This FAQ considers whether a “membrane” is necessary beneath paving projects, and, if so, just what sort of geo-textiles should be used, and where they should be positioned?
I have received several quotes for a block paved drive all within £100 of each other. I was wondering if you could tell me if you recommend geotextile membranes between the sub base and sand course. Contractors who do not use the membrane say it can cause problems later and is not necessary but the ones who do, say it is beneficial.
The Why…
Some contractors are suggesting or even recommending that residential driveways and even patios should incorporate a “membrane” for best results. Unfortunately, quite a few of the contractors recommending the inclusion of a membrane seem to be unsure as to the benefit it brings to the finished pavement. Some claim it is a weed-proofing measure, while others claim it helps reduce the amount of sub-base required, and yet others claim it can eliminate the need for a sub-base completely. We’ve also heard claims that it prevents the settling of bedding sand and even that it prevents flooding!
Several of these claims are spurious, to say the least, and some of them are pure fantasy. The true purpose of a geo-textile used in the construction of a typical block pavement is explained on the Geo-sheets page , but, to summarise, its primary purpose is as a separation membrane , preventing the sub-base or bedding material mixing with the underlying layers, which can result in excess settlement or pumping of a particularly weak sub-grade, which, in turn, can cause settlement and rutting.
Correct use of a separation membrane
The What…
There have also been some reports of less-than competent contractors and some misguided diy’ers using ‘plastic’ or polythene sheeting as a separation membrane within a pavement. This usually turns out to be some form of building film, protective sheeting or a damp proof membrane. One correspondent asked if he could use ‘Bubble Wrap’ misappropriated from his place of work as he could not find a local supplier of a genuine separation membrane!
An impermeable film or membrane will cause all sorts of problems with most types of paving. It will prevent proper drainage of the overlying layers, which can cause saturation of the bedding layer and all the headaches that come with that, such as pumping, settlement and channelisation. Only a genuine separation membrane should be used – not a ” landscape fabric “, not a damp proof membrane, not a layer of polythene, and certainly not a roll of bubble-wrap!
Links to manufacturers of quality geo-textiles are given on the Geo-fabrics page.
The Where…
There also seems to be some confusion and disagreement as to just where a membrane should be placed within the pavement structure, with some contractors claiming it should be placed on top of the sub-base, between the “hardcore” and the sand bedding, and others insisting it should be placed between the sub-grade and the sub-base.
The reasoning behind the ‘sub-base:bedding’ school of thought is that the membrane prevents the bedding from trickling down or being washed into any voids within the sub-base material, while the ‘sub-grade:sub-base’ orthodoxy, in which I include myself, maintains that keeping the sub-base material from mixing with the sub-grade, and reducing or eliminating any risk of pumping (see above) is more relevant.
In the vast majority of light-use constructions (patios, driveways, footpaths, etc) there is absolutely no need for a membrane to be placed between sub-base and the bedding layer: it achieves almost nothing, as a well-selected sub-base material will have what we call a ‘tight’ or ‘close’ finish, that is, there will be very few, if any, significant voids. Further: when constructing a sub-base, the good contractor will ‘blind’ the surface with fines or grit sand to ensure that there are no nasty voids that could gobble up the bedding material.
There are projects where a separation membrane between the sub-base and bedding would be justified. These are often special applications, such as permeable paving, where the sub-base is purposely open in texture to aid drainage, and so a cogent argument could be made to include separation membranes at both interfaces, which would, no doubt, delight the membrane manufacturers, but, if a membrane is to be used, then it should be placed on top of the excavated sub-grade, between the bare earth (or capping layer) and the sub-base.
Obviously, where no sub-base is present, as is often the case with flagged patios and domestic footpaths, then if a membrane is to be used at all, it should be positioned between the sub-grade and the bedding.
The geotextile, if used at all, is best placed between sub-grade and sub-base
The Facts…
So, what about the claims made for the inclusion of membranes within residential paving jobs? Taking the ‘claims’ quoted above one at a time…