Flat Roofs
For practical and cost reasons, flat roofs are a common type of
roof construction for most non-residential buildings.
Overview
If a flat roof is your chosen method of construction then a range
of installation options follow, which allow you the freedom to
insulate not only thermally, but acoustically and with fire
protection.
But a flat roof can be so much more than the traditional
waterproofing of a building. The possibilities are as wide as your
imagination and Knauf Insulation have solutions to support options
like green roofs, car park roofs, patios, single ply, bitumen
waterproofing and roof terraces.
Advantages
Excellent range of insulation solutions, which have:
Knauf Insulation Products
- The Polyfoam Slimline System consists of a high
performance 100% ozone friendly, extruded polystyrene, rigid board
insulation which is lightweight, yet has excellent structural
strength and long term effectiveness. It is supplied with the
Slimline membrane and is specifically designed to reduce the
passage of water and improve the thermal performance of a protected
membrane roof.
- Polyfoam High Impact and Upstand Board is a laminate
of Polyfoam extruded polystyrene, rigid board insulation and a
tough, weather resistant fibre reinforced cement facing board. It
is highly thermally efficient yet provides a tough faced, weather
resistant solution suitable for exterior use. Polyfoam High Impact
and Upstand Board is square edged.
- The range of Rocksilk Krimpact Flat Roof Slabs are
high strength, consistent density, rigid slabs made from rock
mineral wool, manufactured using the new Krimpact technology,
specifically designed for use in flat roofing.
Polyfoam and Rocksilk Krimpact Flat Roof products can both be
utilised in Cut-To-Falls systems which are supplied to meet
individual project specifications.
Summary
Knauf Insulation manufactures products for use in a wide range
of flat roof systems, giving options that will comply with the
Building Regulations and the required level of thermal performance.
These systems include Single Ply, Built-Up Felt, Inverted/Protected
Membrane and Cut-To-Falls and can be used with timber, metal and
concrete decks.
Detailed Design Considerations
Warm and Cold Roofs
Thermal insulation and the flow of heat and water vapour
through a construction are inextricably linked. The position of
thermal insulation, vapour barriers and the need for ventilation
must all be considered together.
The likelihood of condensation occurring will depend on the U-value
of the construction, the relative humidity and temperature of the
internal air and whether an effective vapour barrier has been
provided. To prevent surface condensation, the roof must be kept
above the dew point of the internal air with an appropriate
thickness of insulation, correctly located. To prevent interstitial
condensation, an effective vapour control layer must be provided on
the warm side of the insulation.
The position of the thermal insulation related to the structural
roof deck has created two basic categories of roof design:
A warm roof has the insulation above the structural deck,
which is thus kept warm. The waterproof covering is above the
insulation and a vapour control layer is placed below the
insulation.
An inverted/protected membrane roof is a form of warm roof in which
the insulation is placed above the waterproof covering (which also
acts as a vapour barrier).
A cold roof has the insulation placed below the structural deck,
which thus remains cold. This construction relies on a vapour
barrier working effectively in tandem with a continuous flow of
ventilation air to disperse the water vapour that reaches the cold
deck. Because of the diffculty in ensuring adequate ventilation
Knauf Insulation do not reccomend the construction of Cold Deck
Flat Roof
This section only considers warm roofs as follows:
- warm roof with built-up felt or mastic asphalt roof
covering
- warm roof with single ply membrane
- inverted/protected membrane roof
Building Regulations
Typical U-value requirements for flat roofs is shown
in the table below:
| |
U-value
(W/m2K)
|
|
| England, Wales and NI |
0.20 |
|
| Scotland |
0.20 |
|
| Ireland |
0.20 |
|
Roof Loadings
The roof structure must be capable of supporting the maximum
estimated traffic loads. In addition, the wind pressure acting on
the roof should be
assessed in accordance with
BS 6399: Part 2: 1997.
Drainage
Guidance on the number and size of rainwater outlets can be
obtained from
BS 6367.
The drainage and edge details should be installed as per the
waterproofing membrane manufacturer’s instructions.
|
Metal decks
The profiles of metal deck roofs need to provide sufficient support
for the insulation. The greater the thickness of insulation,
the greater the span it can accommodate on profiled metal decks.
