Garage is being converted to workshop. Internal size is 2.7 x 5.3 M.
Joist dimensions 100 x 50 mm x 2800mm span @ 600mm centres, which
are too small for that span but it's been there for 45yrs with very
little deflection approx 1 cm measured.

So as not to disturb the integrity of the felting and therefore the
waterproofing I want to externally insulate the felt covered roof by
using "Jablite inverted roof" insulation:
http://www.jablite.co.uk/fri/index.php

Unfortunately, to hold the insulation down you need to cover it in 50mm
of gravel which is heavy.
The roof area is approx 15 Sq M, the volume of gravel would be approx
0.75 Cu M which I think, when wet would weigh in the region of 1500Kgms
(distributed load) or approx 100Kgms /Sq M.
Obviously I would need to beef up the joists and I thought to double up
on the joists bolting through a steel flitch plate in between the joists
at say 600mm intervals. Has Anyone tried to do this in situ?

The steel plates , say 100mm x 6mm x 2.7M, and second joists would
not rest on the supporting walls and would be approx 100mm shorter than
existing joists so lateral support would be needed at both ends and
probably in the middle.
Would this be strong enough?

Can anyone help with structural information? and or practical experience
which they can pass on please.

Regards
Don

On 1 Aug, 15:11, Donwill wrote:
Garage is being converted to workshop. Internal size is 2.7 x 5.3 M.
Joist dimensions 100 x 50 mm x 2800mm span @ 600mm centres, which
are too small for that span but it's been there for 45yrs with very
little deflection approx 1 cm measured.

So as not to disturb the integrity of the felting and therefore the
waterproofing I want to externally insulate the felt covered roof by
using "Jablite inverted roof" insulation:
http://www.jablite.co.uk/fri/index.php

Unfortunately, to hold the insulation down you need to cover it in 50mm
of gravel which is heavy.
The roof area is approx 15 Sq M, the volume of gravel would be approx
0.75 Cu M which I think, when wet would weigh in the region of 1500Kgms
(distributed load) or approx 100Kgms /Sq M.
Obviously I would need to beef up the joists and I thought to double up
on the joists bolting through a steel flitch plate in between the joists
at say 600mm intervals. Has Anyone tried to do this in situ?

The steel plates , say 100mm x 6mm x 2.7M, and second joists would
not rest on the supporting walls and would be approx 100mm shorter than
existing joists so lateral support would be needed at both ends and
probably in the middle.
Would this be strong enough?

Can anyone help with structural information? and or practical experience
which they can pass on please.


can you not just insulate inside and save yourself all the agro and
expense??

Cheers

Jim K

On 01/08/2010 15:34, Jim K wrote:
On 1 Aug, 15:11, wrote:

Garage is being converted to workshop. Internal size is 2.7 x 5.3 M.
Joist dimensions 100 x 50 mm x 2800mm span @ 600mm centres, which
are too small for that span but it's been there for 45yrs with very
little deflection approx 1 cm measured.

So as not to disturb the integrity of the felting and therefore the
waterproofing I want to externally insulate the felt covered roof by
using "Jablite inverted roof" insulation:
http://www.jablite.co.uk/fri/index.php

Unfortunately, to hold the insulation down you need to cover it in 50mm
of gravel which is heavy.
The roof area is approx 15 Sq M, the volume of gravel would be approx
0.75 Cu M which I think, when wet would weigh in the region of 1500Kgms
(distributed load) or approx 100Kgms /Sq M.
Obviously I would need to beef up the joists and I thought to double up
on the joists bolting through a steel flitch plate in between the joists
at say 600mm intervals. Has Anyone tried to do this in situ?

The steel plates , say 100mm x 6mm x 2.7M, and second joists would
not rest on the supporting walls and would be approx 100mm shorter than
existing joists so lateral support would be needed at both ends and
probably in the middle.
Would this be strong enough?

Can anyone help with structural information? and or practical experience
which they can pass on please.


can you not just insulate inside and save yourself all the agro and
expense??

Cheers

Jim K

Fair point, I happen to like a warm roof insulation system to avoid
condensation problems and rusting tools, I also want exposed beams in
the workshop for storage etc.
Thanks
Don

On 1 Aug, 15:53, Donwill wrote:

Fair point, I happen to like a warm roof insulation system to avoid
condensation problems and rusting tools, I also want exposed beams in
the workshop for storage etc.

er OK but shurely you will always get condensation of air borne
moisture onto the coldest things in your garage/workshop.

how would a "warm roof" approach be better than other alternatives in
avoiding this?

Cheers
Jim K

Donwill wrote:
Garage is being converted to workshop. Internal size is 2.7 x 5.3 M.
Joist dimensions 100 x 50 mm x 2800mm span @ 600mm centres, which
are too small for that span but it's been there for 45yrs with very
little deflection approx 1 cm measured.

