On 17/11/2014 11:30, Tim Streater wrote:
In article , John
Rumm wrote:

On 17/11/2014 08:18, ARW wrote:
On Saturday I arrived at work to discover this at the customers house

http://wiki.diyfaq.org.uk/index.php?...:Avaulted1.jpg

Yep he has removed the ceiling joists and the back wall.

'kinell, lucky the whole lot did not just splay out at the top and
dump the whole roof into the room under it!

Use the acro in reverse - just needs a couple of volunters to hold it
iin place until it bites

Nice recovery...

http://wiki.diyfaq.org.uk/index.php?...:Avaulted7.jpg

Screw in a few timbers to hold it in place - including a ceiling joist
and it looks OK.

http://wiki.diyfaq.org.uk/index.php?...:Avaulted8.jpg

Dunno what their final solution will be.

Would be worth sticking that quick overview into the wiki to go with
the photos. Nicely illustrates what the "floor/ceiling" joists do in
traditional roof framing.

And, for constructional know-nothings like me, it would be useful to
have a brief overview of how things would be done differently, if
there'd been no extension there and one wanted to build what the DIYer
had been trying to convert it to, i.e., an extension with a vaulted
ceiling.

The main issue is one of "thrust". Nail a couple of bits of wood
together to form an apex, and then add weight to the top (tiles etc),
you will tend to flatten out the apex - pushing the free ends apart (and
if you are unlucky, pushing the walls over into the bargain). Normally a
roof truss or framed roof will have cross members that will act in
tension to resist the spread of the base. So even if the apex joint is a
relatively weak nailed one, it all stays together since the apex joint
is only subject to compression rather than rotational moments.

I'm curious since the house I owned in California had a vaulted ceiling
main living-room.

If you want a vaulted ceiling, then you need to deal with the thrust -
either eliminate it or cope with it.

One way is to modify the ceiling structure to eliminate the thrust -
either with some tie beams across (don't need them on every rafter, and
they can be quite high up making an A frame).

You could use a ring frame of some form around the perimeter at the
base. This works well for hipped vaulted ceilings with 4 sloping sides.
The counters the thrust by placing the 4 hip rafters into compression.

Alternatively with a gable wall at each end you can stick a heavy ridge
beam in to transfer the loads from the tops of all the rafters onto the
game walls. The loads from the bottom end of the rafters is then carried
by the side walls.

All those will eliminate the thrust. An alternative is to design the
supporting structure to be able to cope with the outward thrust, such
that it can supply the restraint to the roof's thrust. (so perpendicular
walls, or other buttress arrangements for the side walls)



--
Cheers,

John.

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