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Description  |
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BACKGROUND OF THE INVENTION
The present invention broadly relates to a flat roof comprising a
substructure, panel-shaped elements which are laid loosely on the
substructure, and corrugated cover members positioned on the panel-shaped
elements.
According to the general definition, the term "flat roof" designates roofs
having a maximum slope of about 20 degrees with reference to a horizontal
plane.
The surface of a flat or slightly sloped roof, i.e. more generally of a
"flat roof", belong to those roofs having surfaces on which the flow of
air, i.e., wind, can produce the greatest vacuum or sub-atmospheric
pressure. The absorption and deflection of the wind force, which acts upon
the flat roof due to the creation of a vacuum and which force is directed
to a lift-off of the roof structure becomes more difficult the lighter the
weight of the roof structure.
In the case of a flat roof having a light weight substructure, or in the
case of an old flat roof, an improvement in the thermal insulation
oftentimes is highly desirable if not required. For such a roof, an
additional layer of thermal insulating material can be applied. The
thermal insulating material layer generally consists of individual panels
of a suitable thermal insulating material. Depending on the substructure,
the individual panels can be mechanically secured to the substructure of
the roof, albeit in a labor-consuming manner.
However, the possiblity of a mechanical attachment to the above described
type of flat roof is excluded for a so-called "upside-down roof" which has
a moisture and vapor resistant barrier membrane placed below the layer of
thermal insulating panels. Such an upside-down roof has the great
advantage that the thermal insulation layer simultaneously serves as
protection for the barrier membrane which ordinarily consists of a
relatively fragile sheet or film, for example of a synthetic resinous
material. The thermal insulation panels are coated with a cementitious
material or mortar, or are covered by a layer of gravel, concrete blocks
or panels on their upper surfaces to protect them from UV-radiation.
Lapped joints may be provided between the individual insulation panels to
allow some pressure compensation between the upper and lower sides of the
panels. This pressure compensation is better, the more similar the
external distribution of pressure on the roof surface becomes to a linear
distribution. At a constant external distribution of pressure, during
gusts of wind, equalization of pressure is practically complete such that
the resulting wind gust loading of the insulation panels is nearly zero.
However, in areas adjacent to or near the outer perimeter of the roof this
external pressure distribution is not linear. In these peripheral areas,
the large resulting wind loads inevitably cause a lift-off of lightweight
insulation panels if they are not reliably secured to the substructure of
the roof by locking or securing members or by a frictional connection. In
principle, the problem could be solved by application of an additional
load, for example by an increase in the amount of gravel or by application
of a layer of concrete of sufficient increased thickness on the insulation
panels. However, such an additional load is not possible for roofs of
light construction or for roofs the carrying capacity of which is already
at its limit, e.g. for an old roof construction which is in need of
retrofitting with an upside-down roof. Furthermore, the retention of
gravel in the critical areas of the roof is not always assured due to
movement of the gravel caused by wind and rain.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a flat
roof comprising a substructure having light-weight insulating panels
loosely positioned on the substructure and in which the insulating panels
are secured against lift-off by means of corrugated cover members even
when an extreme external pressure distribution, caused by a strong wind or
wind gust, exists which acts in a direction causing a lifting-off of the
insulating panels.
More particularly, the invention resides in a flat roof comprising a
substructure, panel-shaped elements loosely positioned on the
substructure, and corrugated cover-members of a substantially rigid
material positioned on the panel-shaped elements at least adjacent to an
outer peripheral region of the roof, said corrugated covering members
having channels which extend from an outer perimeter of the roof towards
the center portion of the roof, and wherein said channels form downwardly
facing portions which are open at their ends facing towards the outer
perimeter of the roof and which are closed at their ends facing toward the
center portion of the roof.
The advantages provided by the present invention are particularly based on
a zone of pressure equalization originating between the bottom surface of
a corrugated cover member and an upper surface of the panel-shaped thermal
insulating elements in which zone a nearly constant subatmospheric
pressure zone or vacuum is created during periods of increased airflow,
i.e. during periods of wind storms or gusts. The magnitude of the vacuum
depends on the external vacuum on the upper surface of the layer of cover
members near the perimeter of the roof. Accordingly, a vacuum created
under the cover member is in large areas greater than the vacuum on the
upper surface of the cover member, i.e. due to the pressure differential
the cover member is pressed onto the underlying insulating panels. Because
the pressure is nearly constant across the upper surface of the insulating
panels, the resulting wind load on the insulating panels is nearly zero.