So as not to disturb the integrity of the felting and therefore the
waterproofing I want to externally insulate the felt covered roof by
using "Jablite inverted roof" insulation:
http://www.jablite.co.uk/fri/index.php

Unfortunately, to hold the insulation down you need to cover it in
50mm of gravel which is heavy.
The roof area is approx 15 Sq M, the volume of gravel would be approx
0.75 Cu M which I think, when wet would weigh in the region of
1500Kgms (distributed load) or approx 100Kgms /Sq M.
Obviously I would need to beef up the joists and I thought to double
up on the joists bolting through a steel flitch plate in between the
joists at say 600mm intervals. Has Anyone tried to do this in situ?

The steel plates , say 100mm x 6mm x 2.7M, and second joists would
not rest on the supporting walls and would be approx 100mm shorter
than existing joists so lateral support would be needed at both ends
and probably in the middle.
Would this be strong enough?

Can anyone help with structural information? and or practical
experience which they can pass on please.

Regards
Don

as Jim has already pointed out, you are going to a ridiculous expense for
nothing.
far better would be to insulate from the inside, I can't see your point WRT
cold spots/condenstaion - the only 'cold spot' would be the joists
themselves and moisture doesn't condense on wood.

I would insulate from undrneath, then cross batten with say, 3 X 2 every
metre or so and use these for your storage.

--
Phil L
RSRL Tipster Of The Year 2008

On 01/08/2010 16:00, Jim K wrote:
On 1 Aug, 15:53, wrote:


Fair point, I happen to like a warm roof insulation system to avoid
condensation problems and rusting tools, I also want exposed beams in
the workshop for storage etc.

er OK but shurely you will always get condensation of air borne
moisture onto the coldest things in your garage/workshop.

Yes you are right, but by having the roof insulated you need less energy
to raise the air temperature above outside temp.By putting the
insulation above the roof the inside of the roof will be warm so no
interstitial condensation.
how would a "warm roof" approach be better than other alternatives in
avoiding this?

What I'm trying to achieve is a relatively constant temp inside the
workshop, one wall of which is in thermal contact with the house, the
other three walls are very well insulated and sealed
The roof has the largest heat loss. Inside the workshop there is a
dehumidifier to lower the dewpoint of the air.
Cheers
Jim K

Thanks
Don

On 01/08/2010 16:21, Phil L wrote:
Donwill wrote:

Garage is being converted to workshop. Internal size is 2.7 x 5.3 M.
Joist dimensions 100 x 50 mm x 2800mm span @ 600mm centres, which
are too small for that span but it's been there for 45yrs with very
little deflection approx 1 cm measured.

So as not to disturb the integrity of the felting and therefore the
waterproofing I want to externally insulate the felt covered roof by
using "Jablite inverted roof" insulation:
http://www.jablite.co.uk/fri/index.php

Unfortunately, to hold the insulation down you need to cover it in
50mm of gravel which is heavy.
The roof area is approx 15 Sq M, the volume of gravel would be approx
0.75 Cu M which I think, when wet would weigh in the region of
1500Kgms (distributed load) or approx 100Kgms /Sq M.
Obviously I would need to beef up the joists and I thought to double
up on the joists bolting through a steel flitch plate in between the
joists at say 600mm intervals. Has Anyone tried to do this in situ?

The steel plates , say 100mm x 6mm x 2.7M, and second joists would
not rest on the supporting walls and would be approx 100mm shorter
than existing joists so lateral support would be needed at both ends
and probably in the middle.
Would this be strong enough?

Can anyone help with structural information? and or practical
experience which they can pass on please.

Regards
Don

as Jim has already pointed out, you are going to a ridiculous expense for
nothing.
far better would be to insulate from the inside, I can't see your point WRT
cold spots/condenstaion - the only 'cold spot' would be the joists
themselves and moisture doesn't condense on wood.

Moisture condenses on any thing that is below the dewpoint of the air,
wood absorbs the condensation so is not readily obvious.
I would insulate from undrneath, then cross batten with say, 3 X 2 every
metre or so and use these for your storage.

I also have a problem with headroom which I forgot to mention.

Thanks
Don

Donwill wrote:
On 01/08/2010 16:21, Phil L wrote:
Donwill wrote:

Garage is being converted to workshop. Internal size is 2.7 x 5.3 M.
Joist dimensions 100 x 50 mm x 2800mm span @ 600mm centres, which are
too small for that span but it's been there for 45yrs
with very little deflection approx 1 cm measured.

