FAQ's
Stucco Home > FAQ's
> Building paper
Q: In a stucco frame-wall assembly, what purpose does building paper
serve?
A:Stucco is known to be a weather
resistant building finish, but it is part of a system. In order
for the wall to resist water penetration effectively, the system
must be properly designed and detailed, then built according to
plans.
The main purpose of building paper is to keep water from contacting
the substrate and structural support members—very commonly
sheathing like plywood or OSB (oriented strand board) and wood or
metal studs—so that these materials stay dry. Metal can rust
and wood can rot. Also, wood is prone to expand and contract with
changes in moisture, so it’s essential to keep sheathing dry
to provide the plaster with a sound substrate. Minimizing the changes
in moisture minimizes the stresses that might be placed on plaster
from behind. In addition to structural considerations, excess moisture
within a wall creates a potential for mold or mildew inside buildings.
Building paper prevents moisture-related problems in stucco walls.
Several industry documents, such as PCA’s Portland
Cement Plaster/Stucco Manual (EB049), ACI’s Guide
to Portland Cement-Based Plaster, and building codes across
the country, recommend 2 layers of paper. During construction, paper
can be damaged. Two layers of paper provide greater assurance that
water won’t get to the sheathing or support members. Paper
should be lapped like siding, meaning that upper layers are placed
over lower layers. This facilitates drainage toward the outside.
Where the edges of paper-backed lath meet, connections should be
lath-to-lath and paper-to-paper.
Building paper should comply with the current requirements of UU-B-790a,
Federal Specifications for Building Paper, Vegetable Fiber (Kraft,
Waterproofed, Water Repellent, and Fire Resistant). This specification
differentiates weather resistive Kraft papers by types, grades,
and styles. Grade D is a water-vapor permeable paper. Grade D paper
with a water resistance of 60 minutes (or more) works well for stucco
applications, and is often preferred to Grade D paper having the
minimum 10-minute resistance required by UU-B-790a.
Some specifiers are turning to house wraps for stucco underlayment.
While these materials may be more rugged than paper—and therefore
less prone to damage during installation—a single layer is
still not adequate according to many industry professionals. At
best, a hybrid system, with the house wrap closest to the sheathing
and covered with the paper, seems to be an acceptable alternative.
Back to Stucco FAQs
|
 |
|