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1. Building Pathology > 05 Roof Defects > Flashcards

05 Roof Defects Flashcards

1
Q

What are the common defects associated with poor design and construction of pitched roofs?

A
  1. Untreated timbers
  2. Inadequately sized timbers
  3. Omitted/removed roof members
  4. Eaves/verge overhang too small
  5. Sarking felt improperly installed
  6. Inadequate flashings/soakers at abutments
  7. Inadequate ventilation
  8. Inadequate pitch
  9. Nail sickness
  10. Delamination of tiles
  11. Bitumastic over-sealing
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2
Q

What is roof spread and how does it usually occur?

A

An outwards movement of the roof ususally caused by:

  1. Collars placed too high or removed completely to facilitate a habitable space within the roof
  2. Ceiling joists inadequately connected to rafters
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3
Q

Why is the roof’s pitch important?

A
  1. If coverings are laid at too low a pitch, rain and snow may penetrate
  2. Laying coverings on steeper pitches (up to the vertical) is not normally a problem, however all tiles must be nailed, as sole reliance on any nibs may be insufficient
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4
Q

What are the problems associated with roof covering laps?

A
  1. If the gauge is extended, thus reducing the lap, it will increase the likelihood of weather penetration
  2. If the gauge is reduced, thus increasing the lap, it will provide greater weather resistance but may result in an excessive load on the structure
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5
Q

What is nail sickness and why is it problematic?

A

Nail sickness is the term for when ferrous nails used to secure slates/tiles inevitably rust in the presence of the moisture present between the roof covering and sarking felt, causing slipped or missing slates/tiles that can lead to damp penetration

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6
Q

How can nail sickness be remediated?

A
  1. Slipped slates can be re-secured using ‘S’ shaped lead/copper clips (aka ‘tingles’), however this can be unsightly
  2. In replacement work, non-ferrous fixings (e.g. copper, aluminium) should always be used - AVOID galvanised steel
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7
Q

What are the common defects associated with slate roof coverings?

A
  1. Delamination
  2. Torching and bedding
  3. Split slates along the line of nail holes
  4. Nail sickness
  5. Broken head-nailed slates caused by wind uplift
  6. Wide joints between slates causing water penetration
  7. Well-laid slates lying close to each other causing water seepage due to capillary action
  8. Verge failure (half slates used at verges instead of slate-and-a-half, the former of which are difficult to secure properly against wind uplift)
  9. Inadequate overhang at verges (water cannot drip away from the face of the wall beneath)
  10. Bitumastic over-sealing (impedes ventilation)
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8
Q

What is torching and bedding and how may it be problematic?

A
  1. Traditional process of filling the gaps between slates and battens with mortar (commonly lime mortar and animal hair) to prevent rain or snow penetration where no boarding or felt was present
  2. Tends to shrink and fall out, leading to water penetration
  3. Also leads to inadequate ventilation to the underside of the slate
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9
Q

What factors would lead you to recommend re-covering a slate pitched roof rather than patch repairing it?

A

The extent of the problem and the condition of the battens beneath

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10
Q

What are the common defects associated with clay tile roof coverings?

A
  1. Delamination
  2. Cambered/non-cambered tiles - cambered tiles (i.e. concave profile across width) more liable to driving rain or snow penetration, non-cambered tiles increase the likelihood of capillary action
  3. Slipped tiles - insufficent nailing (should normally be every fourth or fifth course and around all verges, hips and ridges) or nail sickness
  4. Torching and bedding
  5. Efflorescence
  6. Verge failure (half slates used at verges instead of slate-and-a-half, the former of which are difficult to secure properly against wind uplift)
  7. Inadequate overhang at verges (water cannot drip away from the face of the wall beneath)
  8. Ridges and hips - lime mortar traditionally used to bed ridge and hip tiles loses its ability to bond with age, leading to loose or missing ridge/hip tiles
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11
Q

What defects can occur in a concrete tiled roof?

A
  1. Overloading - extra weight of concrete tiles may cause bowing/buckling of the roof structure where a previous slate/clay tile covering has been replaced
  2. Loss of surface/colour - originally, the finish of concrete tiles was only surface deep, so once weathered or damaged, the concrete beneath is revealed which is unsightly but does not impair performance
  3. Efflorescence
  4. Delamination
  5. Slipped tiles - insufficent nailing (should normally be every fourth or fifth course and around all verges, hips and ridges) or nail sickness
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12
Q

What problems are associated with replacing slated roof coverings with new concrete tiles?

