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Posted by on October 2, 2006, 9:01 am
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.
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Posted by longshot on October 2, 2006, 9:10 am
in steel construction the joists often run across the slope.
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Posted by Kickstart on October 2, 2006, 10:48 am
> 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.
>
you can run across the slope,
I'm interested in your measurements though, your obviously on the metric
system but you refer to wood sizes in inches.
Is this common practice through out the building industry in all metric
orientated countries also ?
kickstart
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Posted by on October 2, 2006, 11:12 am
> you can run across the slope,
> I'm interested in your measurements though, your obviously on the metric
> system but you refer to wood sizes in inches.
> Is this common practice through out the building industry in all metric
> orientated countries also ?
I don't really think of 2x3 as meaning 2"x3" - its just a convenient
nickname for a particular size. I learnt about these things from my
Dad and other older members of my family, and they all use the
`nicknames' to refer to the different sizes, so its easier when talking
to them about 2x4, 2x3, and so on. I would always use the metric size
for buying, using, measuring, etc.
Are there any references on the web about building with joists across
the slope?
dan.
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Posted by Mike on October 2, 2006, 2:38 pm
>
>> 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.
>>
> you can run across the slope,
> I'm interested in your measurements though, your obviously on the metric
> system but you refer to wood sizes in inches.
> Is this common practice through out the building industry in all metric
> orientated countries also ?
>
> kickstart
In Canada we use metric, however wood is sold via 2x4,2x3,2x6 etc and most
construction workers still use inches and feet as this is what they were
taught.
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