plinth

1. Prevent leaking of water into foundation -
To prevent a house from settling it is
critical that no water be leaked into
foundation. Being made of cement, plinth
beam is impervious and therefore prevents
water from rain, flooding, etc. from leaking
into foundation.
2. Provides support for walls - For load
bearing houses, walls carry the weight of
the house. Plinth beam provides solid
footing to raise those walls.
3. Holds house together - Since plinth beam
sits across the periphery of the house, it
provides binding force across rooms. For
example, without plinth beam it is
possible for one of the rooms to settle
thus creating uneven floor level. Plinth
beam prevents that from happening.
Basic process to construct plinth beam is as
follows:
1. Mark-up width - Usually width of plinth is
half that of the foundation. In this case,
foundation is about 18" wide and so
plinth is 9" aligning with the outer edge of
the foundation. Inner 9" of foundation
meshes into the floor level of the house.
2. Lay down the steel beam - As the
adjacent picture shows beams are the
core of the plinth beam.  Beams have a
loop every 6" that holds it together.
3. Setup re-inforcement - Before concrete is
poured, re-inforcement has to be
established to provide rectangular shape
to the beam. This takes majority of effort
as planks have to be nailed properly in
place and once concrete is poured they
need to be removed.
4. Pour the concrete - Next step is to pour
the concrete. As concrete is poured,
mason ensures that it is evenly spread
and smoothens out any edges. He also
needs to ensure the height of the beam is
consistent throughout the periphery.
5. Remove re-inforcement - Concrete turns
solid within 24-hour and final step is to
remove the planks leaving the beam
intact.
This short video explains this process in
action.
Foundation Redone:
To give background context, ground level at
living room area is 1' higher than that at the
guest bedroom level. Typical height of plinth
level is 2.5' that includes 1' of plinth beam
and 1.5' of foundation masonry. Turns out
though that foundation masonry was 2.5' at
living room and 3.5' at guest bedroom. What
this meant was plinth height would be 3.5'
from natural ground level. This was realized
only when plinth beam casting was about to
start. To add to misery, by then Rainwater
tank was already done to align with 3.5' of
plinth !
My architect was clear that plinth level can
be no taller than 2.5' or else house would
look place oddly high. What followed was no
fun - We ended-up undoing a few days of
work. Specifically
Reduce Rainwater tank height
by 1' by chipping away at
just-finished concrete tank
Remove 1-layer of foundation
stone masonry from across
the house periphery to reduce
the foundation height by 1'.
Here is the clip of work being undone. It
could have been worse had plinth beam been
built. Good news was we caught it at a time
that cost us about a week and some cost in
material and labor. Having said that, there
were key lessons learnt -
1. Trust your gut instinct - Even though as
homeowners we may not be in
construction business, you have a feel for
what's going on. I certainly did feel that
foundation was looking tall. While "plinth"
was a new term and I didn't know that it
was 1' in height I concluded I knew no
better. That was the critical mistake. No
question is a dumb question especially
when you are new to the domain. Had I
had surfaced my instinct more strongly or
did some research things could have been
different.
2. Working drawing consultation - I
independently found my architect and
contractor. So they had no prior
relationship between them. When architect
released drawings there was no
consultation that occurred between the
two. So, architect didn't get a chance to
explain the nitty-gritty and contractor felt
drawings were clear-enough. A big no-
no ! It is critical that drawings are
released to contractor only after architect
has had a chance to explain them to the
contractor. It is best to have such
meetings at the site to relate paper
drawings to the actual mapping onsite.
Often times, what looks right on paper
doesn't feel right to the eye. If that's not
possible, have them meet at architect's
office.
3. Inspection schedule - Identify stages at
which architect would come and inspect
the site to validate it is built according to
the specifications. In this case, architect
mentioned a couple of times need to
schedule inspection. However, given that
this was first or second inspection we
were not as diligent as needed.
Again, in retrospect, it was not a terrible
setback.. Like many other things in life, none
of the lessons were new; rather they were re-
inforcement of what you would learn at many
other times. Lessons learnt were critical and
have since been put in practice. There is now
tighter collaboration between me or my wife,
architect and the contractor and crisper
conversation about respective point of view.
So, in the end it definitely had a silver lining !