Why Monsoon Storms Find Weak Spots
Albuquerque doesn't get much rain overall, but monsoon storms deliver it fast. The National Weather Service puts the Albuquerque area's average rainfall for the June 15–September 30 monsoon season at under 5 inches. Individual storms can drop a big share of that at once: NWS has recorded rates of 3 to 4 inches per hour in eastern Albuquerque and more than 2 inches in an hour at a weather station in 2025. Thunderstorm downbursts can reach 50 to 70 mph.
A roof can handle light rain for years with a flaw that only shows when that much water arrives at once, or when wind drives it sideways. As one New Mexico roofer told the Santa Fe New Mexican, some leaks only show up in "a really big storm, or a three-day rain."
Flat Roofs: When Water Rises Above the Flashing
Overwhelmed scuppers and canales. On a parapet roof, water leaves through scuppers and canales. When rain arrives faster than they can drain, or when they're partly blocked with leaves or debris, water builds up against the parapet. JLC has described how a dammed scupper holds water even on a sloped roof. Our canales and flat roof drainage guides cover sizing and clogs.
Low flashing. Once water pools, it can rise above the top of the base flashing at walls and curbs. Industry guidance commonly calls for base flashing at least 8 inches above the roof surface, and Building Enclosure notes standing water is harder on flashings than on the membrane. Older roofs, and roofs built up over the years, often have flashings much lower than that.
Overflow protection. For parapet roofs where water could be trapped, the building code calls for secondary overflow scuppers or drains (IBC 1502.2). Roof consultants typically set overflows about 2 inches above the low point. Many older homes don't have them.
Parapet tops and stucco cracks. Wind-driven rain hits the parapet's outside face and top. Cracks there let water into the wall, and it can show up inside as a roof leak. See why parapet walls leak.
Pitched Roofs: When Wind Drives Rain Under
Unsealed or lifted shingles. Shingles are held down by a sealant strip that needs heat to bond. GAF notes shingles that aren't properly bonded are vulnerable to wind uplift and water infiltration, and its hand-sealing guidance includes wind-blown dust and sand areas. A tab that lifts in gusts lets rain in under it.
Vents and soffits. IBHS testing found wind-driven rain can enter through open soffits and gable-end vents, and the FORTIFIED program notes sideways rain can be blown up under roof overhangs.
Flashing at walls, chimneys and skylights. Flashing that sheds normal rain can be overrun when wind pushes water up and sideways. A missing kick-out where a roof meets a wall is a common example. See where a roof meets a stucco wall, why chimney flashing leaks and skylight leaks.
Low-slope sections. Shingles on slopes from 2:12 up to 4:12 need double underlayment under the code. Without it, water driven under the shingles has less to stop it. See roof underlayment.
Shingle wind ratings (ASTM D3161 and D7158) measure resistance to blow-off once the sealant has bonded. They aren't rain-penetration ratings. A wind-rated shingle can still leak if it never sealed or the flashing is wrong.
When It Isn't the Roof
Wind-driven rain gets into walls and windows too, and the water can travel. The Fenestration and Glazing Industry Alliance notes extreme storms can exceed a window's water-penetration rating, and water can show up well away from where it enters. Common non-roof sources:
- Windows and doors on the side facing the storm.
- Stucco cracks and wall penetrations.
- Swamp cooler overflow or a clogged cooler drain. See swamp cooler roof leaks.
If the stain is near a window or only appears on the side of the house facing the wind, start there.
How These Leaks Are Found
- Note the pattern. Which storms, which direction the wind came from, and where the water showed up.
- Look in the attic during or right after rain. InterNACHI calls the attic a great place to find evidence of roof leaks and recommends a moisture meter to tell fresh leaks from old stains.
- Inspect where water collects. Scuppers, canales, parapet bases, valleys, walls and penetrations.
- Controlled water testing. Wetting one area at a time while watching inside narrows the source, as forensic investigators describe. Windows can be tested to ASTM E1105.
- Moisture surveys on flat roofs. Infrared and impedance scans can map wet insulation under a membrane.
A Free Roof Report includes scuppers, canales, flashing and every penetration. For active leaks, see roof leak repair.