When I originally read your post, I assumed a form of construction
completely different. However it would definitely not a knee wall. It
appears to be a system of braced rafters (2x4's spanning 18' is way below
practice without bracing). I agree with marson and Bob that it might be
better to leave the supports alone. (Sorry for not seeing a better picture
of this.)

However, it still may be possible to add a closet space behind the supports;
but to do so would require the services of an engineer as you will need to
design a header and columns to offset the cut support(s). As long as the
closet door was kept small, 30 inches or less, the header would be framed
like any bearing wall carrying roof load & would most likely be minimal. But
it would have to be engineered as no model code has specific provisions for
this specific type of constriction (at least none I'm aware of) and it's
unlikely the local code official would by off on it with out a PE calc. An
additional consideration is whether the attic joists are heavy enough to
carry the added storage loading. Again, a local engineer would be able to
quickly analyze this for you.



· The rafters are 2X4s, without a grade stamp.
· The rafters are spaced 16" OC.
· The roof slopes at a 45° angle.
· The full span (from the outer walls to the roof point) is approx
18'.
· The current knee wall occurs in the middle of that span at approx
9'.
· Over the rafters (under the shingles) are planks that are 7 1/2"
wide and 3/4" thick.
· The planks run the length of the house (across the rafters).
· On top of the planks, looks to be wood shingles (??).
· Covering that layer, is likely a couple asphalt shingle jobs.

Hope this helps paint a better picture. Thanks for the input so far.