It looks like I may be able to build my new workshop come spring, *just*
within the 15 sq.m limit (so it doesn't have to be 1m from next door's
garage-on-the-boundary, Grrr...), and wonder if anyone has any suggestions
for putting a DPM / DPC in - the slab will (I think) be 6" concrete with 2
layers of steel fabric, sat on 2" of expanded poly (warm feet in the
workshop - ooh, lovely!) - is there any advantage to taking the DPM below
the insulation, assuming I don't poke holes in it? At present I'm hoping the
walls will be concrete block with more insulation/vapour
barrier/ply-or-platerboard on the inside, should the DPM come up from the
slab to surround the first course of blocks, or would it be better/easier to
put the DPM internal to the walls and slab, and screed over? I'd rather
avoid any brittleness from a thin screed as there'll be 2 tons of lathe
(and, funds permitting, a ton and a half of milling machine) sat in the
'shop once it's up, hence the need for a Substantial Slab!

Thanks in advance for any suggestions,
Dave H.
--
(The engineer formerly known as Homeless)

"Rules are for the obedience of fools, and the guidance of wise men" -
Douglas Bader

On 23 Oct, 10:57, "Dave H."
wrote:
snip
the slab will (I think) be 6" concrete with 2
layers of steel fabric, sat on 2" of expanded poly (warm feet in the
workshop - ooh, lovely!) -

er ...how will that be getting "warm"?

is there any advantage to taking the DPM below
the insulation, assuming I don't poke holes in it?

it'lll stop your polystyrene getting damp and losing some insulating
value?

Jim K

"Jim K" wrote...
On 23 Oct, 10:57, "Dave H." wrote:
snip
the slab will (I think) be 6" concrete with 2
layers of steel fabric, sat on 2" of expanded poly (warm feet in the
workshop - ooh, lovely!) -

er ...how will that be getting "warm"?

Well, for values of warm that don't involve heat loss through the floor from
a regularly-used, slightly heated and well-insulated workshop?

is there any advantage to taking the DPM below
the insulation, assuming I don't poke holes in it?

it'll stop your polystyrene getting damp and losing some insulating
value?

That's about what I thought (although the poly should be closed-cell anyway,
apart from any cracks that occur), and it should stop the damp rising
through the slab just as well as if it were directly underneath it, I'd
guess?

Thanks, Dave H.
--
(The engineer formerly known as Homeless)

"Rules are for the obedience of fools, and the guidance of wise men" -
Douglas Bader

Dave H. wrote:
It looks like I may be able to build my new workshop come spring, *just*
within the 15 sq.m limit (so it doesn't have to be 1m from next door's
garage-on-the-boundary, Grrr...), and wonder if anyone has any suggestions
for putting a DPM / DPC in - the slab will (I think) be 6" concrete with 2
layers of steel fabric, sat on 2" of expanded poly (warm feet in the
workshop - ooh, lovely!) - is there any advantage to taking the DPM below
the insulation, assuming I don't poke holes in it? At present I'm hoping the
walls will be concrete block with more insulation/vapour
barrier/ply-or-platerboard on the inside, should the DPM come up from the
slab to surround the first course of blocks, or would it be better/easier to
put the DPM internal to the walls and slab, and screed over? I'd rather
avoid any brittleness from a thin screed as there'll be 2 tons of lathe
(and, funds permitting, a ton and a half of milling machine) sat in the
'shop once it's up, hence the need for a Substantial Slab!

Thanks in advance for any suggestions,
Dave H.

I struggled with this question for ages until someone pointed out that
the DPM under the slab is there to protect the concrete from losing
water, from gaining water (if it rains or the water table rises) and
from gaining unwanted salts from ground-water whilst it is in the
process of going off.

Once the slab has cured, the DPM continues to protect the slab from
sulphates (if you didn't use sulphate-resistant cement, of course) and
chlorides.

So, don't think of the DPM under the slab as being a barrier to stop
rising damp and don't worry about the edges and interfacing with the DPM
in the walls. That's a different DPM for a different purpose.

I would consider:


------------Slab--------------
Mesh - Mesh - Mesh - Mesh
I I I I - Standoffs/spacers for Mesh
------------Slab--------------
===========Jablite============
------------DPM---------------
::::::::::Sand Blinding:::::::
Crushed and compacted hardcore

See also:
http://www.pavingexpert.com/concrete.htm

"Dave Osborne" wrote...

I struggled with this question for ages until someone pointed out that the
DPM under the slab is there to protect the concrete from losing water,
from gaining water (if it rains or the water table rises) and from gaining
unwanted salts from ground-water whilst it is in the process of going off.

