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Flashcards in 05 Roofs Deck (53)
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1

What is the difference between a cold deck and a warm deck roof?

Cold deck - insulation placed immediately above ceiling and between joists

Warm deck - insulation located above the deck, meaning temperature of structure is kept close to temperature inside the building

2

What are the advantages of a cold roof?

  1. Waterproof membrane easily accessible for repair and maintenance
  2. Insulation does not need to take load so wider choice of material available
  3. Do not need to increase height to upgrade insulation, therefore overcoming potential planning restrictions

3

What are the disadvantages of a cold roof?

  1. Relies on ventilation removing moisture that may enter roof space, which is often difficult to provide in sheltered areas where there is less air movement
  2. Difficult to maintain integrity of VCL due to its position (e.g. punctured by light fittings or other services)
  3. Moisture from outside can enter roof space through ventilation gaps and can condensate in cold weather
  4. Likelihood of thermal movement occurring in the deck is increased due to direct exposure to heat from the sun (not protected by insulation as per warm roofs)

4

What different types of warm roof are there and how do they differ?

Warm 'Sandwich' Roof:

  • Insulation is placed above the deck but below the waterproof covering, i.e. forming a 'sandwich' between the deck and the waterproofing

 

Warm 'Inverted' Roof:

  • Insulation is placed above both the deck and the waterproof membrane, i.e. the waterproof layer is inverted from its 'normal' position

5

What are the advantages of a warm 'sandwich' roof?

  1. Roof structure is protected from extremes of heat and cold (potential damage caused by thermal movement is reduced)
  2. Less likelihood of condensation occurring in the warm roof space
  3. Waterproof membrane easily accessible for repair and maintenance

6

What are the disadvantages of a warm 'sandwich' roof?

  1. Any water from leaks or water vapour passing through VCL may get trapped in insulation
  2. Possible that thermal movement within insulation layer could damage waterproof covering (although can be overcome by careful selection of insulation or by placing an isolating layer between insulation and membrane)
  3. Waterproof membrane is exposed to a greater range of temperatures (receives no heat from the building in cold weather and cannot dissipate solar heat in warm weather due to the insulation below)
  4. Insulation must be able to resist impact of foot traffic

7

What are the advantages of a warm 'inverted' roof?

  1. Waterproof membrane protected against degradation from extremes in temperature or punctures from foot traffic
  2. Separate VCL not required (waterproof membrane performs a dual function)
  3. Whole structure (including waterproof membrane) protected from thermal movement due to insulation positioned on outside of the roof
  4. Upgrading the thermal insulation can be easier/cheaper

8

What are the disadvantages of a warm 'inverted' roof?

  1. Extra load due to paving slabs or pebble ballast
  2. Access to maintenance of waterproof membrane difficult
  3. Rainwater has tendency to drain more slowly and less effectively
  4. Insulation limited to materials that resist frost attack, UV light and retain thermal integrity in water, as well as resisting impact of foot traffic
  5. Possibility of condensation forming through rainwater flowing between insulation and waterproof layer (although can be counteracted by installing an extra layer of insulation below the waterproof membrane)
  6. Possibility of grit being washed beneath insulation and damaging the waterproof membrane through a 'scouring' action (although a geo-textile sheeting material can be placed to help prevent this)

9

How can the issue of falls be dealt with on a flat roof?

  1. Joists cut to falls
  2. Joists laid to falls
  3. Firrings with joist run
  4. Firrings against joist run
  5. Cut-to-falls insulation
  6. Concrete and screeds

10

Why do flat roofs need to be protected from the sun?

  • Solar radiation can cause durability problems for flat roofs (particularly built-up felt and mastic asphalt coverings) due to the temperature changes causing constant expansion and contraction
  • This can be particularly problematic in warm-decked roofs as the position of the insulation means solar radiation is focused on the waterproof covering

11

How can flat roofs be protected from the sun?

  1. White stone chippings
  2. Reflective paints

12

How would you construct a cold flat roof and what would you have to allow for complying with the Building Regulations?

  • Insulation placed immediately above ceiling and between joists
  • Void above insulation must be ventilated to prevent moisture vapour condensing on the colder timbers
  • Gap above insulation should be approx. 50mm (60mm for roofs spanning over 5m), with 25mm continuous ventilation gap at the eaves (30mm for roofs spanning over 5m)
    • NB: Building Regulations Part C refers to compliance with BS 5250:2011 'Code of practice for control of condensation in buildings'
  • Vapour Control Layer (VCL) required on warm side of the insulation to help combat condensation

13

What is a 'green roof'?

  • A roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproof membrane
  • May also include additional layers such as a root barrier and drainage and irrigation systems

14

What are the benefits of a green roof?