So as not to disturb the integrity of the felting and therefore the
waterproofing I want to externally insulate the felt covered roof by
using "Jablite inverted roof" insulation:
http://www.jablite.co.uk/fri/index.php

Unfortunately, to hold the insulation down you need to cover it in
50mm of gravel which is heavy.
The roof area is approx 15 Sq M, the volume of gravel would be
approx 0.75 Cu M which I think, when wet would weigh in the region of
1500Kgms (distributed load) or approx 100Kgms /Sq M.
Obviously I would need to beef up the joists and I thought to double
up on the joists bolting through a steel flitch plate in between the
joists at say 600mm intervals. Has Anyone tried to do this in situ?

The steel plates , say 100mm x 6mm x 2.7M, and second joists
would not rest on the supporting walls and would be approx 100mm
shorter than existing joists so lateral support would be needed at
both ends and probably in the middle.
Would this be strong enough?

Can anyone help with structural information? and or practical
experience which they can pass on please.

Regards
Don

as Jim has already pointed out, you are going to a ridiculous
expense for nothing.
far better would be to insulate from the inside, I can't see your
point WRT cold spots/condenstaion - the only 'cold spot' would be
the joists themselves and moisture doesn't condense on wood.

Moisture condenses on any thing that is below the dewpoint of the air,
wood absorbs the condensation so is not readily obvious.
I would insulate from undrneath, then cross batten with say, 3 X 2
every metre or so and use these for your storage.

I also have a problem with headroom which I forgot to mention.


OK going back to your OP, you say you're thinking of using steel, and also
doubling up on joists, but these extra supports, nor the steel would not
reach the walls? - in this case, they are merely adding to the extra weight.
Can't you have a 'post' in the centre of the workshop, considering it's
never going to be used to house vehicles again?
Then you can use a small steel beam, say 50X50 box section, two pieces about
2.8m each, both resting on the wall at each side and touching the underside
of each joist in the middle of the workshop, and propped up by an acrow or
similar in the centre.

--
Phil L
RSRL Tipster Of The Year 2008

On 1 Aug, 15:11, Donwill wrote:
Garage is being converted to workshop. Internal size is 2.7 x 5.3 M.
* *Joist dimensions 100 x 50 mm x 2800mm span @ 600mm centres, *which
are too small for that span but it's been there for 45yrs with very
little deflection approx 1 cm measured.

So as not to disturb the integrity of the felting and therefore the
waterproofing I want to externally insulate the felt covered roof by
using "Jablite inverted roof" insulation:
*http://www.jablite.co.uk/fri/index.php

Unfortunately, to hold the insulation down you need to cover it in 50mm
of gravel *which is heavy.
The roof area is approx 15 Sq M, the volume of gravel would be approx
0.75 Cu M which I think, when wet would weigh in the region of 1500Kgms
(distributed load) or *approx 100Kgms /Sq M.
Obviously I would need to beef up the joists and I thought to double up
on the joists bolting through a steel flitch plate in between the joists
at say 600mm intervals. Has Anyone tried to do this in situ?

* The steel *plates , say 100mm x 6mm x 2.7M, and second joists would
not rest on the supporting walls and would be approx 100mm shorter than
existing joists so lateral support would be needed at both ends and
probably in the middle.
Would this be strong enough?

Can anyone help with structural information? and or practical experience
which they can pass on please.

Regards
Don

If your roof is 45 years old, it's done well. However it can't go on
for much longer, most people are lucky to get 20 yrs out of a flat
roof. I would examine this roof very closely before undertaking this
modification. Hammering and bashing may provoke a leak on something
this old.

On Aug 1, 3:11*pm, Donwill wrote:

Garage is being converted to workshop. Internal size is 2.7 x 5.3 M.
* *Joist dimensions 100 x 50 mm x 2800mm span @ 600mm centres, *which
are too small for that span but it's been there for 45yrs with very
little deflection approx 1 cm measured.

So as not to disturb the integrity of the felting and therefore the
waterproofing I want to externally insulate the felt covered roof by
using "Jablite inverted roof" insulation:
*http://www.jablite.co.uk/fri/index.php

Unfortunately, to hold the insulation down you need to cover it in 50mm
of gravel *which is heavy.
The roof area is approx 15 Sq M, the volume of gravel would be approx
0.75 Cu M which I think, when wet would weigh in the region of 1500Kgms
(distributed load) or *approx 100Kgms /Sq M.
Obviously I would need to beef up the joists and I thought to double up
on the joists bolting through a steel flitch plate in between the joists
at say 600mm intervals. Has Anyone tried to do this in situ?

* The steel *plates , say 100mm x 6mm x 2.7M, and second joists would
not rest on the supporting walls and would be approx 100mm shorter than
existing joists so lateral support would be needed at both ends and
probably in the middle.
Would this be strong enough?

Can anyone help with structural information? and or practical experience
which they can pass on please.

