dpc & dpm How?

[BillySilly]

Hi
I am building a largish shed using 215mm celcon blocks. I have laid the foundations and have trench blocks up to the level of the dpc. I have laid the concrete slab on hardcore-blinding layer of sand-dpm. Some elementary questions on what to do next!
1 Is the preferred order mortar-dpm-mortar-dpc-mortar-first row of blocks?
2 Doesn't this make a rather thick sandwich (especially at the corners with lapping of the dpc): how thin should each mortar layer be?
3 I know to lap the dpc at the corners but what do I do with the dpm at the corners? Do I bunch or cut it and if I cut, how?
4 Do I trim the dpm and if so where?
5 Do I put down the whole first layer of mortar on one side of the building and press the dpm onto it, or go on to lay the second layer of mortar and dpc in one session? Do I then go on in the same session to lay the third layer of mortar and first row blocks on the dpc or wait until the first 2 layers have gone off?
6 What do I do with the dpm where the garage door opening is?
Thanks

 

[mikric]

Cut your dpm off about 10mm in from the outside face of your wall.Corners can be a pain but do your best to fold up, it helps sometimes to nick it.
Then lay your dpc over this.
Dont worry about bedding dpc/membrane just lay your first mortar bed on top of dpc and begin laying your blocks, the weight of the wall will hold it all in place.
Cut off any excess dpm in door opening.
Hope that helps.

 

[BillySilly]

Thanks mikric
Thats helpful. One problem though! The 2 end walls are the loadbearing ones taking the load of the lattice beams and flat roof.
One end wall is 'standalone' until the lattice beams are bolted to it and door and window frames are inserted.
Should I
(i)
take the dpm up the wall, laying the dpc on a bed of mortar on the blocks,
or
(ii)
trim the dpm to 50mm and build it in below the dpc, or
(iii)
take the dpm across the full width of the block without mortar and risk the wall falling
over in a wind!

 

[BillySilly]

Now I am confused!!
I emailed Visqueen (ie British Polythene) asking for current practice and received helpful replies - unfortunately incompatible.
(i) said DPC's should be wet bedded in mortar in accordance with BS 8215& BS 8000.

(ii) said the dpm and dpc should be taped using their 50mm tape - but if you do that you cant put mortar under the dpc. You could put it under the taped dpm but then the only bond is the tape

(iii) said Poly DPM's should not pass through any load bearing masonry
walls as they can act as a slip plain & destabilise a wall.

This (iii) was my fear about the end wall. Does anyone know what the correct practice is for a wall of my type?
I had always thought that the dpm went under the dpc to the full width less 5mm or 10mm. Elsewhere I read that the minimum lap is 50mm, but this still leaves me with a problem. I could just tape the dpm to the inside of the blockwork and mortar in the dpc.
Help wanted before I start!!

 

[hotrod]

(iii) said Poly DPM's should not pass through any load bearing masonry walls as they can act as a slip plain & destabilise a wall.

I find it very hard to believe Visqueen are stipulating this as an MI. It's a standard installation detail!

Elsewhere I read that the minimum lap is 50mm, but this still leaves me with a problem. I could just tape the dpm to the inside of the blockwork and mortar in the dpc.

Bed the DPM then lay DPC as per Mik's instructions above, see this detail for further guidance:-

 

[BillySilly]

Thanks Hotrod for the confirmation of the 50mm.
My (iii) was an exact quote from Visqueen's reply. I had emailed technical support and though they said they were manufacturers and not Building Control it seemed categorical even if it is not followed in practice!

You say to lay the dpm on mortar which is what I suppose I wanted to hear, but then said to follow Miks instructions. Unfortunately my reading of Miks post is that you dont bother with mortar under the dpm or the dpc but only put mortar on top of the dpc. Is this common practice? I fail to see how a wall can be secure if it is just standing on unbonded plastic (dpc) or even worse two layers of plastic (dpm + dpc) even if they may be taped together which I dont think is standard practice!

I'm not a bricklayer but you can see why I am puzzled especially as I don't want to build the walls twice!

 

[noseall]

I think the pudding is being over egged here slightly!

What we do is bed the dpm up onto the brick with a slither of mortar, this insures no voids. Incidentally the dpm lap is trimmed back so that only 50mm or so is sitting on the brick.

Next we lay a full but tight bed of mortar down on top of the dpm and bed the dpc onto this. The dpm is then troweled into the muck and the snot removed.

Never ever have i seen the two (dpc, dpm) taped together.

Just do as above and ignore the white coats.

 

[BillySilly]

Thanks noseall
OK. I will trim the dpm back to 50mm and do as you say. Do you lay your first row immediately or wait till the dpmand dpc mortar have set?

 

[stuart45]

In some parts of the country the DPC isn't bedded, just laid on top of the DPM and brickwork to leave a thinner joint. Years ago the DPM was cut off flush with the slab on many sites.

 

[noseall]

Thanks noseall
OK. I will trim the dpm back to 50mm and do as you say. Do you lay your first row immediately or wait till the dpmand dpc mortar have set?

IMMEDIATELY. We are trying to earn a crust you know!

No probs, just go ahead and build straight away.