A

Overloading - extra weight of concrete tiles may cause bowing/buckling of the roof structure where a previous slate/clay tile covering has been replaced

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13
Q

Regarding sagging of the roof, what defects would you look for in the roof space?

A

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14
Q

What are the common defects associated with metal roofs?

A

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15
Q

What are the potential problems with a roof structure incorporating trussed rafters?

A
  1. Roof trusses inappropriately stored or mishandled (physical damage, distortion, water saturation) leading to inadequate structural performance, water saturation and risk of rot
  2. Inadequate bracing of roof trusses - trussed roof likely to move or tilt sideways as a unit
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16
Q

How do the Building Regulations address the problems associated with trussed rafter roof construction?

A

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17
Q

What are the problems associated with a butterfly roof?

A

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18
Q

Why do flat roofs fail?

A
  1. Physical failure - thermal movement, building movement, poor workmanship, poor materials
  2. Constructional moisture - water trapped by the waterproofing layers (e.g. poured concrete slabs, wet screeds laid to falls etc.)
  3. Condensation - lack of ventilation causing interstitial condensation
  4. Water penetration form other sources - parapets, copings, DPCs, rooflights, openings, gutters, service pipes in roof voids, uninsulated refrigerant pipework etc.
19
Q

What methods would you use to find flat roof faults?

A
  1. Visual inspection
  2. Dye
  3. Electronic Leak Detection (ELD)
  4. Infra-Red Thermography
  5. Endoscopes
20
Q

What would you look for when visually inspecting a leaking flat roof?

A
  1. Obvious signs of water ingress externally
  2. Measurement from internal signs of moisture to find the external source (although source of ingress may not be directly above internal signs of moisture - water can run between layers or in voids beneath the covering)
  3. Detailing (cappings, drips, upstands etc.)
  4. Poor falls leading to ponding
  5. Visible signs of movement in the roof / deflection of roof members
  6. Leaking services (water tanks, pipework etc.)
21
Q

Explain how electronic leak detection (ELD) might be used to detect a leak.

A
  1. A weak electrical field is created on the dampened surface
  2. Any current that ‘earths’ into the building changes the nature of the field locally, pinpointing the penetration (or showing when water penetration is not caused by a roof leak)
22
Q

Explain how thermography might be used to detect a leak.

A
  1. Shows the overall thermal continuity of the roof, highlighting areas of heat loss from poor insulation (possibly as a result of being wet) and cold bridging
  2. Useful when source of ingress may not correspond to signs of moisture internally
23
Q

What is ponding and why is it problematic?

A

Collection of water on a flat roof, which can cause:

  1. Stress on the roof covering - water will absorb heat , increasing the surface temperature of the covering which can lead to cracking (especially of built-up felt) - continual freeze/thaw process could also have a detrimental effect
  2. Water can attract insects and larvae which in turn may attract birds whose pecking can damage fabric-based coverings
  3. Any leak in the ‘pond’ area will result in a greater amount of water penetration than just normal rain
  4. Severe cases may affect the structural integrity of the roof
24
Q

What type of common defects with felt flat roofs are you aware of?

A
  1. Ponding
  2. Splits, tears and cracks
  3. Blistering
  4. Lifting of lap joints
  5. Failure of solar reflective paint
  6. Water ingress where pipes/vents pass through roof
  7. Falling away skirting upstands at abutments
25
Q

What type of common defects with asphalt flat roofs are you aware of?

A
  1. Ponding
  2. Superficial crazing
  3. Distortion (rippling, creeping, sagging)
  4. Cracking that penetrates thickness
  5. Blistering
  6. Chemical damage
26
Q

What are the common defects associated with sheet lead coverings?

A
  1. Ponding
  2. Splits and cracks
  3. Dents, punctures and rips
  4. Lifting around edges
  5. Corrosion shown by white streaks
  6. Pitting and corrosion of surface
  7. Pitting and corrosion of underside
27
Q

How would you advise your client on remediation for the failure of a flat roof?

A
  1. Patch repair / re-cover depending on extent of damage
  2. Rebond lifting laps using hot bitumen
  3. Provide expansion joints where thermal/structural movement of substrate is involved
  4. Clean / prime surface and reapply solar reflective paint if failed
  5. Install layer of 10mm light-coloured stone chippings to reduce solar damage
  6. Remake inadequate skirtings to minimum height of 150mm above finished roof level
  7. Eradicate condensation (insulation, vapour barrier, ventilation etc.)
28
Q

What guidance is available in relation to flat roof coverings?

A

RICS Information Paper ‘Flat Roof Coverings’ (2011) contains guidance in the common defects associated with different flat roof coverings and how to remediate them

1. Building Pathology (7 decks)

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