05 Roofs Flashcards
1
Q
What is the difference between a cold deck and a warm deck roof?
A
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
Q
What are the advantages of a cold roof?
A
- Waterproof membrane easily accessible for repair and maintenance
- Insulation does not need to take load so wider choice of material available
- Do not need to increase height to upgrade insulation, therefore overcoming potential planning restrictions
3
Q
What are the disadvantages of a cold roof?
A
- 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
- Difficult to maintain integrity of VCL due to its position (e.g. punctured by light fittings or other services)
- Moisture from outside can enter roof space through ventilation gaps and can condensate in cold weather
- 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
Q
What different types of warm roof are there and how do they differ?
A
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
Q
What are the advantages of a warm ‘sandwich’ roof?
A
- Roof structure is protected from extremes of heat and cold (potential damage caused by thermal movement is reduced)
- Less likelihood of condensation occurring in the warm roof space
- Waterproof membrane easily accessible for repair and maintenance
6
Q
What are the disadvantages of a warm ‘sandwich’ roof?
A
- Any water from leaks or water vapour passing through VCL may get trapped in insulation
- 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)
- 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)
- Insulation must be able to resist impact of foot traffic
7
Q
What are the advantages of a warm ‘inverted’ roof?
A
- Waterproof membrane protected against degradation from extremes in temperature or punctures from foot traffic
- Separate VCL not required (waterproof membrane performs a dual function)
- Whole structure (including waterproof membrane) protected from thermal movement due to insulation positioned on outside of the roof
- Upgrading the thermal insulation can be easier/cheaper
8
Q
What are the disadvantages of a warm ‘inverted’ roof?
A
- Extra load due to paving slabs or pebble ballast
- Access to maintenance of waterproof membrane difficult
- Rainwater has tendency to drain more slowly and less effectively
- Insulation limited to materials that resist frost attack, UV light and retain thermal integrity in water, as well as resisting impact of foot traffic
- 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)
- 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
Q
How can the issue of falls be dealt with on a flat roof?
A
- Joists cut to falls
- Joists laid to falls
- Firrings with joist run
- Firrings against joist run
- Cut-to-falls insulation
- Concrete and screeds
10
Q
Why do flat roofs need to be protected from the sun?
A
- 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
Q
How can flat roofs be protected from the sun?
A
- White stone chippings
- Reflective paints
12
Q
How would you construct a cold flat roof and what would you have to allow for complying with the Building Regulations?
A
- 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
Q
What is a ‘green roof’?
A
- 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
Q
What are the benefits of a green roof?
A
- Water - retain up to 80% of summer rainfall (35% in winter), reducing demand on drainage systems
- Noise - considerably reduce external sound within a building
- Energy conservation - minimises internal temperature fluctuations (although less effective in the winter as the substrate is often damp)
- Roof longevity - protects roof from temperature extremes
- Air quality - absorbs carbon dioxide and produces oxygen, filtering out dust and smog particles
- Green space - encourages biodiversity
15
Q
What is the purpose of a parapet wall?
A
- Originally used to defend buildings from military attack
- Slow the spread of fire
- Common in London as a result of the Building Act 1707, when overhanging eaves were considered a fire hazard
- Act as guard rail to protect people/objects falling
- Conceals the rainwater channels and hides the roof
16
Q
Name some different types of flat roof coverings.
A
- Built-Up Mineral Felt
- Mastic Asphalt
- Single-Ply
- Lead Sheeting
- Zinc Sheet
- Copper Sheet
- Steel Sheet
- Asbestos/Fibre Cement Sheet
- Profiled Aluminium Sheeting
- Liquid Material Coverings
17
Q
Describe the build-up of a traditional 3-layer built-up felt system.
A
- 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
Q
Describe the build-up of a modern 2-layer built-up felt system.
A
- 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
Q
How can built-up membranes be applied to flat roofs?
A
- Pour-and-roll
- Torch-on
- Self-adhesion
- Cold adhesion
20
Q
What is the typical life expectancy of a built-up felt roof?
A
- 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
Q
What is mastic asphalt?
A
- 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
