Building Pathology - Back Church Lane

Question 1

What was the roof build-up?

Answer 1

• Concrete interlocking tiles
• Timber battens
• Poor condition felt underlay
• Timber joists / trusses
• Insulation at loft level

Question 2

What is the age of the building?

Answer 2

• 1990s

Question 3

How did you know this?

Answer 3

• Use of balconies and columns
• Various colours of brickwork with cavity walls
• Aluminium windows
• Concrete tiled roofs
• Circular windows as features

Question 4

What was the cause of the roof defects?

Answer 4

• Water ingress over time resulted in the roof underlay membrane to disintegrate, leaving large holes
• Various instances of poor temporary repairs to parapet upstands and lead flashing using incorrect mastic

Question 5

What mastic had been applied?

Answer 5

• Internal silicone mastic sealant usually used for bathrooms

Question 6

Why was this incorrect?

Answer 6

• Silicone sealant is not designed for heavy weight bearing items, such as roof tiles, coping stones

Question 7

What is a correct mastic to use?

Answer 7

• Sealants using CT1 – they contain no solvents and will not shrink

Question 8

How were the tiles fixed together?

Answer 8

• Interlocking – no nails, only a single lap is required
• Hung on the battens (38mm x 25mm)
• Headlaps – 75mm

Question 9

What roofing underlay did you specify? Why?

Answer 9

• Marley vapour permeable membrane with BS 5534 25x38 treated battens
• Breathable membrane to assist with potential of internal condensation

Question 10

How are coping stones mechanically fixed?

Answer 10

• Each coping stone drilled to accommodate 6mm pins, holes drilled 20mm from bottom of cope, at least 60mm from the sides using 7mm masonry drill
• Place coping stone onto wall, mark out where holes must be drilled on wall to secure bracket
• Drill holes using 8mm masonry drill, the fit mechanical plugs – brackets then screwed into position with stainless steel screws, resin bond pins into the holes
• Coping stones fitted to allow for 10mm mortar joint
• Coping stones installed with drip channel on the underneath each side, at least 30mm away from wall

Question 11

What did you specify to repair the box guttering?

Answer 11

• 2-layer liquid system with reinforcement mesh
• Terminating at 150mm on the upstands of the parapets
• Replacement of lead flashing to code 5 lead, with CT1 mastic sealant

Question 12

What internal repairs did you recommend?

Answer 12

• Following period of rainfall, no further water ingress
• Proceeded to undertake minor filling to plasterwork and decorations

Question 13

Back Church Lane pitched roof defects – can you explain a cross section of the pitched roof system installed?

Answer 13

• Typical detail – rafters / trusses, sarking felt / breathable membrane, tile batten tiles
• Eave details: extended 200mm beyond external face of wall, guttering beneath lower tile course, fascia, soffit underneath
• Ridge detail: half-round ridge tile with timber ridge batten, ridge union fixing at the top, ventilation at the bottom of either side of tile
• Ventilation – low level ventilation accepted through soffit vents, high level ventilation releases stale warm air through ridge tiles
• When might you need to use a counter batten?
  o Counter battens run vertically behind the tile batten course
  o To create clear path for rainwater to run
  o For roofs insulated at rafter level
  o Allows ventilation of the batten space