On Wed, 12 Nov 2008 21:24:32 -0800, "Ivan Vegvary"
wrote:


"Paul Franklin" wrote in message
.. .
On Wed, 12 Nov 2008 11:21:30 -0800 (PST), Fotheringay-Phipps
wrote:

I have a question about the structure of my attic roof. As it stands
now, the roof beams are given some additional support from beams
(2x4)
that are parallel to the floor and which are nailed into the roof
beams on either side, about half-way between the attic floor and the
peak of the roof. IOW the roof is structured like an A, although the
crossbeams are about 1 every 4 linear feet (versus the roof beams
themsevels, which are 16" apart).

The purpose of these cross-beams is presumably to stop the weight of
the roof from pushing the roof down and the exterior walls of the
house apart.

However, I would like to make some more use of the attic space, and
these cross beams are in the way. So I'm casting about for another
method of achieving the same support in a less obstrusive manner.

Most specifically, I'm wondering if I can remove these beams and
instead nail vertical beams into the ceiling beams and into the floor
joists of the attic. So that instead of supporting the roof by
binding
the two halfs of the roof to each other, I would be supporting it by
propping up the roof against the 2x8s that form the attic floor.

Does this make sense? Does anyone know and/or is there some way for me
to determine this without hiring a structural engineer?

Well, you can hire one, or you can become one, or you can take a
chance. How lucky do you feel?

What you are describing sounds like collar ties. And you're right,
they are there to help resist the outward force on the walls caused by
the weight of the roof pressing outward on the top plate of the walls.
The roof doesn't just press down, it also presses out because of the
shape.

There are at least three common ways to resist the roof forces.

You can have a structural ridge beam. The ridge is the top peak of
the roof. With a structural ridge beam, a heavy, strong beam runs
down the ridge and is supported at the ends (and sometimes at one or
more points along the length) with strong enough structure to carry
the full load down to the foundation. With the ridge beam holding up
the center of the roof, the rafters no longer push out on the walls.

You can have a stick framed roof, with a thin ridge board, and rafters
running down to the top plate of the walls. With this, the attic
floor joists (ceiling joists of the floor below) run all the way from
the rafter ends on one side to the rafter end on the other side. These
joists essentially form a triangle with the two rafters and the joist
resists the outward force of the roof rafters. Sometimes it isn't
possible for the joists to run all the way from one rafter end to the
opposite rafter end. There may be an opening in the floor, or the
framing pattern doesn't allow this to be done. Collar ties can help
resist the outward load in this case. This is probably what you have,
based on your description.

The third common roof framing method is to use trusses. Doesn't sound
like you have them based on your description.

What you are proposing to do is to convert a stick framed roof to a
sort of structural ridge. It can be done, but not the way you are
proposing, and not without proper engineering. What you propose will
transfer roof load to the center of the floor joists. If there isn't
adequate supporting structure under the center of the joists, you
will create a dangerous situation.

This isn't the kind of project to take a chance on. The last thing
you want to hear is the sound of nails ripping out of lumber as your
roof collapses the first time it snows or you get a big windstorm.

HTH,

Paul F.

Paul, while I have seen plenty of layouts and diagrams, I have never seen a
better VERBAL description of roof framing than how you wrote it above. BTW
I am a licensed engineer. Your description was concise, to the point and
accurate.

Ivan Vegvary

Thanks Ivan, appreciate the kind words.