Roof assembly Documents

BSD-013: Rain Control in Buildings by John Straube — last modified 2008/08/20

Moisture is one of the most important agents leading to building enclosure deterioration. Understanding and predicting moisture movement within and through the enclosure is therefore of fundamental importance to predicting and improving building enclosure performance, particularly durability. Since driving rain deposition on walls and roofs is quantitatively the largest single source of moisture for most walls and roofs, it is no surprise that controlling rain penetration is one of the most important parts of a successful moisture control strategy. In fact, failure to control rain is likely the oldest and most common serious building enclosure performance problem. Commentators as long as Vitruvius (70 BC) bemoaned the challenges of controlling rain penetration. This document will consider rain control from a general to a specific level. The following sections will cover: basic moisture control principles that should be employed in the design of above-grade building enclosures; driving rain as a moisture load on walls; a classification system of the various rain control strategies available for walls; and finally, good design practises for walls. The rain control of roofs will be covered in more detail in another BSD.

BSD-018: The Building Enclosure by John Straube — last modified 2008/08/20

That part of any building that physically separates the exterior environment from the interior environment(s) is called the building enclosure or building envelope. Environmental separator is another term used to describe the enclosure, but note that this generic term also applies to separators of two different interior environments. The term building enclosure is preferred to the term building envelope largely because it is considered both more general and more precise. Also note that the building enclosure may contain, but is not the same as, the so-called thermal envelope, a term that is used to refer to the thermal insulation within the enclosure. The enclosure, the loadings it must resist, and its functions are addressed in this digest.

BSD-102: Understanding Attic Ventilation by Joseph Lstiburek — last modified 2008/08/20

Attics or roofs can be designed and constructed to be either vented or unvented in any hygro-thermal zone (Map 1). The choice of venting or not venting is a design and construction choice not a requirement determined by the physics or by the building code. The model building codes allow both vented and unvented roof assemblies. The applicable physics impacts the design of attic or roof systems as does the applicable building code but neither limit the choice.

BSD-105: Understanding Drainage Planes by Joseph Lstiburek — last modified 2008/08/20

Controlling rain is the single most important factor in the design and construction of durable buildings and in the control of mold. Drainage planes are used in the design and construction of building enclosures to control rain. All exterior claddings pass some rainwater. Siding leaks, brick leaks, stucco leaks, stone leaks, etc. As such, some control of this penetrating rainwater is required. In most walls, this penetrating rainwater is controlled by the drainage plane that directs the penetrating water downwards and outwards.

BSD-106: Understanding Vapor Barriers by Joseph Lstiburek — last modified 2008/08/21

The function of a vapor barrier is to retard the migration of water vapor. Where it is located in an assembly and its permeability is a function of climate, the characteristics of the materials that comprise the assembly and the interior conditions. Vapor barriers are not typically intended to retard the migration of air. That is the function of air barriers.

BSD-115: Wood Pitched Roof Construction by John Straube — last modified 2008/08/20

Pitched roofs of either wood rafter and joist or truss construction are used in the construction of literally millions of homes and small commercial buildings each year. There are variations in these roofs, but there are relatively few primary options. The following digest describes the most common types of wood pitched roofs, their enclosure functions, and common modes of failure.

BSD-119: Summer Condensation Problems in Ice Arenas by John Straube — last modified 2008/08/20

Ice rinks and arenas are a common building type in many communities. The trend over the last 25 years has been to operate these arenas for greater periods of the year, often throughout the summer. Also, an increasing number of such buildings are being built in areas with warm, humid summer weather. The result has been an increase in the number of reported moisture problems, most of which revolve around summer condensation. This digest will describe the causes and discuss potential retrofit solutions for summer condensation in ice arenas.

BSD-135: Ice Dams by John Straube — last modified 2008/08/20

Ice dams are a common roof performance problem in buildings that experience snowfall and at least a month of below freezing temperatures. The combination of sufficient roof pitch, adequate insulation just above the exterior wall, and air sealing at the wall-roof assemblies transition are all essential to prevent ice dams. But ice dams can occur even in properly detailed roof assemblies from differential solar snow melt. This digest outlines both the causes and solutions to ice dam problems.