I know aircrete coursing blocks should not be used for loadbearing,
and probably not as the first course on top of a lintel, but is there
any reason they should not be used directly under a roof plate (2x4
the rafters join onto) ?
Thanks,
Simon.

sm_jamieson wrote:
I know aircrete coursing blocks should not be used for loadbearing,
and probably not as the first course on top of a lintel, but is there
any reason they should not be used directly under a roof plate (2x4
the rafters join onto) ?
Thanks,
Simon.

Simon,

If the blocks are not classed as "load bearing" then that is a very good
reason for not sticking around 20 tons of roof on top of them (by the way,
there is a distinct lack of information here as to the type of roof that you
are talking about) - and it's that weight the wallplate transfers to the
wall.

Cash

Cash wrote:
sm_jamieson wrote:
I know aircrete coursing blocks should not be used for loadbearing,
and probably not as the first course on top of a lintel, but is there
any reason they should not be used directly under a roof plate (2x4
the rafters join onto) ?
Thanks,
Simon.

Simon,

If the blocks are not classed as "load bearing" then that is a very
good reason for not sticking around 20 tons of roof on top of them
(by the way, there is a distinct lack of information here as to the
type of roof that you are talking about) - and it's that weight the
wallplate transfers to the wall.

Cash

TWENTY tons? - what roofs have you seen that got to even half that weight?

To the OP: - it's the crush value that matters here, not load bearing (the
entire weight of the roof is bearing down on the blocks anyway as that is
what the walls are built with!) - you can use normal bricks, either 1,2 or 3
courses at the top of the internal walls to sit the wallplate on (this
normally requires a course of block being knocked off if you've already
reached ceiling height)

--
Phil L
RSRL Tipster Of The Year 2008

Phil L wrote:
Cash wrote:
sm_jamieson wrote:
I know aircrete coursing blocks should not be used for loadbearing,
and probably not as the first course on top of a lintel, but is
there any reason they should not be used directly under a roof
plate (2x4 the rafters join onto) ?
Thanks,
Simon.

Simon,

If the blocks are not classed as "load bearing" then that is a very
good reason for not sticking around 20 tons of roof on top of them
(by the way, there is a distinct lack of information here as to the
type of roof that you are talking about) - and it's that weight the
wallplate transfers to the wall.

Cash

TWENTY tons? - what roofs have you seen that got to even half that
weight?
To the OP: - it's the crush value that matters here, not load bearing
(the entire weight of the roof is bearing down on the blocks anyway
as that is what the walls are built with!) - you can use normal
bricks, either 1,2 or 3 courses at the top of the internal walls to
sit the wallplate on (this normally requires a course of block being
knocked off if you've already reached ceiling height)

Stick on a heavy, wet snow loading, and 20 tons is very obtainable on a
roof!

Crush value and load bearing are in the same context here - the weight of
the roof is transferred to the inner skin via the wall plate - and then
straight down to the foundations in a compression (crushing) load - and this
can cause problems with low, or non-load bearing materials, particularly in
buildings of more than 1 storey.

Now if Simon will tell us whether he is taking about a 'main' roof (and it's
construction [which will also have bearing on its weight]) or a simple flat
or lean-to roof of an extension, then perhaps a definitive answer can be
given.

BTW, with regards to your first paragraph, there are roofs that weigh around
the 20 ton mark - usually on older houses using traditional materials.

Cash

Cash wrote:

Stick on a heavy, wet snow loading, and 20 tons is very obtainable on
a roof!

Crush value and load bearing are in the same context here - the
weight of the roof is transferred to the inner skin via the wall
plate - and then straight down to the foundations in a compression
(crushing) load - and this can cause problems with low, or non-load
bearing materials, particularly in buildings of more than 1 storey.

Now if Simon will tell us whether he is taking about a 'main' roof
(and it's construction [which will also have bearing on its weight])
or a simple flat or lean-to roof of an extension, then perhaps a
definitive answer can be given.

It's a single storey extension, he's been building it at his own house


BTW, with regards to your first paragraph, there are roofs that weigh
around the 20 ton mark - usually on older houses using traditional
materials.
Cash

I find that impossible to accept, unless you are talking about huge mansions
or stately homes?

