Hi,

Thanks for the previous help with cement bag sizes, that bit is in
place now.

Q1. The beams are to support a light weight corrugated flat roof of
internal size 8' x 15'. Beam spacing will be a little under every 2
foot with the 5 beams running the length of the 15 foot with two sets
of bracing noggins at 5' in from each ends. Unsupported span is 15
feet.

Ends will be firmly fixed down to structure and protruding through
walls.

Will 5" x 2.5" (actual sawn size) be adequate?

Q2. What is a good cost effective alternative to asbestos cement
corrugated roof panels?





--

Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

Harry Bloomfield wrote:
Hi,

Thanks for the previous help with cement bag sizes, that bit is in
place now.

Q1. The beams are to support a light weight corrugated flat roof of
internal size 8' x 15'. Beam spacing will be a little under every 2
foot with the 5 beams running the length of the 15 foot with two sets
of bracing noggins at 5' in from each ends. Unsupported span is 15
feet.

Ends will be firmly fixed down to structure and protruding through
walls.

Will 5" x 2.5" (actual sawn size) be adequate?


15ft unsupported span is far too long, the beams will sag, is there no
possibility of going the opposite way and using 8ft beams?....failing that,
I fear it will require a purlin in the centre, possibly steel.

Q2. What is a good cost effective alternative to asbestos cement
corrugated roof panels?

OSB boards, overlaid with torch-on roof felt

Harry Bloomfield wrote:
Q2. What is a good cost effective alternative to asbestos cement
corrugated roof panels?

Corrugated iron over Sterling board. You could use Onduline
or similar, but... well, I really wouldn't.

The message
from Harry Bloomfield contains these words:

Q1. The beams are to support a light weight corrugated flat roof of
internal size 8' x 15'. Beam spacing will be a little under every 2
foot with the 5 beams running the length of the 15 foot with two sets
of bracing noggins at 5' in from each ends. Unsupported span is 15
feet.

Ends will be firmly fixed down to structure and protruding through
walls.

Will 5" x 2.5" (actual sawn size) be adequate?

Probably. If it was a floor you would want 9" x 2" joists to span 15'
but you don't want to load it with anything more than wind and your own
weight as you fix it down.

Q2. What is a good cost effective alternative to asbestos cement
corrugated roof panels?

I think you will find that modern corrugated sheet does not contain asbestos.

Plastic covered steel is an alternative. I can't remember what it is
called but it has a section something like (IIRC)
_____ ______
_/ \_____/ \_

and is a doddle to fit.

You don't say which way the roof slopes but the beam layout suggests it
is down the short dimension. However should you want it to slope down
the long dimension the steel sheets can be ordered in in quite long
lengths, I think in excess of 15'.

--
Roger Chapman

Harry Bloomfield wrote:

Hi,

Thanks for the previous help with cement bag sizes, that bit is in
place now.

Q1. The beams are to support a light weight corrugated flat roof of
internal size 8' x 15'. Beam spacing will be a little under every 2
foot with the 5 beams running the length of the 15 foot with two sets
of bracing noggins at 5' in from each ends. Unsupported span is 15
feet.

Ends will be firmly fixed down to structure and protruding through
walls.

Will 5" x 2.5" (actual sawn size) be adequate?

Q2. What is a good cost effective alternative to asbestos cement
corrugated roof panels?

sagulator will give you a figure for how much sag you can expect under
any given load.
http://www.woodbin.com/calcs/sagulator.htm
Then you can decide of the amount of sag will look ok. Over 15' it
would have to sag a foot or so before breaking.

You dont say whether this needs to be BR compliant, if so there are
different rules to follow.

Dont forget to calculate for snow loading, roofs must survive that ok.


NT

"Harry Bloomfield" wrote in message
...

Q1. The beams are to support a light weight corrugated flat roof of
internal size 8' x 15'. Beam spacing will be a little under every 2
foot with the 5 beams running the length of the 15 foot with two sets
of bracing noggins at 5' in from each ends. Unsupported span is 15
feet.

