Chimney service in Delridge, Seattle

Shannon & Wilson's landslide study for the city describes West Seattle as two glacier-shaped ridges running north and south, split by Longfellow Creek, which it calls one of Seattle's longest and lowest-gradient streams. The historical society names what that geography brought: landslide potential, periodic flooding, ravines and creeks, the reasons it gives for Delridge's slow development. It also credits a decade-long city effort with restoring the creek and adding a trail along it.

The study is specific about the eastern side of the valley. Two dozen slides are on record for the hillside rising east of Delridge Way SW toward 21st Avenue SW, the earliest trouble noted in 1914, and names Pigeon Point among the city's chronic slide areas. Beneath that hillside, the engineers describe deep, loose silty clay resting on firmer clay, wet most of the year.

Citywide, the engineers judged that water had a hand in almost all the slides on file. For chimneys that means two things: water attacking the stack from above through the long wet season, and ground that can shift beneath it after a soaked winter. Both show up first as small cracks, which is why we look closely at the masonry and the plumb of every stack we inspect in the valley.

World War II brought thousands of defense workers to the Harbor Island plants, and by 1945, HistoryLink writes, the peninsula had twice its prewar headcount, upward of 70,000 people. Delridge's share was a stopgap project of 450 homes for those workers, put up in 1943 and turned over to low-income tenants by 1953, according to HistoryLink's history of the Seattle Housing Authority. The historical society remembers temporary war-worker housing crowding onto playfields and vacant lots, and a high rate of home ownership through the 1950s that later gave way to more renting.

Present-day Census tallies for the valley's ZIP area, a zone that spills south beyond the city line, show the layers: about 13 percent of units built before 1940, 21 percent from 1940 to 1959, and nearly 32 percent in 2000 or later, so the middle house dates from 1981, later than in any other part of West Seattle. Masonry flues from the first half of the century, prefabricated fireplaces and metal chimneys from its second half, and gas units from the recent wave may sit within a few blocks of each other, so we identify the system before we recommend anything.

After the 2001 Nisqually earthquake, Booth, Wells and Givler surveyed about 60,000 chimneys block by block across the greater Seattle area and found 1,556 visibly damaged. Their 2004 paper, published through the USGS, says the damage was strongly clustered, with West Seattle the leading cluster, where damage had also been mapped after 1965. What they found runs against intuition for anyone on a wet lot. Neither epicentral distance, nor soft soil, nor the lay of the land or the way a slope faced predicted the broken chimneys; the worst blocks traced strands of the Seattle fault zone, and a block or two past each strand's end the damage nearly vanished.

So a valley lot is not automatically at greater risk in an earthquake, and a ridge lot is not automatically safe. None of the sources we read places Delridge inside or outside the damage zone. What is clear is how to repair a damaged brick chimney. A 2015 FEMA recovery advisory cautions that putting the same brick column back exactly as before leaves it exposed to the same failure; its preferred paths swap the upper masonry for a metal flue, or replace firebox and chimney altogether. For new or rebuilt masonry chimneys in the D seismic design categories, Seattle's residential code calls for four continuous No. 4 vertical bars and steel straps anchoring the chimney at floor, ceiling and roof lines more than 6 feet above grade.