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Historic Preservation - Technical Procedures

Spectitle:

Types Of Flat Roofing And Factors Affecting Its Deterioration

Procedure code:

0750002S

Source:

Hspg Prepared For Nps - Sero

Division:

Thermal And Moisture Protection

Section:

Membrane Roofing

Last Modified:

02/24/2012

Details:

Types Of Flat Roofing And Factors Affecting Its Deterioration



TYPES OF FLAT ROOFING AND FACTORS AFFECTING ITS DETERIORATION


TYPES OF MEMBRANES FOR FLAT ROOFING

1.   Built-up Roof Membrane (BUR):

    -    Assembled in place using multiple plies of asphalt-
         impregnated felt bedded in bitumen.

    -    Asphalt or coal-tar is applied hot in order to merge with
         the saturant bitumens in the felt and form a single-piece
         membrane.

    -    The felt is laminated in overlapping layers to form a
         membrane that is two to four plies thick.

    -    The membrane is protected from sunlight and physical wear
         by applying a layer of aggregate (such as crushed stone
         or other mineral granules) embedded in the surface ply.

2.   Elastomeric/Plastomeric Roof Membrane:  

    -    Sheet materials applied to the roof in a single layer.

    -    They require less on-site labor than built-up roofing and
         are usually more elastic and, therefore, less prone to
         cracking and tearing.  

    -    They may be affixed to the roof with adhesives, by the
         weight of a gravel ballast, by fasteners concealed in the
         seams between sheets, or with mechanical fasteners that
         do not penetrate the membrane.

    -    Some types of elastomeric/plastomeric roof membranes
         include the following:

         1.   Neoprene (polychloroprene):  

              -    A high-performance synthetic rubber compound
                   applied in sheets ranging from 0.030 to 0.120
                   inches thick and joined at the seams with an
                   adhesive.  

              -    Vulnerable to attack by ultraviolet light;
                   therefore, it is usually coated with a
                   protective layer of chlorosulfonated
                   polyethylene.

              -    Vulnerable to aromatic solvents and strong
                   oxidizing chemicals

              -    It may be fully adhered to the roof deck or
                   partially adhered, with aggregate ballast to
                   prevent wind uplift.

         2.   EPDM (ethylene propylene diene monomer):

              -    The most widely used material for single-ply
                   roof membranes.

              -    A synthetic rubber manufactured in sheets
                   ranging from 0.030 to 0.060 inches thick and
                   joined at the seams with an adhesive.

              -    Vulnerable to petroleum products and plastic
                   roofing cement.

              -    It may be fully or partially adhered, or used
                   in a protected membrane roof.

         3.   PVC (polyvinyl chloride):

              -    A thermoplastic compound commonly known as
                   vinyl.

              -    Relatively inexpensive.

              -    PVC sheets for roofing range from 0.032 to
                   0.060 inches thick and are joined at the seams
                   either by solvent welding or hot air welding.

              -    Vulnerable to ultraviolet radiation, petroleum
                   products, and coal tar.

              -    It may be fully or partially adhered, or used
                   as a protected membrane.

         4.   Chlorinated polyethylene and chlorosulfonated
              polyethylene sheets:

              -    Vulnerable to petroleum products.

              -    Not compatible with PVC.

              -    Highly resistant to ultraviolet deterioration
                   and can be manufactured in light, heat-
                   reflective colors.

              -    Used on roofs where aggregate ballasting is
                   unacceptable for reasons of appearance or
                   excessive slope.

         5.   Polymer-modified bitumens:  

              -    Vulnerable to petroleum products, hydro-
                   carbons, and some chemicals.

              -    Formed into composite sheets with various
                   other materials.  

              -    Some are intended to be laid loose, others to
                   be adhered to the roof deck or insulation.

3.   Fluid-applied Roof Membrane:

    -    Used primarily for domes, vaults and other complex shapes
         that are difficult to roof by conventional means.  

    -    Applied usually in several coats using a roller or spray
         gun.  When it cures, it forms a rubbery membrane.


FACTORS CONTRIBUTING TO FLAT ROOF DETERIORATION

1.   Sun:  

    -    Hot sunshine on a roof causes the volatile ingredients of
         tar or asphalt to evaporate.  

    -    The asphalt oxidizes and becomes brittle.  

    -    The roof mat eventually loses its elasticity, the surface
         coating becomes checked and flakes off, exposing the felt
         below.

2.   Water:  

    -    Water can seep into a dry roof through cracks and cause
         a leak.  

    -    This moisture can turn to ice in freezing temperatures
         and can cause the roof to tear or heave.

3.   Wind:  

    -    A strong wind can drive rain into defective joints in the
         mat or parapet, can cause the roof to tear at loose seams
         and can cause the roof structure to sway.

4.   Temperature Changes:  

    -    Expansion and contraction place strains both on the roof
         structure (deck and walls) and strain the flashings.
         These strains can cause the roof mat to tear and mortar
         in coping joints to crack, providing sources for water
         entry.

5.   Settlement:  

    -    As walls settle, extra strains may be exerted on
         flashings, or the roof may settle below the level of the
         drain pipe.  This will either cause a backup of flood
         water, or a leak through the crack around the drain.

6.   Outside Interference:  

    -    Roof mats are not designed for extra accessories such as
         signs and electric wires, nor are they intended for
         regular foot traffic.  Anchorage planks are spiked or
         lagged to the deck, piercing both the mat and the deck,
         causing serious damage.

    -    Pollutants, acids and saturated animal fats can
         potentially damage membrane roofing.  Protection from
         these are provided through coatings and/or ballast
         covering.

                             END OF SECTION