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Indoor airPLUS

Technical Guidance to the Indoor airPLUS Construction Specifications

1. Moisture Control

Sections 1.7 - 1.10: Water-Managed Roof Assemblies

1.7 Direct Roof Water Away from House

Direct roof water away from the house using gutters and downspouts that empty into lateral piping that deposits water on a sloping finish grade a minimum of 5 ft. from the foundation. Roofs designed without gutters are acceptable if they are designed to deposit rainwater to a grade-level rock bed with waterproof liner and drain pipe that deposits water on a sloping finish grade, as specified above. When lot space limits or prevents required grading, direct roof water to an underground catchment system (not connected to the foundation drain system) that deposits water a minimum of 10 ft. from the foundation. Rainwater-harvesting systems may be used to meet this requirement when they are designed to properly drain overflow, meeting discharge distance requirements above.

Exception: Dry climates, as shown in IECC Figure 301.1.

Detailed Illustrations

Click on the image for a full-page version

Lateral drainage from guttering

Lateral Drainage From Guttering

Installation of above-ground lateral drains from guttering

Installation of above-ground lateral drains from guttering

Alternative for depositing water from gutters/downspouts

If down spouts deposit rainwater next to the house, a wet foundation can result.

Underground catchment system

Underground catchment system

At-grade roof drainage system - alternative to gutters

Roof drainage installed below grade-alternative to gutters

At-grade drainage system empties to lateral piping beyond mulch bed - alternative to gutters

Integrating drainage pipes with landscaping beds

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Why

During heavy or prolonged rains, the large quantity of water that runs off roofs can saturate the soil around a home, sometimes resulting in wet basements and crawl spaces and mold and musty odors. In some cases, even structural damage may occur. However, a properly installed system of gutters, downspouts and lateral piping will direct roof water away from the foundation. Gutters are not necessary in dryer climates, where a substantial roof overhang will create a drip-line that will deposit occasional rainwater and melted snow away from the structure. A gentle slope to the finish grade next to the house is required by Moisture Control Section 1.1.

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How

Gutters and downspouts for residential applications are available in various sizes. The gutter channels are available in 4-, 5-, and 6-in. sizes (5-in. channels are popular, and larger, custom sizes are available) and are referred to as either “K” or “U” style based on their shape. The downspouts are either 2-by-3 or 3-by-4 in. The larger the components, the more water the system can carry. Homes in areas subject to tropical storms often need even larger capacity systems. The appropriate size is determined by (1) the number of inches of rain per hour expected from an intense downpour in the region where the house is located and (2) the area and pitch of the house’s roof. A gutter system’s capacity can be increased by using larger gutters and downspouts or by increasing the number of downspouts per length of gutter.

If downspouts deposit rainwater next to the house, a wet foundation can result. Two of the most frequent causes of wet basements are the lack of correct grading around the home (see Moisture Control Section 1.1) and gutter systems that deposit roof water too close to the foundation.

Although “splash blocks” are frequently placed under the outfall of a downspout to help prevent the stream of water from eroding the soil, their short length (typically 2 ft.) deposits the water close to the foundation. For more protection, the water from the downspout should be carried much farther from the house before it is deposited into the soil. For example, solid corrugated drainpipe (or similar pipe or open gutter) can be attached to the bottom of the downspout and sloped away from the house for at least 5 ft. If pipe is used, the lateral can be buried and directed to surface or sub-surface disposal. It can be laid directly on top of the sloping final grade (see Moisture Control Section 1.1) and covered with landscaping mulch to hide its location, with just the end protruding beyond the landscape bedding. Connectors are available that fit rectangular downspout pipe to 4-in. round corrugated pipe, and proprietary piping products for this application are available in the marketplace.

If the size of the yard is limited, the water can be piped to a swale that will carry it to an area where it can dissipate in the soil; a water garden provides an excellent deposit point. (Note: the more limited the yard space next to the house, the steeper the grade away from the house should be because the water must be moved away faster.) The water can also be piped to an underground catchment system such as a dry well. Rain-harvesting systems also can capture roof runoff from downspouts; however, barrels can fill quickly in a heavy rain so they should be designed so that once filled, the overflow is piped at least 5 ft. from the house.

Some designers prefer that a home not have gutters and downspouts. The design must still provide a means of preventing large quantities of roof water from being deposited at the foundation. One approach is to move the guttering to grade level; however, it must still be sloped to an outflow point at least 5 ft. from the foundation. First, excavate an area to a depth of at least 16 in. and extending out from the home’s foundation wall and sloping slightly to a point at least 1 ft. beyond the drip line of the eave. This wide, shallow trench should accommodate the roof's drip line at all points and slope slightly over its length toward the ultimate point of water discharge (most likely 5 ft. or more from one corner of the home). Then, cover the bottom of the excavated area with a layer of water-impervious sheeting, such as EPDM (ethylene propylene diene M-class rubber) roofing. Next, lay perforated corrugated drainpipe wrapped in filter cloth along the outermost edge of the trench and fill the excavated area with clean stone (preferably of a color to complement the landscaping or the home's facade). When approaching what will be the final grade, cover the area with a layer of filter cloth and cover the cloth with the final inches of stone to bring the area to the desired final grade elevation. The pipe should drain to daylight.

Areas that have little rainfall do not require gutter systems. Refer to the Map of Climate Zones, to determine “Dry Climates,” but also consult code officials when local/regional climate conditions might dictate the use of systems to collect and direct roof water.

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References/Additional Information

  • International Residential Code, R801.3, Roof Drainage. Get a copy of the Codes at www.iccsafe.org/ exiting epa.
  • BSP-040, READ THIS: Before You Design, Build or Renovate. Building Science Corporation, 2006. Get a copy at http://www.buildingscience.com/ exiting epa
  • Architectural Sheet Metal Manual, Sheet Metal and Air Conditioning Contractors' National Association, Inc., (SMACNA) Sixth Edition - Sept., 2003. See www.smacna.org/ exiting epa.

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