Roofing Code: Section 1507- Roof Assemblies

1507.2.8.2, Ice Dam Membrane

In areas where there has been a history of ice forming along the eaves causing a backup of water, a membrane that consists of at least two layers of underlayment cemented together or of a self-adhering polymer modified bitumen sheet shall be used in lieu of normal underlayment and extend from the eave’s edge to a point at least 24 inches (610 mm) inside the exterior wall line of the building.

Exception: Detached accessory structures that contain no conditioned floor area.

Code Interpretation

This section implies that additional underlayment is required the first 24 inches (610 mm) up slope from the roof edge. This type of material is typically referred to as eaves flashing or Ice and Water Shield. The code allows the use of two adhered underlayments (15 pound base sheet) or a single layer of manufactured Ice and Water Shield.

The roof eave is defined as the horizontal, lower edge of a sloped roof. Eaves flashing is an additional layer of roofing material applied at the eaves to help prevent damage from water backup.

Proper eaves flashing application can be completed by applying manufactured eaves flashing material to deck a minimum of 24 inches up slope. Most eaves flashing materials are self-adhered sheets that are attached to deck after removal of release film (polyethylene). Application in high-velocity wind zones may require fastener attachment. Check local codes and the manufacturer’s installation requirements prior to application.

Eaves flashing can be manufactured in the field through the use of two layers of 15 pound base sheet. In these circumstances the bottom layer of felt is attached to the deck with appropriate nails and nail pattern. The top layer of felt is set in a continuous and even application of roof adhesive (plastic cement).

WaterproofingCode: Section 1807- Dampproofing and Waterproofing

1807.4, Subsoil Drainage System

Where a hydrostatic pressure condition does not exist, dampproofing shall be provided and a base shall be installed under the floor and a drain installed around the foundation perimeter. A subsoil drainage system designed and constructed in accordance with Section 1807.1.3 shall be deemed adequate for lowering the ground water table.

Code Interpretation

This section implies that a subsoil drainage system is required in instances where hydrostatic pressure does not exist. The subsoil drain shall be installed by applying a base layer under the floor and installing a drain around the foundation perimeter.

Proper below-grade waterproofing design must include a system for collecting, draining, and discharging groundwater away from the structure. The most effective way to properly collect and discharge groundwater is through the use of foundation drains. Foundation drains can be field-constructed drainage systems or prefabricated soil drainage systems.

Field-constructed drainage systems consist of a perforated pipe (typically PVC) that is set in a bed of gravel at the bottom of the foundation. The perforation in the pipe is applied downward to allow the water to flow into the gravel bed. A drainpipe is installed next to the structure slightly above the bottom of the foundation to prevent the soil under the foundation from washing away. The pipe is set to slope the water towards drain fields, bare soil or sump pits. A layer of coarse gravel is set around the drainage pipe for additional water accumulation. In some cases, meshes and/or mats can be applied over the top gravel layer to prevent soil build-up from interfering with water flow to the drainage system. The biggest disadvantage with these systems is that they rely on proper field construction and over time they may become clogged with dirt, soil and contaminants.

It is the designer’s responsibility to review the site engineering analysis for water table conditions and soil analysis to determine if and what type of dampproofing or waterproofing is required.

The section also implies that the ground-water table must be lowered by implementing a drainage system that is designed to meet the following engineering principles:

1. Permeability of the soil.
2. Rate at which water enters the drainage system.
3. Rated capacity of the pumps.
4. Head against which pumps are to operate.
5. Rate capacity of the designed area of the system.