November 2006
Search    

Vented base sheets | Manual Elements

Manual Elements not ratednot ratednot ratednot ratednot rated

Vented base sheets

by James R. Kirby, AIA
Be the first to comment


An important but less commonly used detail in the roofing industry is for proper installation of base flashing with a new lightweight insulating concrete (LWIC) or poured gypsum roof deck. The NRCA Roofing and Waterproofing Manual, Fifth Edition, provides a detail for installation of a built-up roof system's base flashing where a wall meets an LWIC or poured gypsum deck in BUR-5A, "Base flashing for vented base sheet."

The unique and critical element of the detail is proper installation of the vented base sheet horizontally under the membrane and vertically behind the base flashing. The purpose of the vented base sheet is to facilitate venting and drying of residual moisture vapor (generally not liquid moisture) from the hydration process of an LWIC or poured gypsum roof deck. A roof assembly that incorporates LWIC or poured gypsum needs to be detailed and constructed to allow moisture to travel horizontally in the roof's field and vertically at base flashings; therefore, a vented base sheet cannot be fully adhered to a deck. NRCA recommends a vented base sheet be mechanically attached to an LWIC or poured gypsum roof deck with suitable fasteners.

Even though NRCA recommends LWIC and poured gypsum decks be vented from the underside—through slotted metal form decks or permeable form boards—edge details that allow venting should be installed to allow top side venting of excess moisture. The primary method of venting an LWIC or poured gypsum deck is to provide ventilation from the underside. Therefore, adequate interior ventilation should be provided below a deck to remove excess moisture from an LWIC or poured gypsum deck. Lateral and vertical venting become even more critical when LWIC is installed over nonvented substrates, such as structural concrete.

Detail BUR-5A from The NRCA Roofing and Waterproofing...



To read the article in its entirety, please log in or register (registration is free).

Log in or register for FREE access to this article and other Professional Roofing online content.

Not a professionalroofing.net user?

Register now for free access
  • Full access to every article
  • Online Web exclusive information
  • Photo gallery
  • Breaking news
  • Online classified ads

Already a professionalroofing.net user?

Log in now

User name:

Password:

 


Login help
Click here to have your user name and password emailed to you.




Comments (0) Login to post a comment or rating
There are no comments posted.

NRCA NRCA