Inspection Policy for Foundation Drain sand Damp Proofing
Design, Construction and Development Community
Aaron Bowman – Acting Chief Building Official
February 15, 2024
The City of Missoula will be updating its inspection policy for foundation drains and damp proofing on March 1, 2024.
To meet State requirements, the City of Missoula is implementing the following inspection protocols: Foundation Drains and Damp Proofing.
Foundation Drain Inspection
Foundation Drain IRC requirements
IRC Section 405 Foundation Drainage. The 2021 version of the code, including diagrams and tables, can be found here: https://up.codes/viewer/montana/irc-2021/chapter/4/foundations#R405
Foundation Drain IBC requirements:
IBC Section 1805.4.2 Foundation Drain. The 2021 Version of the code can be found here:https://up.codes/viewer/montana/ibc-2021/chapter/18/soils-and-foundations#1805.4.2
The code states: A drain shall be placed around the perimeter of a foundation that consists of gravel or crushed stone containing not more than 10-percent material that passes through a No. 4 (4.75 mm) sieve. The drain shall extend not less than 12 inches (305 mm) beyond the outside edge of the footing. The thickness shall be such that the bottom of the drain is not higher than the bottom of the base under the floor, and that the top of the drain is not less than 6 inches (152 mm) above the top of the footing. The top of the drain shall be covered with an approved filter membrane material. Where a drain tile or perforated pipe is used, the invert of the pipe or tile shall not be higher than the floor elevation. The top of joints or the top of perforations shall be protected with an approved filter membrane material. The pipe or tile shall be placed on not less than 2 inches (51 mm) of gravel or crushed stone complying with Section 1805.4.1 and shall be covered with not less than 6 inches (152 mm) of the same material.
When to schedule a Foundation Drain Inspection
Foundation Drain inspection shall be scheduled and completed after the drain is completed and ready for backfill and prior to any backfill operations. Foundations drains are NOT required in all areas of the City. Foundation drains need only be installed in areas known to have expansive soils or when specified on plans.
Foundation Drain inspections may be scheduled at the same time as the Damp Proofing inspection. Should you have any questions about the requirements of foundation drains, please reach out to a Missoula Building Inspector or Plans Examiner.
Missing or uninspected foundation drains will require excavation of the foundation, as determined by the Building Inspector.
Damp Proofing Inspection
Damp Proofing IRC requirements:
IRC Section 406. The 2021 version of the code can be found here: https://up.codes/viewer/montana/irc-2021/chapter/4/foundations#R406
R406.1 Concrete and masonry foundation damp proofing.
Except where required by Section R406.2 to be waterproofed, foundation walls that retain earth and enclose interior spaces and floors below grade shall be dampproofed from the finished grade to the higher of the top of the footing or 6 inches (152 mm) below the top of the basement floor. Masonry walls shall have not less than 3/8-inch (9.5 mm) Portland cement parging applied to the exterior of the wall. The parging shall be dampproofed in accordance with one of the following:
1. Bituminous coating.
2. Three pounds per square yard (1.63 kg/m2) of acrylic modified cement.
3. One-eighth (1/8) inch (3.2 mm) coat of surface-bonding cement complying with ASTM C887.
4. Any material permitted for waterproofing in Section R406.2.
5. Other approved methods or materials.
Exception: Parging of unit masonry walls is not required where a material is approved for direct application to the masonry.
Concrete walls shall be damp proofed by applying any one of the listed damp proofing materials or any one of the waterproofing materials listed in Section R406.2 to the exterior of the wall.
Damp proofing IBC Requirements:
IBC Section 1805.2 Damp Proofing. The 2021 Version of the code can be found here:https://up.codes/viewer/montana/ibc-2021/chapter/18/soils-and-foundations#1805.2
1805.2 Damp proofing.
Where hydrostatic pressure will not occur as determined by Section 1803.5.4, floors and walls for other than wood foundation systems shall be damp proofed in accordance with this section. Wood foundation systems shall be constructed in accordance with AWC PWF.
1805.2.1 Floors.
Damp proofing materials for floors shall be installed between the floor and the base course required by Section 1805.4.1, except where a separate floor is provided above a concrete slab.
Where installed beneath the slab, damp proofing shall consist of not less than 6-mil (0.006 inch; 0.152 mm) polyethylene with joints lapped not less than 6 inches (152 mm), or other approved methods or materials. Where permitted to be installed on top of the slab, damp proofing shall consist of mopped-on bitumen, not less than 4-mil (0.004 inch; 0.102 mm) polyethylene, or other approved methods or materials. Joints in the membrane shall be lapped and sealed in accordance with the manufacturer’s installation instructions.
When to schedule a Damp Proofing Inspection:
Damp Proofing inspection shall be scheduled and completed after the damp proofing procedures are completed and ready for backfill and prior to any backfill operations.
Damp Proofing inspections may be scheduled at the same time as the Foundation Drain inspection. Should you have any questions about the requirements of foundation drains, please reach out to a Missoula Building Inspector or Plans Examiner.
Missing or uninspected damp proofing will require excavation of the foundation, as determined by the building inspector.
Guidance and regulations for installations of Drainage and Damp Proofing systems provided in this memo for reference and are not all inclusive. The installing contractor is responsible for adherence to the currently adopted code at time of installation.
Questions pertaining to the procedures shall be directed to the City of Missoula Development Services Reception at 406-552-6630.