Accordingly, the insulating panels cannot be lifted, even at high wind
speeds. The higher the speed of the wind, the greater becomes the vacuum
or subatmospheric pressure between the cover member and the underlying
insulating panels, i.e. the greater also become the forces which press the
cover member and the insulating panels against the substructure of the
roof.
If desired, the cover members can be fixed with respect to the roof
structure by any additional, mechanical securing means which, for example,
can be positioned at the corners of the flat roof. In such case, care must
be taken that the sensitive barrier membrane of the upside-down roof is
not damaged.
DESCRIPTION OF THE DRAWINGS
The invention will now be explained in greater detail wih reference to the
accompanying drawings in which:
FIG. 1 is a vertical cross-sectional view of a flat roof, specifically, an
upside-down roof.
FIG. 2 is a graphic presentation of an external pressure distribution
(c.sub.p ex) above a corner area of a flat roof and of the pressure
distribution (c.sub.p int) under a layer of the corrugated cover members.
FIG. 3 is a graphic diagram of the lifting forces of air pressure,
represented as the change of the pressure coefficient c.sub.p of the
pressure above the standard area of a portion of the surface of the flat
roof. One curve (c.sub.p ex) relates to a common unprotected roof surface
and the other one (c.sub.p res) relates to a roof surface protected by a
corrugated covering layer.
FIG. 4 is a perspective view of a corner of a flat roof.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 illustrates schematically the general construction, in
cross-section, of a flat upside-down roof. A layer of a roof sealant or
sealing compound is applied or laid on a roof substructure 1. The layer of
roof sealant 2 generally consists of a layer of an elastomeric material
such as, for example, a sealing compound of a rubber or latex based
material, or a sheet or film of a synthetic resinous material. Thermal
insulation panels (3) are laid on top of the roof sealant (2).
A layer of corrugated cover members (4), is positioned on top of the
insulating panels (3) such that the corrugations in the cover members
extend in a direction perpendicular to the width of the cover members. The
cover members serve the purpose of holding the insulating panels (3) in
position on the roof substructure. Each of the cover members (4) is
provided with channel-shaped deformations or grooves (5a) which are shown
in cross-section in FIG. 1. In a preferred embodiment, the cross-section
of the channels or grooves in FIG. 1 are trapezoidal. Other
cross-sectional shapes are useful as well. However, periodically recurring
channels should exist which are open in a downwardly facing direction,
i.e. open toward the roof substructure (1) and the roof seal (2) and which
are closed upwardly.
The channels (5a) are open in a direction facing the insulation panels (3)
and should have a cross-section sufficiently large to allow for an
unhindered run off of moisture or liquid or a diffusion of vapour from
above the roof substucture. As illustrated by FIG. 1, the channels (5a)
form grooves (5b) between the channels which are open in an upwardly
facing direction. These grooves (5b) are filled with a ballast such as
gravel (5c), or the like, the weight of which additionally secures the
position of the insulating panels. When gravel is used, the grooves (5b)
also prevent movement of the gravel due to wind or rain. Such movement
inevitably takes place on conventional gravel-covered flat roofs in which
gravel of the same granular size is used.
FIG. 4 is a perspective view of an upper surface of a corner of a flat roof
comprising a layer of corrugated cover members (4). The roof is surrounded
by a parapet (9). The central area of the roof is covered only by the
insulation panels (3) which are loosely positioned on top of the roof
sealing layer 2, not shown. Along the perimeter of the roof, i.e. adjacent
to the parapet (9), the corrugated cover members (4) are arranged such
that the channels (5a) and the grooves (5b) extend in a direction
perpendicular to the perimeter of the roof or parapet and in a direction
generally towards the center of the roof. The cover members are positioned
such that the open ends of the channels (5b) are adjacent the perimeter of
the roof whereas the ends of the channels facing towards the center of the
roof are sealed or closed by means of a sealing element or closure (7).
The corrugated cover members (4) can be secured to the roof substructure by
means of a fastening member such as nails, screws, or the like, as
illustrated by reference member (6) in FIG. 1. For this purpose, the
fastening member (6) can be driven through the bottom of a groove (5b) of
a cover member 4 into an underlying insulation panel (3). The fastening
forces caused therby are generally sufficient for preventing movement of
the cover member (4).