Ends will be firmly fixed down to structure and protruding through
walls.

Will 5" x 2.5" (actual sawn size) be adequate?

I've just built a garage with 16ft span, using 8x2 at 4ft centres. My
gut feeling is that 5" is a bit light, and the joists will sag visibly
under their own weight. Bearing in mind that deflection is inversely
proportional to the cube of the depth, even a modest increase in depth
makes a big difference (e.g. 5" deflects 4 times as much as 8" for the
same total load). If you decide to do proper calcs, the deflection and
maximum stress are the easy bits - deciding on the wind load is the
tricky bit.


Q2. What is a good cost effective alternative to asbestos cement
corrugated roof panels?

Box section steel sheet is about 6 quid a square metre, and is available
in long lengths, so you could get away with no end laps at all. Minimum
suggested pitch is 5 degrees iirc, or about 1 in 12. Fibre cement
sheets are only available in shorter lengths ( 3m ?) and really need a
steeper pitch, particularly if they're laid such that there are end
laps, despite the practice of makers of cheap concrete garages.

The problem with steel sheets is that the insulation value is minimal,
and you'll get condensation dripping off the underside in some
conditions. The design of insulated roofs with vapour barriers etc is
quite a complicated business: my compromise was to deck the roof with
secondhand 18mm ply then cover it in polythene, then clad in steel.

The SLE Cladding site is helpful.


--
Kevin Poole
**Use current month and year to reply (e.g. )***

Autolycus wrote:
"Harry Bloomfield" wrote in message
...

Q2. What is a good cost effective alternative to asbestos cement
corrugated roof panels?

Box section steel sheet is about 6 quid a square metre, and is available
in long lengths, so you could get away with no end laps at all. Minimum
suggested pitch is 5 degrees iirc, or about 1 in 12. Fibre cement

I'm not sure what you refer to here, can you give us a link or
soemthing?

thanks, NT

wrote in message
ups.com...
Autolycus wrote:
"Harry Bloomfield" wrote in message
...

Q2. What is a good cost effective alternative to asbestos cement
corrugated roof panels?

Box section steel sheet is about 6 quid a square metre, and is
available
in long lengths, so you could get away with no end laps at all.
Minimum
suggested pitch is 5 degrees iirc, or about 1 in 12. Fibre cement

I'm not sure what you refer to here, can you give us a link or
soemthing?



http://coruspanelsandprofiles.co.uk/

then

Products and services Roofs Site assembled Standard roof profiles
View Profiles R32-1000

takes you (I hope) to some basic data on one of the most common
profiles. I was wrong: it's 4 degrees (1 in 15). Note that it's also
often found in 0.5mm thickness, which imho is a bit thin unless you
support in continuously, as I did.


--
Kevin Poole
**Use current month and year to reply (e.g. )***

Autolycus wrote:
wrote in message
ups.com...
Autolycus wrote:
"Harry Bloomfield" wrote in message
...

Q2. What is a good cost effective alternative to asbestos cement
corrugated roof panels?

Box section steel sheet is about 6 quid a square metre, and is
available
in long lengths, so you could get away with no end laps at all.
Minimum
suggested pitch is 5 degrees iirc, or about 1 in 12. Fibre cement

I'm not sure what you refer to here, can you give us a link or
soemthing?



http://coruspanelsandprofiles.co.uk/

then

Products and services Roofs Site assembled Standard roof profiles
View Profiles R32-1000

takes you (I hope) to some basic data on one of the most common
profiles. I was wrong: it's 4 degrees (1 in 15). Note that it's also
often found in 0.5mm thickness, which imho is a bit thin unless you
support in continuously, as I did.

thanks Kevin. What a horrible website.


