As noted
earlier, modern roof pavers and board systems have been
developed to serve as ballast for roofing single-plies,
and to provide additional benefits such as insulation
and aesthetics. Paver systems covered under the SPRI
specification for roof pavers have been extensively
tested for wind resistance and contain several features,
which enhance their performance such as:
INTERLOCKING
JOINTS
DRAINAGE
CHANNELS
STAGGARD
SEAMS (Step & Repeat Technology)
Results: The only products on the market
today that offer these three things are the “Play-Land” and “Soft-Land” series
as offered by “Unity” Surfacing Systems.
As a result
of the experience gained from many building installations
and from the substantial number of wind tests to which
lightweight roof paver systems have been subjected,
it has become evident that the literal calculation of
wind forces, as defined by Codes and Standards, it NOT
applicable to the design and evaluation of the wind
performances of loose-laid systems, such as rubberized
roof pavers.
Tests,
and actual paver installations which have undergone
exposure to very high winds during hurricanes: HUGO,
JERRY, GLORIA and most recently KITRINA, to name a few,
have shown that roof paver systems resist structural
load wind uplift forces which far exceed their own weight.
The reason for this apparent mystery may be explained
in terms of the following two physical effects:
Results
PAVER INTERLOCKS (found within the
product itself and not connected by any other means
such as pins,
clips or fasteners, i.e. foreign objects): – The mechanical
inter-connection of one paver to the next throughout
an assembly, creates
a cohesive array of units, which tends to behave in unison.
Because the design wind pressures, calculated as described
above, correspond to maximum values occurring within relatively
small areas, and are constantly moving within generalized
roof regions, the large forces they may generate on one
or several pavers are readily distributed through the interlock
system, in effect considerably spreading the tributary area
or paver weight which resists such localized forces.
PRESSURE EQUALIZATION: Pressure equalization
contributes to augment their resistance to uplift or displacement
due to wind forces. Pressure equalization results in a substantial
net reduction in the uplift acting upon roof paver systems,
and it is characterized by the semi-instantaneous transmission
of air pressure through the paver system joints, from top
to bottom of the loose laid elements.
As an example, in the case in which wind
forces impinging on a building resulted in disturbances,
such that, negative pressures of say 50 psf would be acting
on perimeter areas of the roof, this pressure would first
be imposed upon the top surface of pavers, which for the
purpose of this illustration would weigh 6 psf. Within fractions
of a second after this generation of the 50 psf pressure
on the paver tops, air under the pavers rushes through the
pavers themselves as well as the joints to equalize the
pressure differential, so that rather than having 50 psf
of upward suction acting on the pavers, the negative pressure
transmitted to their underside is simultaneously pulling
then down with a nearly equal force.
Experimental evidence shows that the degree
of pressure equalization available to a paver system is
a function of, among other things, the rate of fluctuation
(turbulence) of the air disturbances; that is to say, the
amount of downward suction which tends to cancel uplift
(equalization), decreases as turbulence increases. In our
example, if the fluctuating component of the wind allows
a pressure equalization effect of only 80%, rather than
the maximum 100% possible, the 50 psf on top of the paver
would be counteracted at any one time by an opposing suction
force underneath the pavers of 40 psf, so that the net uplift
acting on the ballast becomes only 6 psf.
OTHER DESIGN PARAMETERS: As seen above,
paver interlock, and pressure equalization, effects contribute
to give modern roof paver systems extremely effective wind
uplift resistance. Proper engineering, design, and installation
of paver ballasts in however essential for a well executed
project, and must contemplate the effect of the following
parameters, as further elaborated in the SPRI Wind Design
guides:
Basic
wind speed
Terrain
exposure / upwind structures
Roof
height above men ground elevation
Parapet
/ gravel stop height
Air
permeability of the roof-deck and edge flashing
Weight
capacity of the roof structure
Roof
geometry and discontinuities
Perimeter
restraints to couple interlock with roof structure
References:
American
National Standards Institute (ANSI) – “Minimum Design
Loads for Buildings and Other Structures” ANSI standard
A58.1
Bienkiewicz,
B. – Meroney, R. N. – “Wind Tunnel Study of Ballast
Paver” – Civil Engineering Dept., Colorado State University
Project 2-96460, Report CER 85-86 BB RNM13, 1985
Kind,
R. L. – Savage, N.G. – Wardlaw, R. L. – “Tests on Loose-laid
Roofing Systems” – National Research Council of Canada
Report LTR-LA-295, 1986
Pardo, J. – “Wind
Performance Limits of Roof Pavers”, Report RCP-88702-National
Concrete Masonry Assoc., 1988
Single Ply
Roofing Institute (SPRI) – “Guide to Specifications
for Roof Pavers” and “Wind Design Guides”
For a complete
copy (set) of the actual test results, please contact “Unity” Surfacing
Systems directly at: (1-877) 41-UNITY |