Once the slab has cured, the DPM continues to protect the slab from
sulphates (if you didn't use sulphate-resistant cement, of course) and
chlorides.

So, don't think of the DPM under the slab as being a barrier to stop
rising damp and don't worry about the edges and interfacing with the DPM
in the walls. That's a different DPM for a different purpose.

I would consider:


------------Slab--------------
Mesh - Mesh - Mesh - Mesh
I I I I - Standoffs/spacers for Mesh
------------Slab--------------
===========Jablite============
------------DPM---------------
::::::::::Sand Blinding:::::::
Crushed and compacted hardcore

See also:
http://www.pavingexpert.com/concrete.htm


Thanks Dave, I wasn't aware that that was the reason for the DPM! I'll
probably go with 2 layers of mesh as a belt-and-braces measure to be sure
the slab's rigid enough to take any movement / the loads applied and bring
the slab forward at the door so I can make a slight ramp for moving
machinery in and out. Being the last build on the estate, our garden's
mostly crushed and compacted hardcore already...

I looked at the pavingexpert site, useful but nothing that exactly fits -
lots of garage and shed slabs without insulation, or insulated with cavity
walls! I think I may have to go with a DPM on top of the concrete slab and
up to a DPC and then a thickish (2"?) screed or concrete on top of that - a
bit of a sod, as it'll involve 2 pours/levellings and the top layer might be
a bit more brittle than the slab - a consideration when rolling 2 tons about
on scaffold poles and applying prybars to lift and manoeuvre it!

Dave H.
--
(The engineer formerly known as Homeless)

"Rules are for the obedience of fools, and the guidance of wise men" -
Douglas Bader

On Oct 23, 4:26*pm, "Dave H."
wrote:
"Dave Osborne" wrote...

I struggled with this question for ages until someone pointed out that the
DPM under the slab is there to protect the concrete from losing water,
from gaining water (if it rains or the water table rises) and from gaining
unwanted salts from ground-water whilst it is in the process of going off.

Once the slab has cured, the DPM continues to protect the slab from
sulphates (if you didn't use sulphate-resistant cement, of course) and
chlorides.

So, don't think of the DPM under the slab as being a barrier to stop
rising damp and don't worry about the edges and interfacing with the DPM
in the walls. That's a different DPM for a different purpose.

I would consider:

------------Slab--------------
* Mesh - Mesh - Mesh - Mesh
* * I * * *I * * *I * * *I * * * *- Standoffs/spacers for Mesh
------------Slab--------------
===========Jablite============
------------DPM---------------
::::::::::Sand Blinding:::::::
Crushed and compacted hardcore

See also:
http://www.pavingexpert.com/concrete.htm

Thanks Dave, I wasn't aware that that was the reason for the DPM! I'll
probably go with 2 layers of mesh as a belt-and-braces measure to be sure
the slab's rigid enough to take any movement / the loads applied and bring
the slab forward at the door so I can make a slight ramp for moving
machinery in and out. Being the last build on the estate, our garden's
mostly crushed and compacted hardcore already...

I looked at the pavingexpert site, useful but nothing that exactly fits -
lots of garage and shed slabs without insulation, or insulated with cavity
walls! I think I may have to go with a DPM on top of the concrete slab and
up to a DPC and then a thickish (2"?) screed or concrete on top of that - a
bit of a sod, as it'll involve 2 pours/levellings and the top layer might be
a bit more brittle than the slab - a consideration when rolling 2 tons about
on scaffold poles and applying prybars to lift and manoeuvre it!

Dave H.

Why would you want a DPM on top of the concrete?
If for some reason you really do, bitumen paint would be better re
firm support of a thin layer above.


NT

Tabby wrote:
On Oct 23, 4:26 pm, "Dave H."
wrote:
"Dave Osborne" wrote...

I struggled with this question for ages until someone pointed out that the
DPM under the slab is there to protect the concrete from losing water,
from gaining water (if it rains or the water table rises) and from gaining
unwanted salts from ground-water whilst it is in the process of going off.
Once the slab has cured, the DPM continues to protect the slab from
sulphates (if you didn't use sulphate-resistant cement, of course) and
chlorides.
So, don't think of the DPM under the slab as being a barrier to stop
rising damp and don't worry about the edges and interfacing with the DPM
in the walls. That's a different DPM for a different purpose.
I would consider:
------------Slab--------------
Mesh - Mesh - Mesh - Mesh
I I I I - Standoffs/spacers for Mesh
------------Slab--------------
===========Jablite============
------------DPM---------------
::::::::::Sand Blinding:::::::
Crushed and compacted hardcore
See also:
http://www.pavingexpert.com/concrete.htm
Thanks Dave, I wasn't aware that that was the reason for the DPM! I'll
probably go with 2 layers of mesh as a belt-and-braces measure to be sure
the slab's rigid enough to take any movement / the loads applied and bring
the slab forward at the door so I can make a slight ramp for moving
machinery in and out. Being the last build on the estate, our garden's
mostly crushed and compacted hardcore already...