In exceptional cases such as, for example, in the case of a very high
building and a very large roof surface, a form-fit fastening of the layer
of corrugated cover members (4) can be provided in the corners of the roof
by a rod, bar, or the like, which is attached to the inner walls of the
parapet (9) or to the border of the roof. The rod (8) can be made of, for
example, metal, wood, or a synthetic resinous material. The rod (8) is
laid on the upper side of the layer of corrugated cover members (4) and
thereby maintains the layer (4) in position on the roof substructure.
As illustrated by FIG. 4, a layer of the corrugated cover members (4) is
laid at a distance from the outer perimeter of the roof or from the inside
edge of the parapet, so that a gap is formed (measured perpendicularly to
the roof perimeter) between the roof perimeter or parapet on one side and
the cover members (4) on the other side which gaps should be narrow
compared to the width of the cover members (4) themselves. Generally, the
width of the cover members (4) should amount to at least five times the
width of this gap.
The corrugated cover members (4) can be made of any suitable material such
as, for example, a sheet of metal or a synthetic resinous material.
The mode of operation of a layer of the corrugated cover members will how
be described with particular reference to FIG. 2 wherein a corner of a
flat roof is taken into consideration. The edges of the corner are 0.1 B
units long, based on a width B of the entire surface of the roof. The air
pressure distribution above this corner illustrates that substantial
subatmospheric pressure can exist, especially near the perimeter of the
roof. If, in the corner of the roof, a layer of the corrugated cover
members (4) is placed on top of the insulation panels (3), and if the
channels of the cover members (4) are closed at their inner ends, i.e.
towards the center of the roof, and are open in a direction facing the
perimeter of the roof, a nearly constant vacuum occurs in the volume which
is essentially bounded by the channels (5a) which are open in a downwardly
facing direction. This vacuum depends on the external pressure
distribution near the roof perimeter. Accordingly, the vacuum is, over
large areas of the roof surface under the cover members (4), higher than
above the cover members. Thus, the harder the wind blows, i.e. the greater
the air speed and pressure, the greater the vacuum, i.e. subatmospheric
pressure, on the roof surface and correspondingly, the greater the vacuum
(subatmospheric pressure) underneath the cover members (4). Therefore, it
is surprising, but due to the foregoing physical explanations an
understandable, phenomenon that the layer of cover members is better
protected from lift-off the higher the wind-created suction forces are
near the roof surface. Furthermore, the position of the insulation panels
is secured since the pressure on their upper surfaces is maintained nearly
uniform.
As illustrated in the perspective view of FIG. 2, the suction coefficient
c.sub.p int is about minus 2 under the cover member (4), i.e. in the
predominate part of the corner of the roof, a vacuum or subatmospheric
pressure is generated which is greater than the vacuum or pressure on the
outer surface of the cover member.
This behaviour is more clearly shown in the graph of FIG. 3 which
illustrates the coefficients for the external pressure c.sub.p ex which
exists on a flat roof in the corner area of an unprotected roof surface
(broken curve) and for the resultant pressure c.sub.p res on a roof
surface covered by a layer of cover members (4) (solid line). The values
c.sub.p ex and c.sub.p res were obtained by wind tunnel measurements on a
model of correct scale. In the fashion the mean values of pressure
coefficients c.sub.p ex and c.sub.p res have been calculated for a square
corner surface (A .sub.eck) of which the length of the edges is varied
from 0 to 0.06 B. The building had a rectangualr cross section (width B).
FIG. 3 shows that the resultant force is directed downwardly if the corner
area is larger than 0.0015 B.sup.2. Accordingly, it is sufficient to
secure a relatively small area by means of a layer of the corrugated cover
members (4).
Due to the relatively high flexural strength of the cover members (4), to
which the channel-shaped grooves also contribute, the load acting in the
direction of lift-off above a relatively small area can be compensated by
the load directed downward which acts upon the rest of the cover members.
According to the embodiment of FIG. 4, the height of the parapet of the
flat roof is a multiple of the height of the corrugated cover member (4).
While not mandatory, an optional parapet on the perimeter of the roof
should be higher than the upper surface of the cover member (4).
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