Tree Roots and Foundation
In his answer to the question about whether tree roots can dry
out the soil beneath a foundation enough to cause the concrete
to crack (Q&A, 7/09), Bill Palmer refers to
expansive soils. But in fact all nonexpansive clayey soils will
also shrink as tree roots “suck” the moisture out
of the soil — a process known as desiccation. When
desiccation occurs with roots that extend beneath the bottom of
the footing, the foundation will indeed settle, usually
resulting in cracking.
When we investigate a settlement problem in an area known to
have clayey soils, trees are the first things we look for. If a
tree canopy extends over the roof or is even near the wall line
in the area that is settling, then dessication is the most
likely cause, since the tree’s root system has about the
same diameter as the tree canopy. If the settlement is not too
severe, it can be stopped by removing the tree, which is the
least costly solution. If the tree is not removed, settlement
will continue until the foundation will ultimately require
costly underpinning with hydraulic or helical piles.
Desiccation can be confirmed with soil testing. In some cases,
the geotechnical engineer might be able to predict how much
additional settlement will occur based on the moisture content
of the clay. It should be noted that it is not possible to
water the grass and hope that water will be reintroduced into
nonexpansive clay — clay is relatively impervious and
will not accept water by gravity alone.
Stuart Jacobson, P.E., S.E.
In “Reroofing With Asphalt Shingles” (7/09), the
author says he uses a single-layer self-adhering membrane for
underlayment on roofs having a roof pitch between 2/12 and
4/12. But the IRC — section R905.2.2, which applies to
asphalt roofs — says that a double layer of underlayment
is required for slopes between 2/12 and 4/12.
Blue Springs, Mo.
A page later, IRC section 905.2.7, which addresses ice
barriers where there is a probability of ice dams, allows for a
self-adhering polymer modified bitumen sheet — a membrane
like the one referred to in the article — to be used
instead of two layers of felt cemented together. According to
Andrew Visser, a technical rep for W.R. Grace, maker of Grace
Ice & Water Shield, “If one layer of a self-adhered
underlayment is good enough for the ice-dam areas, then by
extension a single layer is sufficient as the underlayment for
the entire roof.” — The Editors
Don’t Mix Plastics
The article “Condensing Storage Water Heaters”
(6/09) was interesting and informative, but I was appalled that
professional plumbers would glue ABS pipe to PVC because they
ran out of ABS. I’m not a fan of ABS-to-PVC transition
cement because it’s significantly weaker than either ABS
or PVC cement. With the expansion rate of ABS 25 percent higher
than that of PVC, there’s a good chance the joint will
eventually break and leak, due to the heating and cooling cycle
every time the hot water heater runs. I’ve seen this
several times in home inspections and service calls on waste
Jim Nordstrom, P.E.
Overland Park, Kan.
Site-Built Arched Trusses
Who makes the truss system used to create the enclosure in the
article “Roofing Under Cover” (Backfill,
Niels Kampmann, Architect
The temporary roof framing was designed by the GC and
assembled on site. Here’s another view (above). —