Regards
Don


Surely its far easier to put your polystyrene on the outside then your
chosen regular roof covering over it rather than 2" of gravel. Sounds
like youre making your life hard.

The roof joists arent too small, as proven by your experience.

As someone said, 45 yrs from a flat roof is unusually long, if you're
considering reroofing you might look at it and see if its rotting,
usually they are by 45 yrs.


NT

On 01/08/2010 18:38, Tabby wrote:
On Aug 1, 3:11 pm, wrote:


Garage is being converted to workshop. Internal size is 2.7 x 5.3 M.
Joist dimensions 100 x 50 mm x 2800mm span @ 600mm centres, which
are too small for that span but it's been there for 45yrs with very
little deflection approx 1 cm measured.

So as not to disturb the integrity of the felting and therefore the
waterproofing I want to externally insulate the felt covered roof by
using "Jablite inverted roof" insulation:
http://www.jablite.co.uk/fri/index.php

Unfortunately, to hold the insulation down you need to cover it in 50mm
of gravel which is heavy.
The roof area is approx 15 Sq M, the volume of gravel would be approx
0.75 Cu M which I think, when wet would weigh in the region of 1500Kgms
(distributed load) or approx 100Kgms /Sq M.
Obviously I would need to beef up the joists and I thought to double up
on the joists bolting through a steel flitch plate in between the joists
at say 600mm intervals. Has Anyone tried to do this in situ?

The steel plates , say 100mm x 6mm x 2.7M, and second joists would
not rest on the supporting walls and would be approx 100mm shorter than
existing joists so lateral support would be needed at both ends and
probably in the middle.
Would this be strong enough?

Can anyone help with structural information? and or practical experience
which they can pass on please.

Regards
Don


Surely its far easier to put your polystyrene on the outside then your
chosen regular roof covering over it rather than 2" of gravel. Sounds
like youre making your life hard.

The roof joists arent too small, as proven by your experience.

As someone said, 45 yrs from a flat roof is unusually long, if you're
considering reroofing you might look at it and see if its rotting,
usually they are by 45 yrs.


NT

I think the felting & probably the 20mm ply has been replaced but I
think the 4x2s are the original beams according to the neighbours who
have lived there since the houses were built, we arrived only 6yrs
ago. No rot visible.
Regards
Don

On 01/08/2010 21:28, John Rumm wrote:
On 01/08/2010 15:11, Donwill wrote:
Garage is being converted to workshop. Internal size is 2.7 x 5.3 M.
Joist dimensions 100 x 50 mm x 2800mm span @ 600mm centres, which are
too small for that span but it's been there for 45yrs with very little
deflection approx 1 cm measured.

Given its holding up a roof and not a floor for the room above, that
is not too surprising!

So as not to disturb the integrity of the felting and therefore the
waterproofing I want to externally insulate the felt covered roof by
using "Jablite inverted roof" insulation:
http://www.jablite.co.uk/fri/index.php

Tis one way I suppose.

Unfortunately, to hold the insulation down you need to cover it in 50mm
of gravel which is heavy.
The roof area is approx 15 Sq M, the volume of gravel would be approx
0.75 Cu M which I think, when wet would weigh in the region of 1500Kgms
(distributed load) or approx 100Kgms /Sq M.

Which is basically the force used in calculations for standard floor
loading...

Obviously I would need to beef up the joists and I thought to double up
on the joists bolting through a steel flitch plate in between the joists
at say 600mm intervals. Has Anyone tried to do this in situ?

Not tried it, but it would be relatively easy to do. You would need a
face mounting shoe at each end (which could be added after the plate
is in and the beams are bolted through) that supports the end of the
new joist, flitch plate, and allows the existing joist to pass through
the back of it.

The steel plates , say 100mm x 6mm x 2.7M, and second joists would not
rest on the supporting walls and would be approx 100mm shorter than
existing joists so lateral support would be needed at both ends and
probably in the middle.
Would this be strong enough?

So you have and extra 1500kg supported on say 9 joists, plus you ought
to allow another 75kg/m^2 for snow and access etc, which give you an
all up loading of about 291kg/joist. At 2.7m each that is 108kg/m or
1080N/m linear UDL

fx: fires up superbeam

with 2 of 100x50 and a full height 6mm flitch plate, that gives a
total deflection of just under 5mm which looks ok. The only unknown in
that calculation is the weight of the existing roof materials - but at
a guess you would appear to have reasonable margins to play with.



Thank you John for the reassuring information.
Does the "Superbeam" give any guidance on the spacing and positioning of
the bolts in the flitch beam?
Can you explain the purpose/ function of the "shoe" and how does it play
a part in the strength of the roof?

Most grateful to you, it was exactly the type of information I was seeking.
Cheers
Don