Membrane, is it necessary?

[KeithP]

Hi,

I was just wondering when building a Block paving drive whether its important to lay down some membrane, to prevent the bases mixing together or prevents plant growth from underneath?, would you recommend it?

Also where should the membrane be laid, in correlation to the seperate layers beneath the actual paving?

Many Thanks
Keith Porter

(Edited by KeithP at 9:25 am on Aug. 28, 2002)

[84-1093879891]

No, it's not essential, unless you're on really bad ground, and then its role is as a separation membrane, not a weed barrier.

There's more about the use of membranes on the Geo-Sheets page.

[KeithP]

Ive received a quote from a paving company which includes a plastic liner between the sub-base and the bedding layer. Is this OK?, by the sounds of it, it isnt the membrane mentioned above.

(Edited by KeithP at 6:57 pm on Aug. 28, 2002)

[84-1093879891]

Have they given a reason why they think a plastic liner is necessary, because I can't think of one.

I can't understand why a 'liner' or a membrane would be used between the sub-base and the bedding layer in the first place - if a membrane is to be used, it is placed over the sub-grade and beneath the sub-base.

There's a definite whiff of bovine excrement about this. Call them and ask for their reasoning.

[KeithP]

They told me that it was to stop weeds, is this correct? and if they do lay it could it cause any problems (the drive is on a slight slant)

Thanks

(Edited by KeithP at 8:15 am on Aug. 29, 2002)

[84-1093879891]

How will it stop weeds? Think about it - there should be at least 100mm of well-compacted granular sub-base (crushed rock) which only the most pernicious of weeds could possibly penetrate. If such weeds (eg, Japanese Knot Weed) are present, then the membrane should be between the sub-grade and the sub-base, to keep it out of the sub-base altogether.

Weeds tend not to grow through block paving - they settle into the sand joints and grow into the paving, and no membrane can prevent this. For that, you need a sealant or joint stabilising compound.

I still think they're trying to baffle you with science. There is absolutely no way a membrane between the sub-base and bedding layer will prevent weeds colonising the surface of the paving, or prevent the sub-base being contaminated by sub-grade pumping.

The way they have proposed to use a membrane is a total waste of money. Have they given a type of membrane, or a brand name?

[KeithP]

Hi,

Thanks for the reply

They showed me the material, and it is just plastic sheeting (not membrane). I think its because I was asking about membrane they thought they could make me happy by adding this layer (although by the looks of it, I dont need membrane anyway as the ground is pretty good).

I was just wondering whether there would be any negative result by putting this plastic layer in? (such as flooding)?

Thanks again for all your help
Keith

[84-1093879891]

Get rid of these eejits, pronto, Keith! Lead them back to their horses, saddle them up and send them packing! They wanted to use a plastic sheet?!?!? Beneath the paving?!?!? That's a more or less guaranteed recipe for disaster!


Using an impermeable sheet in this manner means the bedding layer cannot drain, so it becomes saturated, and starts to behave like a fluid, with the blocks floating on the surface. In next to no time, the whole pavement will have rutted, blocks will have spalled, and the jointing sand will start to 'pump', ie, be forced out of the paving.

They've no idea, have they? Send 'em packing!

[KeithP]

We will probably change builder, all the other local builders have said that the bedding layer will be stone dust instead of Coarse sand is this OK ?(Best to check).

Thanks again for all the advice and help
Keith

[84-1093879891]

Stone dust is fine, usually. I know it's popular in some parts of the country and, if all the local trade are saying it's fine, then it probably is. :)

[KeithP]

Excellent :-), Thanks for all your help.

Keep up the good work

I will post some pictures of the drive when its done and see what you think.

Thanks once again
Keith