessment is a lump sum. Repairs are itemised per member or connection; strengthening and plant frames are lump sums; purlin replacement is by length and usually done with re-roofing. Termite management recurs annually. RAAC remediation is a lump-sum project with propping as an immediate cost.

Strengthening for added loads is code compliance; repairs are repairs and maintenance; recoating steel is lifecycle; the assessment is the check line that sizes everything else.

• Australia. The National Construction Code calls up AS 1684 for timber framing, AS 1720.1 for timber structures, AS 4440 for nail-plated trusses, AS 4100 and AS/NZS 4600 for steel, and AS/NZS 1170 for actions; termite management follows AS 3660.1 and inspections AS 4349.3.
• United Kingdom. Approved Document A and the Eurocodes (BS EN 1995 for timber, BS EN 1993 for steel, BS EN 1991 for actions) apply; the Institution of Structural Engineers publishes guidance on identifying and assessing RAAC, and the government has published guidance for building owners.
• United States. International Building Code Chapters 16 (loads), 22 (steel) and 23 (wood), with ASCE 7 for actions and ANSI/TPI 1 for trusses; the International Existing Building Code governs alterations and added loads.

• Structural drawings and truss certificates.
• Engineering for plant, solar arrays and tanks on the roof.
• Termite inspection and treatment records.
• Records of re-covering, alterations and loft or roof-space conversions.
• RAAC survey for buildings of the relevant type and era.
• Leak history, since leaks drive most decay and corrosion.