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Building a flat roof with joists running across the slope

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Building a flat roof with joists running across the slope public 10-02-2006
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Posted by marson on October 2, 2006, 7:06 pm



there should be no problem with joists running across the slope. i
would fasten a plate on top of the block with concrete anchors, toenail
the joists to this, and run a row of blocking above the plate. if you
are concerned about a tornado coming along and taking the roof off, you
can add some sort of hurricane ties from the joist to the plate.


Posted by Kickstart on October 2, 2006, 7:28 pm
> there should be no problem with joists running across the slope. i
> would fasten a plate on top of the block with concrete anchors, toenail
> the joists to this, and run a row of blocking above the plate. if you
> are concerned about a tornado coming along and taking the roof off, you
> can add some sort of hurricane ties from the joist to the plate.
>

If your concerned about tornados you should have a tornado shelter,
If one hits directly, anchors and tie downs wont help anyway.
Use your span table for spacing and sizing


kickstart



Posted by DT on October 3, 2006, 9:29 am
public@towner.org.uk says...
>
>Hi all,
>
>I have a block-built outbuilding, 3m x 5.5m, which has a flat roof with
>a 5 degree slope running down its length. The current roof is
>corrugated asbestos cement, supported on joists running across the
>shorter span, at about 1m centres. The joists are embedded in narrow
>slots in the inner face of the wall.
>
>I want to replace the roof with a solid roof. Everything I've read so
>far details the construction of such a roof based upon the joists
>running down the length of the slope, rather than across the slope. Now
>for my building, having the joists running that way would mean I need
>5.5m lengths of 3"x9". If I ran the joists the other way (as they are
>currently) I could use smaller joists (2"x6"), or greater centres,
>either of which makes the whole construction cheaper.
>
>The difficulty I see with running the joists across the slope, rather
>than down its length, is securing them. I can't reuse the existing
>joist slots - they are too narrow, and widely spaced. Instead, I would
>have to position the joist on top of the walls (actually quite
>desirable, since this gives me more headroom inside the building). If I
>attached the joists directly to the top of the wall, then they would be
>off vertical by 5 degrees, so I would have to position blocks of timber
>to fit into the gaps between the joists to provide sideways support.
>Alternatively, I could run a timber bar along the top of the wall, and
>then cut vertical notches in that timber to support the joists in an
>upright position. I would then have to cut the top off each joist, to
>provide a 5 degree slope to which I could attach the plyboard deck.
>
>Is building a roof with the joists running across the slope feasible,
>or should I conform to the practice of running the joists down the
>length of the slope?
>
>thanks,
>
>dan.
>


Posted by DT on October 3, 2006, 9:47 am

>The difficulty I see with running the joists across the slope, rather
>than down its length, is securing them. I can't reuse the existing
>joist slots - they are too narrow, and widely spaced. Instead, I would
>have to position the joist on top of the walls (actually quite
>desirable, since this gives me more headroom inside the building). If I
>attached the joists directly to the top of the wall, then they would be
>off vertical by 5 degrees, so I would have to position blocks of timber
>to fit into the gaps between the joists to provide sideways support.
>Alternatively, I could run a timber bar along the top of the wall, and
>then cut vertical notches in that timber to support the joists in an
>upright position. I would then have to cut the top off each joist, to
>provide a 5 degree slope to which I could attach the plyboard deck.


Sorry about the blank reply :-)

I would run a header (what you call a timber bar) along the tops of each wall.
This would normally be a doubled joist, if you are using 2 x 6 rafters, then
the header would be (2) 2 x 8's. It is fully supported, so you could get away
with 2 x 6's, but I like to use a larger size. If you want a higher ceiling,
use a wider header, 2 x 10's or whatever.

Layout your rafters on 16" centers. Attach them to the header at each end with
metal joist hangers set vertically, there is no need to notch anything. You can
indeed bevel the top edge of each rafter to 5 degrees, this is a nice touch and
the correct way to do it, although I suspect most people would just leave them
and support the roof on the corner of each rafter. Bevel the rafters first,
before you position the hangers and put things together.

Fasten your roof panels to the rafter being sure that all ends of the roof
panels fall directly over the center of a rafter, so each panel is supported.
Note that the first rafter spacing will have to be less than 16", so the roof
panels will start at the very edge of the roof and end up on the center of a
rafter.

--
Dennis


Posted by DT on October 3, 2006, 9:59 am
One more thing - when I mentioned the rafters on 16" centers, that is because
it is standard for construction in the US with 48" x 96" x 1/2" thick plywood
or OSB sheets. If your plywood panels are metric lengths, obviously you need to
set the rafter spacing to accomodate your panels. If your panels are thicker
than 1/2" (13mm) then you may be able to use wider spacing on the rafters.
Check with the building center where you buy your materials for customary
spacing in your area.

--
Dennis


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