An average 3 bed semi takes about 500 tiles (slates are much lighter, but
people don't usually have these nowadays) and depending upon which tile you
choose, they weigh in at about 3.5 to 4.5 kilos each, but to work things out
easier, lets say they are 5kg apiece.
Felt will weigh approx 18kg a roll and it will take about 3 rolls
3kg of nails
Timber for a main roof (remember the purlins are built into the wall and are
not resting on any wallplates and are therefore not part of the roof) is
normally around 7X2 and say 5m long - these will weigh in at around 40kg
each and there will be about 15 on the front and 15 at the back, not to
mention a ridge board @ say 15kg.
Lead flashing is quite heavy though and a normal roof will take a few 25kg
rolls and not forgetting the wall plates themselves, say 2 X 6m lengths of
4X2 @ about 15kg each, so lets recap:

Tiles = 2500
felt = 36
nails = 3
main timbers = 1200
ridge board = 15
lead = 50
wallplates = 30

Total is 3384kg, or 3.3 tonnes, even if you round it up to 4000kg for
sundries like ridge tiles, velux windows and roof battens, it's still well
short of 20 tonnes.

And if you are talking about a 5 or 6 bedroom detatched house roof, there is
still nowhere near double the amount of materials as mentioned aove and even
if there were, it would still only get to 8 tonnes, but all this is academic
anyway as roof trusses weigh very little and a new build roof even on a 5
bed det would be closer to 3 tonnes overall weight, and I can't remember
ever seeing 17 tonnes of snow on a roof


--
Phil L
RSRL Tipster Of The Year 2008

On 21 May, 21:58, "Cash"
wrote:
Phil L wrote:
Cash wrote:
sm_jamieson wrote:
I know aircrete coursing blocks should not be used for loadbearing,
and probably not as the first course on top of a lintel, but is
there any reason they should not be used directly under a roof
plate (2x4 the rafters join onto) ?
Thanks,
Simon.

Simon,

If the blocks are not classed as "load bearing" then that is a very
good reason for not sticking around 20 tons of roof on top of them
(by the way, there is a distinct lack of information here as to the
type of roof that you are talking about) - and it's that weight the
wallplate transfers to the wall.

Cash

TWENTY tons? - what roofs have you seen that got to even half that
weight?
To the OP: - it's the crush value that matters here, not load bearing
(the entire weight of the roof is bearing down on the blocks anyway
as that is what the walls are built with!) - you can use normal
bricks, either 1,2 or 3 courses at the top of the internal walls to
sit the wallplate on (this normally requires a course of block being
knocked off if you've already reached ceiling height)

Stick on a heavy, wet snow loading, and 20 tons is very obtainable on a
roof!

Crush value and load bearing are in the same context here - the weight of
the roof is transferred to the inner skin via the wall plate - and then
straight down to the foundations in a compression (crushing) load - and this
can cause problems with low, or non-load bearing materials, particularly in
buildings of more than 1 storey.

Now if Simon will tell us whether he is taking about a 'main' roof (and it's
construction [which will also have bearing on its weight]) or a simple flat
or lean-to roof of an extension, then perhaps a definitive answer can be
given.

BTW, with regards to your first paragraph, there are roofs that weigh around
the 20 ton mark - usually on older houses using traditional materials.

Cash

Folks, I'm not talking about crush strength for uniformly distributed
loads, more for concentrated loads under lintels etc. I read somewhere
(can't find it now !) that if you need to use aircrete coursing
blocks, dont put them on the course on which is bearing a lintel,
joist hanger etc. Thats what I meant by "load bearing" in this
situation - some type of concentrated load.
I guess the reason is that coursing blocks fairly easily snap, since
for aircrete, its strength is in its homogeneous nature. Just like a
very thin sliver of brick should not bear point loads, neither should
a moderately thin piece of aircrete. If the mortar bed under a
coursing block is slightly difficient, a point-type load on it could
cause it to snap, but a whole block would never snap in that
situation.

If you look at buldings in construction around the place, you often
see a row of coursing blocks below the top row of whole blocks at
lintel height for standard height windows and doors. But you don't see
the coursing blocks above the whole blocks.
That's what lead me to believe that what I read was correct.

With a roof, the roof plate on top of the blocks will even out the
load, so I don't think there would be a problem, but the question is
what is good practice ?

Simon.

wrote:

Folks, I'm not talking about crush strength for uniformly distributed
loads, more for concentrated loads under lintels etc. I read somewhere
(can't find it now !) that if you need to use aircrete coursing
blocks, dont put them on the course on which is bearing a lintel,
joist hanger etc. Thats what I meant by "load bearing" in this
situation - some type of concentrated load.
I guess the reason is that coursing blocks fairly easily snap, since
for aircrete, its strength is in its homogeneous nature. Just like a
very thin sliver of brick should not bear point loads, neither should
a moderately thin piece of aircrete. If the mortar bed under a
coursing block is slightly difficient, a point-type load on it could
cause it to snap, but a whole block would never snap in that
situation.