Where the butt joint of Krimpact Flat Roof Insulation slabs occurs
along the top of the profiles then the maximum allowable span is
(a), as shown in the table. Where the butt joint occurs over the
troughs of the profiles then the maximum allowable span is (b), as
shown in the table.
Note: Polyfoam solutions will perform at least as well as the
Krimpact Flat Roof insulation slabs.
|
Maximum allowable span
| Insulation Thickness (mm) |
Max Span 'a' (mm) |
Max Span 'b' (mm) |
| 140 |
400 |
280 |
| 120 |
360 |
240 |
| 100 |
300 |
200 |
| 70 |
170 |
140 |
| 50 |
170 |
120 |
|
 |
Warm Roofs - Built-up Felt and Mastic Asphalt
Concrete, timber and metal deck flat roofs can all be designed
as warm roofs.
Vapour control layers
The vapour control layer may take the form of a bitumen
bonded, a nailed felt layer or a reinforced polyethylene sheeting.
If in doubt please contact our Technical Advisory Centre.
Bitumen felt vapour control layers
The deck joints and the deck must be coated with bitumen
primer to
BS 3416:
1991, when the vapour barrier is bitumen bonded to the
substrate.
Bitumen felt vapour control layers should always be installed as
described in
BS
8217: 1994.
Care should be taken to ensure continuity at joints, upstands and
roof penetrations. A range of suitable grades of roofing felts are
available with
BS 747
type 5U preferred for its greater resistance to accidental
perforation. It is important to seal any exposed edges of the
boards, e.g. at roof vents, upstands, etc, with waterproofing or
hot bitumen in accordance with normal practice. To prevent moisture
entrapment on or in the insulation boards it is essential to
protect them during laying, before the application of the roof
waterproofing, or to lay the roof covering at the same time as the
board.
Timber decks
The vapour barrier is either nailed to the deck as defined in
BS 8217: 1994, or
bitumen bonded using traditional techniques. Hot bitumen is mopped
over the vapour control layer and the insulation board is bedded
into it as work proceeds.
Inverted/Protected Membrane Roofs
With insulation placed above the waterproof membrane the
latter is protected from the extremes of thermal stress and will
normally result in an extended life. As an inverted/protected
membrane roof places additional requirements on the thermal
insulation which needs to have the following properties:
- low water absorption in service (including freeze/thaw
cycles)
- high compressive strength to withstand ballast load and support
surface traffic
Roof Loadings
Inverted/protected membrane roofs can be constructed on
concrete, timber or metal decks. The structure must be capable of
supporting the weight of the loading layer to the satisfaction of
the structural engineer.
The loading layer can be provided by washed, graded and rounded
gravel of nominal diameter 20-40mm free from fines, by dense
concrete paving slabs or by a green/garden roof system.
A loading layer is necessary to prevent the insulation boards being
lifted by high winds or floating as water drains from the roof. It
also protects the insulation from the effects of
UV
light and gives the roof its fire rating.
For heavily trafficked areas, concrete flags should be used to form
the loading layer.
Where gravel ballast is used paving slabs are required at exposed
edges (ie edges without upstands or abutments) to negate the risk
of wind uplift or wind scour (as shown in the design details
overleaf).
The table below sets out typical loadings for roofs insulated with
the Polyfoam Slimline system. For further details on wind uplift,
dead and imposed loads refer to
BS 6399.
Weight and thickness of
ballast.
|
Insulant
Gravel
Concrete Slabs
thickness
thickness
weight
thickness weight
(mm) (mm) (kg/m
2) (mm)
(kg/m2)
|
|
|
|
all
50
80
50
125 |
|
|
Example Details for Protected Membrane Roofs
Roof waterproofing and falls
BS 6229 recommends
inverted/protected membrane roofs are designed with a 1:40 fall to
ensure a minimum fall of 1:80 in actual site application. The
maximum recommended fall is 1:20.
For roofs with a slope of up to 5º the waterproofing membrane
should meet
BS 8218:
1998 for mastic asphalt,
BS 8217: 1994 for built-up felt or be
an approved single ply membrane.
For roofs with slopes below 1:80 the waterproofing membrane must be
tanking grade and a separating membrane placed between the
waterproofing and the insulation.
As the Polyfoam Slimline system increases the rate of run off of
surface water above the insulation care should be taken to ensure
the capacity and location of gulleys and outlets is correct.
