Your ending question hit the nail on the head. It does NOT matter
what you call the structural member but as the definition says,
whether it impedes heat flow. If the top chord is deep enough to be
readily identified as obstructed when it is a joist then it IS
obstructed construction.
The root cause of our problem is that there is nothing that
quantifies how deep is too deep for unobstructed. I attempted to get
this straightened out in either 99 or 02 edition and the TC response
was that it is defined in the Annex. When I pointed out with a
comment what the Annex quantities and it isn't depth DOWN from the
ceiling the TC still failed to comprehend. You can lead a horse to
water but you can't make it drink (DROWNING it is an option though).
The annex identifies 4 inches deep as part of what differentiates a
joist from a beam (rightly called thickness and width in this
section) and a bar joist from a truss (called depth in this section)
but it is the distance ACROSS the structural member. Nothing
discusses distance down from the ceiling.
Few would call a 2 x 6 joist unobstructed but what about a 2 x 4
stringer? AS a REASONABLE point of demarcation, I use the 2x4.
Looking at the figures in the annex of a Truss, it looks like a 2x4.
We know some amount of protrusion is acceptable and a 2x6 joist is
generally accepted as obstruction so WALLAH, a 2x4 is unobstructed
and anything beyond it REGARDLESS of the name of the member is
obstructed (assuming the spacing is as defined of course).
Being a member of 13 and discussing such an ambiguous issue, I have
to state the NFPA caveat that this is not to be considered a formal
interpretation of the NFPA or its technical committees .
Roland
On Jun 21, 2007, at 9:59 AM, Bob Knight wrote:
I have a situation in an attic space that is using a standard wood
truss w/
2x6 top cords, but in addition to this there is another 2x6
sistered to the
top cord so that the top cord is now 11" deep. Per 13, truss
construction
is considered unobstructed, but I'm having a difficult time
considering this
unobstructed. This is a dry system. If this is unobstructed
construction,
then the deflector is to be 1" to 12" down which leaves only 1" of
play for
sloping the branch lines. At a 1/2" per 10' there is not enough
play to
provide proper slopeage. Can this be considered obstructed
construction due
to the depth of the double top cord?
As a side question, I've never understood why a truss with 2x4 or
2x6 top
cord is considered unobstructed when a joist that is a 2x4 or 2x6 is
considered obstructed. What's the difference?
Thank you,
Bob Knight, CET
(208) 495-2057
[EMAIL PROTECTED]
www.firebyknight.com
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