  1. Water - retain up to 80% of summer rainfall (35% in winter), reducing demand on drainage systems
  2. Noise - considerably reduce external sound within a building
  3. Energy conservation - minimises internal temperature fluctuations (although less effective in the winter as the substrate is often damp)
  4. Roof longevity - protects roof from temperature extremes
  5. Air quality - absorbs carbon dioxide and produces oxygen, filtering out dust and smog particles
  6. Green space - encourages biodiversity

15

What is the purpose of a parapet wall?

  1. Originally used to defend buildings from military attack
  2. Slow the spread of fire
  3. Common in London as a result of the Building Act 1707, when overhanging eaves were considered a fire hazard
  4. Act as guard rail to protect people/objects falling
  5. Conceals the rainwater channels and hides the roof

16

Name some different types of flat roof coverings.

  1. Built-Up Mineral Felt
  2. Mastic Asphalt
  3. Single-Ply
  4. Lead Sheeting
  5. Zinc Sheet
  6. Copper Sheet
  7. Steel Sheet
  8. Asbestos/Fibre Cement Sheet
  9. Profiled Aluminium Sheeting
  10. Liquid Material Coverings

17

Describe the build-up of a traditional 3-layer built-up felt system.

  • Base layer: underfelt, partially bonded to allow for differential movement by either:
    • Providing a perforated underfelt laid loose over the deck/insulation
    • Nailing the underfelt to the deck (if the deck is timber)
  • Intermediate layer: fully bonded waterproof bitumen membrane
  • Top layer: aka 'cap sheet' - fully bonded waterproof bitumen membrane incorporating a protective finish (e.g. mineral granules, reflective paint, foil-faced)

18

Describe the build-up of a modern 2-layer built-up felt system.

  • Base layer: fully bonded waterproof polyester-reinforced membrane with elastomeric bitumen (to allow for differential movement)
  • Top layer: aka 'cap sheet' - fully bonded waterproof polyester-reinforced bitumen membrane incorporating a protective finish (e.g. mineral granules, reflective paint or foil-faced to provide solar reflectance)

19

How can built-up membranes be applied to flat roofs?

  1. Pour-and-roll
  2. Torch-on
  3. Self-adhesion
  4. Cold adhesion

 

20

What is the typical life expectancy of a built-up felt roof?

  • Traditional systems: 10-15 years
  • Modern RBM roofs: 15-30 years
  • High performance RBM roofs: up to 50 years
  • NB: life expectancies largely depend on quality of installation

21

What is mastic asphalt?

  • Waterproofing material originally consisting of lake asphalt mixed with limestone aggregate naturally impregnated with bitumen
  • Modern high-performance asphalts are polymer-modified products which perform better in thermal shock and low temperature bending tests

22

What type of roof decks does mastic asphalt perform better on and why?

  • Can degrade far quicker on traditional warm-decked 'sandwich' roofs, as solar gain is focused into the waterproofing
  • Therefore, performance is better on cold decked roofs or inverted warm roofs

23

How is mastic asphalt applied to flat roofs?

  1. Comes to site as solidified blocks where they are re-melted
  2. Laid over an isolating membrane (usually black sheathing felt) which is laid loose with 50mm minimum overlaps
  3. Usually laid in two coats of approximately 10mm thick each (20mm total thickness) in bays about 2.5 - 3 metres wide
  4. Application is usually spreading the liquid asphalt with a trowel
  5. Joints between the bays are staggered but fuse together to produce a seamless covering
  6. Sand should be rubbed into the topcoat in order to break up any surface build-up of bitumen, thus reducing damage by solar radiation
  7. Should be laid to the recommendations of BS 8218:1998 (Code of Practice for Mastic Asphalt Roofing)

24

What is the typical life expectancy of a mastic asphalt roof?

  • 50 years on cold deck roof
  • NB: life expectancy largely depends on quality of installation

25

What is a single-ply roof covering?

  • Polymeric single-ply waterproofing is single sheet material in which synthetic polymers are a major constituent
  • Many different types ranging from thermoplastic materials (e.g. polyvinyl chloride, aka PVC) to new modern elastomeric products (e.g. thermoplastic polythene, aka TPE)

26

Name some benefits of single-ply roof coverings.

  1. Formulated to provide maximum resistance to environmental degradation (e.g. from chemicals, ultraviolet light and ozone)
  2. Do not generally require surface treatment or maintenance

27

What is the main disadvantage of single-ply roof coverings?

Susceptible to damage as they are very thin (approx. 1.2 - 1.5mm) and only of a single layer

28

How are single-ply roof coverings applied to flat roofs?

  1. Mechanical fastening (designed to resist movement occurring under wind loads)
  2. Fully adhered
  3. Loose laid and ballasted

29

What is the typical life expectancy of a single-ply roof?

  • In excess of 25 years
  • NB: life expectancy largely depends on quality of installation

30

Name some benefits of lead sheet roof coverings.

  1. Extremely malleable and easily dressed into shape
  2. Develops its own carbonate over time that helps protect it