Further details can be found in
BS EN 12056:Part 3.
Filter fabric
A filter layer, in this case the Polyfoam Slimline membrane,
should be placed between the insulation and ballast layer to
prevent fine grit being washed down to the waterproof membrane. The
incorporation of the high performance Slimline layer allows the
thickness of the ballast layer to be reduced, thus reducing the
overall weight of the roof.
Roof Maintenance
As with all other types of roof the inverted/protected
membrane roof will benefit from a regular schedule of inspection
and routine maintenance. Areas of special concern are the membrane,
rainwater outlets, gutters, flashings and other detail work.
The inspection should also include checks to ensure that the
insulation and its loading layer are still positioned as installed.
Eradicate weed-growth with a water-based weed killer.
Correction Procedure for Inverted/protected membrane
Roofs
Correction procedures are given for inverted roofs due to
rainwater flowing between the insulation and the waterproofing
membrane.
The following procedures are applicable only to insulation made
from
XPS.
Correction Due to Water Flowing between the Insulation and the
Waterproofing Membrane:
ΔU is calculated to two decimal places.
ΔU less than 0.01 is considered as zero.
For the Polyfoam Slimline system using a single layer of insulation
above the membrane with an open covering such as gravel f·x =
0.001.
Note: This value is the tested and certified value of the
Polyfoam Slimline system, any other solution would require material
and system specific results to have comparable performance.
Otherwise a value of 0.04 for f·x should be used for a single layer
of insulation with open covering.
Control of Condensation
Because the roof structure and membrane are both kept warm in
an inverted/ protected membrane roof, the risk of condensation is
virtually eliminated.
Nevertheless, condensation might occur on the underside of the deck
if the deck is thin and has been cooled by water run off during
heavy and continuous rain or melting snow. This condensation will
disperse when the heating system overcomes the temperature
difference.
To help prevent the condensation from occurring ensure that the
deck has a minimum R-value of 0.15 m
2K/W. If the deck
has a high thermal capacity it will not be rapidly cooled by
rainwater. The proposed design should be assessed for condensation
risk using the methods and information given in
BS 6229: 1982,
BS 5250: 2002 and
BS 13788.
Symbols & Units
| Symbol Quantity |
Unit |
|
|
f Drainage
factor giving the fraction of p
reaching the
waterproofing
membrane
|
- |
|
|
p Average rate of
rain during the heating season,
based upon data relevant for the location, eg,
weather station, or given through local, regional or
national regulation
|
mm/day
|
|
|
x Factor for
increased heat loss caused by
rainwater flowing on the membrane
|
(W-day)/
(m2Kmm)
|
|
|
Ri Thermal
resistance of the layer of XPS insulation
above
the waterproofing membrane
|
m2K/W
|
|
|
Rt Total
thermal resistance of the construction |
m2K/W |
|
|
ΔU Correction to the calculated
U value of the
construction, to take into account the extra heat
loss
caused by rainwater flowing through joints in
the insulation and reaching the waterproofing
membrane
|
W/m2K
|
|
Specify
SPRA
The SPRA
represents membrane manufacturers, associated component
manufacturers and specialist subcontractors and aims to ensure the delivery of best value
single ply roofing systems, through a quality assured
partnership.
By specifying products and specialist installation by
SPRA
Manufacturer, Associate and Contractor members you can be assured
that all parties meet strict quality criteria. Compliance with
these criteria and with the Code of Conduct is assessed at
application, by annual audit and by random spot checks.
For further information and to obtain copies of the
SPRA Design Guide and
other documents visit
http://www.spra.co.uk/
Warm Roofs - Single Ply
When insulation is placed over the deck and under the
waterproof
membrane (warm roof), it becomes an integral part of the
waterproofing. A thermally stable insulant should be selected with
adequate mechanical and bond strength to resist the compressive and
tensile loads during installation and in service. Differential
movement can be avoided by laying a single ply waterproof membrane
and mechanically fixing it through the insulation to the deck,
using an approved fully bonded system or leaving loose and
ballasting.
Fixing of insulation/membrane
Where the insulation is mechanically fixed rather than bonded
to the roof deck, the wind uplift capability of the roof finish
depends entirely on the mechanical fixing.
When mechanically fixed, the roof membrane may be temporarily
lifted by wind suction. The pressure difference between the
cavities above and below the boards will quickly equalise.
