Manual of Low Slope Roof Systems

Question 1

Which is more susceptible to moisture, Extruded or Expanded polystyrene insulation?

Answer 1

Expanded polystyrene, it is highly vulnerable in its resistance to moisture. Especially in lower densities whereas extruded polystyrene retains about 90% of its “dry” thermal resistance after 400 days of wetting test, the lowest density expanded polystyrene retains only 25%.

Question 2

What is the proper way to install a vapor retarder on a steel deck?

Answer 2

First Install a 1/2 in. leveling board and then 2-ply bituminous vapor retarder. Insulation over that and the the roofing membrane.

Question 3

How to you decide whether or not to use a vapor retarder?

Answer 3

As a basis for deciding whether to use a vapor retarder, calculate the location of the dew point and the rate of vapor migration under the wost winter condition (see example at end of chapter). If vapor-retarder location is below depot point, do not specify it. Chapter 6.

Question 4

Is it advised to use a vapor retarder on a roof destined for future penetrations?

Answer 4

No, a vapor retarder on a roof destined for future penetrations is highly vulnerable to damage and likely to fail. Chapter 6.

Question 5

True or False.

If in doubt, specify a vapor retarder.

Answer 5

False. If in doubt do not specify a vapor retarder. Use a vapor retarder only if a study of your conditions indicates a positive need for it. (See fig. 6.6 for critical interior RH as a guideline). Chapter 6.

Question 6

True or False.

If unhumidified exterior air is drawn into the building, a vapor retarder is is required.

Answer 6

False.

If unhumidified exterior air is drawn into the building, a vapor retarder is normally not required, unless the interior relative humidity is 60 percent or more. Chapter 6.

Question 7

True or false.

Never specify a vapor retarder between a poured structural deck and poured insulating concrete fill. (it will seal in moisture).

Answer 7

True. Chapter 6.

Question 8

True or False.

Do not specify a vapor retarder in the roof over an unheated interior.

Answer 8

True. Chapter 6.

Question 9

True or False.

Require flashing and enveloping of a vapor retarder at all roof penetrations.

Answer 9

True. Chapter 6.

Question 10

What should you do prior to mechanically fastening vapor retarders?

Answer 10

Check the vapor retarder’s ability to take nailing or other anchorage punctures and still maintain a satisfactory perm rating. Chapter 6.

Question 11

True or False.

It is important to always check a vapor retarder for its effect on the roof assembly’s fire rating.

Answer 11

True.

It is important to always check a vapor retarder for its effect on the roof assembly’s fire rating (see Chapter 8, Fire Resistance). Chapter 6.

Question 12

What should you do when a vapor retarder is hot-mopped to a deck.

Answer 12

For a vapor retarder hot-mopped to a deck, specify ASTM Type III (steep) asphalt or Type IV (special steep). Specify rosin-sized paper, felt, or tape over joints in wood sheathing, plywood, or other prefabricated units to be nailed. Chapter 6.

Question 13

True or False.

You can use a vapor retarder as a temporary roof.

Answer 13

False. Chapter 6.

Question 14

What could prohibit installation of light, flexible plastic vapor retarders?

Answer 14

Windy conditions. Chapter 6.

Question 15

Why was the Roofing Industry Committee on Weather Issues (RICOWI) created in November 1989?

Answer 15

To promote and coordinate research.

To accelerate the promulgation of new consensus standards for wind design and testing.

(And most importantly), to educate the building industry about wind problems. Chapter 7.

Question 16

Tall parapet walls have what effect on wind uplift?

Answer 16

Tall parapets can drastically reduce win-uplift pressure. A 5ft parapet can cut the uplift coefficient by a factor of nearly 3. Chapter 7.

Question 17

Low parapet walls have what effect on wind uplift?

Answer 17

Low parapets in the 18 inch range, may increase uplift pressures. Chapter 7.

Question 18

In regards to Fire Resistance, class A roof coverings are?

Answer 18

Effective against SEVERE fire exposures. Chapter 8.

Question 19

In regards to Fire Resistance, class B roof coverings are?

Answer 19

Effective against MODERATE fire exposure. Chapter 8

Question 20

In regards to Fire Resistance, class C roof coverings are?

Answer 20

Effective against LIGHT fire exposure. Chapter 8.

Question 21

Type I Fire Resistance is?

Answer 21

Noncombustible, protected. Chapter 8.

Question 22

Type II Fire Resistance is?

Answer 22

Noncombustible, protected and unprotected. Chapter 8.

Question 23

Type III Fire Resistance is?

Answer 23

Noncombustible, exterior walls, interior building elements of any permitted material. Chapter 8.

Question 24

Type IV Fire Resistance is?

Answer 24

Noncombustible exterior walls, interior building elements of heavy timber. Chapter 8.

Question 25

Type V Fire Resistance is?

Answer 25

Combustible, i.e., any materials permitted by the code. Chapter 8.

Question 26

What size void is required to create a blister?

Answer 26

Generally it takes a 3/4 inch void (size of a dime) for a blister to originate.

The void needed to originate a blister can be smaller if it contains liquid moisture than if it contain only an atmospheric air-water-vapor mixture. Chapter 10.

Question 27

A “foaming crater” suggest what type of BUR failure?

Answer 27

A foaming crater (1/8 in or so in diameter) provides circumstantial evidence that liquid moisture was present during the felt-laying operation. Chapter 10.

Question 28

A “Shining bitumen surface” suggests what type of BUR failure?

Answer 28

A shiny bitumen surface may (but does not necessarily) indicate a lack of interply mopping cohesion dating from the original application. Chapter 10.

Question 29

What does so called “legs, or stalactites and stalagmites suggest in a BUR?

Answer 29

So-called stalactites and stalagmites usually form in alligator-ed pattern when the adjacent felts are pulled apart. These legs reveal the heated bitumen’s resistance to the internal blister pressure prying adjacent felts apart (or prying a bottom felt off its substrate) Rising membrane temperature increases the blisters internal pressure and simultaneously reduces the bitumen’s tensile strength. Chapter 10.

Question 30

A “bare, uncoated felt spot” suggest what type of BUR failure?

Answer 30

A bare, uncoated felt spot often marks the original blister void. A radial stair like uncoated felt pattern emanating from the original void indicates the sudden boiling of liquid moisture in the bare spot when it was hit by 300 to 400 degree F asphalt. Chapter 10.

Question 31

What is the most common cause of BUR membrane splitting?

Answer 31

Movement of unanchored insulation boards. Chapter 10.

Question 32

Insulation must be installed so that the ends are at least _____ inches from the flutes.

Answer 32

1.5 inches. Chapter 10.

Question 33

What is slippage?

Answer 33

Slippage can be defined as relative lateral movement between felt plies. A membrane plagued by slippage often assumes a randomly wrinkled appearance, like a badly laid carpet. Chapter 10.