NT

Harry Bloomfield wrote:
Hi,


Q1. The beams are to support a light weight corrugated flat roof of
internal size 8' x 15'. Beam spacing will be a little under every 2
foot with the 5 beams running the length of the 15 foot with two sets
of bracing noggins at 5' in from each ends. Unsupported span is 15
feet.

Depending on the time and trouble you want to take, you might want to
use a structure like designed by the mathematician John Wallis

"Wallis tackled a practical problem - how to provide a flat structure
such as a roof to span a large square open space, using only wooden
beams much shorter than the required span and only supported around the
edges. He devised a repeating pattern of short but interlocking beams,
which could be extended to span any sized space."

See URL:http://www.soue.org.uk/souenews/issue4/wallis.html

As it turned out, this design wasn't used for the internal roof of the
Sheldonian Theatre, Oxford - Wren used a different technique - a
composite roof truss.

Cheers,

Sid.

wrote:
Harry Bloomfield wrote:
Hi,


Q1. The beams are to support a light weight corrugated flat roof of
internal size 8' x 15'. Beam spacing will be a little under every 2
foot with the 5 beams running the length of the 15 foot with two sets
of bracing noggins at 5' in from each ends. Unsupported span is 15
feet.

Depending on the time and trouble you want to take, you might want to
use a structure like designed by the mathematician John Wallis

"Wallis tackled a practical problem - how to provide a flat structure
such as a roof to span a large square open space, using only wooden
beams much shorter than the required span and only supported around the
edges. He devised a repeating pattern of short but interlocking beams,
which could be extended to span any sized space."

See URL:http://www.soue.org.uk/souenews/issue4/wallis.html

As it turned out, this design wasn't used for the internal roof of the
Sheldonian Theatre, Oxford - Wren used a different technique - a
composite roof truss.

Actually - you might not - further investigation here shows:

"However, if the roof structure had been used, it may not have stood
the test of time. Every wooden beam in the structure was vital because
it supported the weight of other wooden beams. If one beam failed, the
whole structure would collapse. So if woodworm nibbled away, or a
lightning strike or earthquake damaged one of the beams, the roof would
fail. Most other roof structures can withstand a bit of a battering,
but in John Wallis's design every beam was doing a vital job and none
of them could afford to be damaged."

from URL:http://www.noisenet.ws/pages/m_carticle2.php?cid=9&aid=216

wrote:
wrote:
Harry Bloomfield wrote:
Hi,


Q1. The beams are to support a light weight corrugated flat roof of
internal size 8' x 15'. Beam spacing will be a little under every 2
foot with the 5 beams running the length of the 15 foot with two sets
of bracing noggins at 5' in from each ends. Unsupported span is 15
feet.

Depending on the time and trouble you want to take, you might want to
use a structure like designed by the mathematician John Wallis

"Wallis tackled a practical problem - how to provide a flat structure
such as a roof to span a large square open space, using only wooden
beams much shorter than the required span and only supported around the
edges. He devised a repeating pattern of short but interlocking beams,
which could be extended to span any sized space."

See URL:http://www.soue.org.uk/souenews/issue4/wallis.html

As it turned out, this design wasn't used for the internal roof of the
Sheldonian Theatre, Oxford - Wren used a different technique - a
composite roof truss.

Actually - you might not - further investigation here shows:

"However, if the roof structure had been used, it may not have stood
the test of time. Every wooden beam in the structure was vital because
it supported the weight of other wooden beams. If one beam failed, the
whole structure would collapse. So if woodworm nibbled away, or a
lightning strike or earthquake damaged one of the beams, the roof would
fail. Most other roof structures can withstand a bit of a battering,
but in John Wallis's design every beam was doing a vital job and none
of them could afford to be damaged."

from URL:http://www.noisenet.ws/pages/m_carticle2.php?cid=9&aid=216

Perhaps one could use 2 layers of this structure, with woodwork bolted
together at each crossing of top and bottom layer.

NT