It is advisable with mechanically fixed membranes, to ensure that
all boards are fixed to the structural deck to avoid the boards
moving out of place.
The exact fixing pattern should be determined with reference to the
solution used, the waterproof membrane manufacturer and
BS 6399: Part 2: 1997.
Suitable mechanical fixings are available for all types of deck
structure.
When using fully bonded membranes, the insulation boards should
be
mechanically fixed to withstand the wind load calculated using
BS 6399: Part 2: 1997.
The waterproof membrane may then be fully bonded to the
insulation.
PVC membranes
Where
XPS
insulation boards are used, a separating layer must be placed
between the PVC membrane and the insulation to prevent plasticiser
migration out of the membrane. This may take the form of a glass
fibre or polyester fleece with a minimum density of 100
g/m
2, or a foil facing to the insulation board. An
alternative is to use a membrane with an integrally bonded fleece
backing. EPDM and polyolefine membranes do not require a separating
layer.
Example Details for Single Ply Roofs
Specification clauses
1a) Warm deck - Single ply membrane - Polyfoam
Roofboard
The vapour control membrane and Polyfoam Roofboard Extra*/ Polyfoam
Foil Faced Roofboard*, ......mm thick, to be mechanically
fixed*/adhered* to the roof deck. (*delete as required)
The single ply membrane should be applied over the insulation in
accordance with the appropriate manufacturer’s recommended
specification.
1b) Warm deck - Single ply membrane - Rocksilk Krimpact
Flat Roof Slab
The vapour control membrane and Rocksilk Krimpact Flat Roof
Slab*/Rocksilk Krimpact Flat Roof Slab Extra* thickness ......mmto
be mechanically fixed*/ adhered*to the roof deck. (*delete as
required)
The single ply membrane should be applied over the insulation in
accordance with the appropriate manufacturer’s recommended
specification.
2a) Warm Deck - Car Park Roof – Polyfoam
Roofboard
The whole of the roof area covered with a waterproofing layer to be
insulated using Polyfoam Roofboard Super ....mm thick. The
insulation to be covered with a .....mm reinforced concrete slab.
Slab to be finished to withstand vehicle traffic.
2b) Warm Deck - Protected Membrane Green/garden Roof –
Polyfoam Roofboard
The whole of the roof area, lined with a waterproof membrane to be
insulated with Polyfoam Roofboard Standard*/Extra*/Super*,......mm
thick, covered with Polyfoam Slimline membrane. The Slimline
membrane to be overlaid with a root barrier/moisture reservoir as
specified ensuring no gaps and edges are overlapped. This to be
covered with a filtration layer, covered with growing matter as
specified to client requirements.
2c) Warm Deck - Protected Membrane Roof Terrace
–Polyfoam Roofboard
The whole of the roof area, lined with a waterproof membrane to be
insulated with Polyfoam Roofboard Extra*/ Super*, ......mm thick.
The insulation to be overlaid with Polyfoam Slimline membrane and
covered with minimum 50mm thick paving slabs on spacers.
2d) Warm Deck - Protected Membrane Ballasted Roof
–Polyfoam Roofboard
The whole of the roof area, lined with a waterproof membrane to be
insulated with Polyfoam Roofboard Standard*/Extra*/Super*, ......mm
thick. The insulation to be overlaid with Polyfoam Slimline
membrane, covered with gravel ballast.
The gravel to be washed, rounded and graded, free from sand and
grit with a nominal diameter of 20 to 40mm and a minimum of 50mm
deep.
3) Warm Deck - Built-up Felt and Mastic Asphalt –
Rocksilk Krimpact Flat Roof Slab
The vapour control layer and Rocksilk Krimpact Flat Roof
Slab*/Krimpact Cut-To-Falls*/Krimpact Flat Roof Slab Extra*/
......mm thick to be fully bonded, with hot bitumen*/PU adhesive*.
(*delete as required)
Rocksilk Krimpact Cut-To-Falls should be laid in accordance with
the laying scheme supplied.
The waterproof membrane should be applied in accordance with the
appropriate manufacturer’s recommended specification.
NBS Plus
NBS clauses covering the use of Knauf Insulation products in
all these applications are available in three sections covering
each waterproofing methods:
- Mastic Asphalt: J21
- Built-up felt: J41
- Single ply membrane: J42