At Unity Surfacing Systems, we realized a need for an improved ballast paver material, which would take into consideration the effect of roof pressures and wind uplift. The goal was to develop the best possible design of roof paver, which would perform at high wind speeds, and bring improved ballast qualities to the roofing industry. Extensive research and wind tunnel testing was done at our own manufacturing facility. The results: studied, and slowly the double tile-mat-block with our unique double interlocking system emerged as our "Play-Land" and "Soft-Land" series.

The "Play-Land" and "Soft-Land" series is the only ballast material, which enables the transfer of air pressures. It's patented design offers superior wind uplift resistance in a high quality rubberized roof paver. The water permeable products we offer provides air vents, which enables rapid transfer of over paver to under paver pressure forces, while still maintaining a UV barrier against color fading.

There are preset interlocking tabs throughout the products that are designed to snap and hook each and every product together along the joints. Adhesive is also applied to the interlocking tabs for extra strength and security. Out step & repeat technology stops an individual paver from being the weakest link. Under pressure drainage has been provided, and incorporates with the air vents to assist with the pressure distribution and thus, less with uplift force.

Results: Our rubberized safety surfacing, flooring and paver products in the tile-mat-block form can be installed on a protected membrane system for ballasting and will accommodate playground and pedestrian use.

What is Wind Uplift?

Wind is deflected at roof edges; roof peaks or obstructions will cause a large drop in air pressure immediately above the paver's surface in certain zones of the roof, (i.e., near corners and walls). Other types of ballast paver's, which are not specifically designed for optimal wind resistant properties, would allow this under paver air volume to collect. If the air pressure underneath the paver is not transferred, the resultant pressure difference creates an uplift force. With no way for the pressure to equalize and no true interlock for one paver to the next, the uplifting force can only be countered by the weight of the individual paver itself. Therefore, ballast dislodgement will occur, resulting in roof ballast failure.

Results: Our rubberized safety surfacing, flooring and paver products in the tile-mat-block form have an authentic double interlocking system and weights at least 6 pounds per square foot to prevent wind up-lift. A feature that no one else in the industry has or offers.

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