If you look at buldings in construction around the place, you often
see a row of coursing blocks below the top row of whole blocks at
lintel height for standard height windows and doors. But you don't see
the coursing blocks above the whole blocks.
That's what lead me to believe that what I read was correct.

With a roof, the roof plate on top of the blocks will even out the
load, so I don't think there would be a problem, but the question is
what is good practice ?

Simon.

What do you mean by 'coursing' blocks? - are you talking about the smaller
blocks used to get heights correct? - these are usually cut on site by the
brickie, or more often his labourer, and no, they should not be used under
lintels, what you are probably seeing are in fact concrete bricks, which
have a higher N rating than blocks at around 20N, wheras aircrete are 2.9N.

--
Phil L
RSRL Tipster Of The Year 2008

On 21 May, 23:29, "Phil L" wrote:
wrote:
Folks, I'm not talking about crush strength for uniformly distributed
loads, more for concentrated loads under lintels etc. I read somewhere
(can't find it now !) that if you need to use aircrete coursing
blocks, dont put them on the course on which is bearing a lintel,
joist hanger etc. Thats what I meant by "load bearing" in this
situation - some type of concentrated load.
I guess the reason is that coursing blocks fairly easily snap, since
for aircrete, its strength is in its homogeneous nature. Just like a
very thin sliver of brick should not bear point loads, neither should
a moderately thin piece of aircrete. If the mortar bed under a
coursing block is slightly difficient, a point-type load on it could
cause it to snap, but a whole block would never snap in that
situation.

If you look at buldings in construction around the place, you often
see a row of coursing blocks below the top row of whole blocks at
lintel height for standard height windows and doors. But you don't see
the coursing blocks above the whole blocks.
That's what lead me to believe that what I read was correct.

With a roof, the roof plate on top of the blocks will even out the
load, so I don't think there would be a problem, but the question is
what is good practice ?

Simon.

What do you mean by 'coursing' blocks? - are you talking about the smaller
blocks used to get heights correct? - these are usually cut on site by the
brickie, or more often his labourer, and no, they should not be used under
lintels, what you are probably seeing are in fact concrete bricks, which
have a higher N rating than blocks at around 20N, wheras aircrete are 2.9N.

Coursing blocks are what you say. I cut a whole lot when I needed
them, but you can buy them ready cut. I am talking only about
aircrete. Cavity wall lintels are usually sat on aircrete one side,
brick t'other. But the question is: why is it OK to sit a lintel on a
whole aircrete block, but not on an aircrete coursing block ?
I agree aircrete is often not sufficient to carry a heavy beam without
a padstone, but cavity wall lintels do not usually need a padstone.
Cheers,
Simon.

Cash wrote:
Phil L wrote:
Cash wrote:
sm_jamieson wrote:
I know aircrete coursing blocks should not be used for loadbearing,
and probably not as the first course on top of a lintel, but is
there any reason they should not be used directly under a roof
plate (2x4 the rafters join onto) ?
Thanks,
Simon.
Simon,

If the blocks are not classed as "load bearing" then that is a very
good reason for not sticking around 20 tons of roof on top of them
(by the way, there is a distinct lack of information here as to the
type of roof that you are talking about) - and it's that weight the
wallplate transfers to the wall.

Cash
TWENTY tons? - what roofs have you seen that got to even half that
weight?
To the OP: - it's the crush value that matters here, not load bearing
(the entire weight of the roof is bearing down on the blocks anyway
as that is what the walls are built with!) - you can use normal
bricks, either 1,2 or 3 courses at the top of the internal walls to
sit the wallplate on (this normally requires a course of block being
knocked off if you've already reached ceiling height)

Stick on a heavy, wet snow loading, and 20 tons is very obtainable on a
roof!

He's not wrong. 20 tins is not that much really.

Crush value and load bearing are in the same context here - the weight of
the roof is transferred to the inner skin via the wall plate - and then
straight down to the foundations in a compression (crushing) load - and this
can cause problems with low, or non-load bearing materials, particularly in
buildings of more than 1 storey.

Now if Simon will tell us whether he is taking about a 'main' roof (and it's
construction [which will also have bearing on its weight]) or a simple flat
or lean-to roof of an extension, then perhaps a definitive answer can be
given.

BTW, with regards to your first paragraph, there are roofs that weigh around
the 20 ton mark - usually on older houses using traditional materials.

He's right there too.

Cash