Thickest polythene sheet.

[epsilonGreedy]

What is the thickest polythene waterproof membrane sheet generally available at BMs for a sensible price?

 

Some sheet is described as nnnn grade others are quoted with a thickness. Some internet searching this morning leads me to this product as a best candidate:

 

CAPITAL VALLEY DAMP-PROOF MEMBRANE 1200GA which works out as £0.66 per m2.

https://www.screwfix.com/p/capital-valley-plastics-ltd-damp-proof-membrane-black-1200ga-4-x-15m/59191

[PeterW]

1200ga is normal for DPM. Cheaper as a bigger roll. 

[Russell griffiths]

What’s it for ?

thicker may not be user friendly, if you need to fold it in corners neatly it can be a bugger to work with. 

[epsilonGreedy]

24 minutes ago, Russell griffiths said:

What’s it for ?

 

 

(1) My footings blockwork which is two courses high at ground level has not progressed since September and before any real hard frost arrives in the new year I want to reduce the moisture levels by laying a temporary dpc roof about 350mm across the top of the cavity and blocks.

 

(2) I want to restack about 400 blocks to tidy up the site and also sit the block stacks on a layer of membrane to prevent rising damp.

 

(3) I have had a few trench floods from surface runoff during heavy rain. The trenches a now dry with help of a sump pump and some manually dug sumps within the footings oversite. So I now intend to create of water runoff deflection bund about 300mm high long a 20m length of the site boundary . The core of the bund will be a length of blocks with membrane overlapping these blocks and all buried under subsoil that is still onsite from the original foundation dig.

[PeterW]

For the footings I’d just get a roll of 450 or 600 DPC and roll it out - can be reused. 

 

Blocks need stacking on pallets first then covered with a tarp - stops them getting clagged up. 

[Jeremy Harris]

Damp won't rise up blocks stacked on wet ground to any significant degree, but as @PeterW says, they are best left stacked on their pallets to keep them clean, anyway. 

 

The mechanism involved for getting moisture to rise is capillary action, and at most you might get damp to rise up a few  mm above ground level.  Also, blocks are made using concrete with a pore blocking additive to stop any tendency for water to rise through them when they are sat on a wet surface, not just to stop so called "rising damp" (which is largely mythical, anyway) but primarily to stop them sucking the water out of mortar.

 

[Russell griffiths]

Any reason you can’t backfil your trenches, if you are up to dpc level you won’t need to go back into them. 

[Russell griffiths]

As @PeterW said really, have you got to fit a cavity tray ?

if so buy a roll of the dpc you will use and just roll it out on top of wall, put a couple of bricks on it and that’s done, 

core fill your cavity up to the height in your design, and then back fill both sides of the trench

two jobs done and no need to buy that sheet. 

[epsilonGreedy]

20 hours ago, Russell griffiths said:

Any reason you can’t backfil your trenches, if you are up to dpc level you won’t need to go back into them. 

 

 

Not up to dpc yet. I have an enviro recommendation for an extra couple of courses of bricks below damp, i.e 300mm above ground level. Think the brickies will appreciate open trenches when the build continues. 

[epsilonGreedy]

13 hours ago, Russell griffiths said:

As @PeterW said really, have you got to fit a cavity tray ?

if so buy a roll of the dpc you will use and just roll it out on top of wall, put a couple of bricks on it and that’s done, 

core fill your cavity up to the height in your design, and then back fill both sides of the trench

two jobs done and no need to buy that sheet. 

 

I am coming around to the idea of using dpc roll, I had not looked at the price before and assumed it would be too expensive for a temporary measure. A quick ScrewFix search indicates dpc roll is a cheap option.

 

There is no need for a cavity tray on my site but I wonder if 450mm dpc roll would be suitable on the inner block wall with the surplus 350mm lapped down the side of the block to meet the upstand of the damp membrane over the block & beam floor? One concern I have is whether dpc plastic is too thick for a 90 degree bend over the top edge of a block?

[PeterW]

DPC will bend 90 degrees - its a fairly standard detail.