Roofs Flashcards
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
What are the advantages of a cold roof?
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
What are the disadvantages of a cold roof?
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)
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
What are the advantages of a warm ‘sandwich’ roof?
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
What are the disadvantages of a warm ‘sandwich’ roof?
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
What are the advantages of a warm ‘inverted’ roof?
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
What are the disadvantages of a warm ‘inverted’ roof?
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)
How can the issue of falls be dealt with on a flat roof?
Joists cut to falls Joists laid to falls Firrings with joist run Firrings against joist run Cut-to-falls insulation Concrete and screeds
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
How can flat roofs be protected from the sun?
White stone chippings
Reflective paints
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
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
What are the benefits of a green roof?
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
What is the purpose of a parapet wall?
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
Name some different types of flat roof coverings.
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
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)
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)
How can built-up membranes be applied to flat roofs?
Pour-and-roll
Torch-on
Self-adhesion
Cold adhesion
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
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