I looked at the pavingexpert site, useful but nothing that exactly fits -
lots of garage and shed slabs without insulation, or insulated with cavity
walls! I think I may have to go with a DPM on top of the concrete slab and
up to a DPC and then a thickish (2"?) screed or concrete on top of that - a
bit of a sod, as it'll involve 2 pours/levellings and the top layer might be
a bit more brittle than the slab - a consideration when rolling 2 tons about
on scaffold poles and applying prybars to lift and manoeuvre it!

Dave H.

Why would you want a DPM on top of the concrete?
If for some reason you really do, bitumen paint would be better re
firm support of a thin layer above.


NT

here it was slab, DPM curled up at the edges, insulation also up at the
dges.. screed with mesh, done.

On 23 Oct, 20:26, The Natural Philosopher
wrote:
Tabby wrote:
On Oct 23, 4:26 pm, "Dave H."
wrote:
"Dave Osborne" wrote...

I struggled with this question for ages until someone pointed out that the
DPM under the slab is there to protect the concrete from losing water,
from gaining water (if it rains or the water table rises) and from gaining
unwanted salts from ground-water whilst it is in the process of going off.
Once the slab has cured, the DPM continues to protect the slab from
sulphates (if you didn't use sulphate-resistant cement, of course) and
chlorides.
So, don't think of the DPM under the slab as being a barrier to stop
rising damp and don't worry about the edges and interfacing with the DPM
in the walls. That's a different DPM for a different purpose.
I would consider:
------------Slab--------------
* Mesh - Mesh - Mesh - Mesh
* * I * * *I * * *I * * *I * * * *- Standoffs/spacers for Mesh
------------Slab--------------
===========Jablite============
------------DPM---------------
::::::::::Sand Blinding:::::::
Crushed and compacted hardcore
See also:
http://www.pavingexpert.com/concrete.htm
Thanks Dave, I wasn't aware that that was the reason for the DPM! I'll
probably go with 2 layers of mesh as a belt-and-braces measure to be sure
the slab's rigid enough to take any movement / the loads applied and bring
the slab forward at the door so I can make a slight ramp for moving
machinery in and out. Being the last build on the estate, our garden's
mostly crushed and compacted hardcore already...

I looked at the pavingexpert site, useful but nothing that exactly fits -
lots of garage and shed slabs without insulation, or insulated with cavity
walls! I think I may have to go with a DPM on top of the concrete slab and
up to a DPC and then a thickish (2"?) screed or concrete on top of that - a
bit of a sod, as it'll involve 2 pours/levellings and the top layer might be
a bit more brittle than the slab - a consideration when rolling 2 tons about
on scaffold poles and applying prybars to lift and manoeuvre it!

Dave H.

Why would you want a DPM on top of the concrete?
If for some reason you really do, bitumen paint would be better re
firm support of a thin layer above.

NT

here it was slab, DPM curled up at the edges, insulation also up at the
dges.. screed with mesh, done.

15 sq m - that's 5 x 3m which is the size of the workshop I have had
for 20 years. At that size you are, I would have thought, be highly
unlikely to have any really heavy equipment - I doubt somehow that you
are going to install a Wadkins Bursgreen RS wood turning lathe
weighing nearly a ton, so what is with this 6" of concrete and re-
inforcement for. Totally, totally unnecessary - you are seriously
over-engineering. 4" good plain concrete on proper base is all that
you need, with the dpm between the blinded base and the concrete.
I've done that now 5 times on sheds of about that size and I still use
3 of them with no signs of failure.

And why the added complication of insulation when you can go to Costco
and get interlocking rubber floor mats which £30's worth would give
you a good 1 cm's worth of real foot comfort - ie your concrete could
be warm (how is it going to be warm in a workshop?) but it is still
bloody hard.

I've never looked into the sums for the insulating of a concrete
floor, but I would need to be convinced the heat loss is all that
great in comparison to that through windows and, in a workshop's, case
the walls and ceiling.

So - why the over-engineered concrete floor, and have you done the
sums to see if the effort and cost of floor insulation is justified
when the roof, wall and window heat losses are far more serious.

A little bit of realism needs to be applied about what a workshop is
and what basic building techniques apply to it.

Rob