Why would one want to lay weed fabric?
Weeds are strong. In the right conditions weeds can break right through concrete simply by feeding off the moisture and light sourced through the tiniest of cracks.
If one is laying a path (see our project on Creating a Garden Path) or base of some kind in the garden, a roll of >weed fabric, also called landscaping fabric, is a cost worth laying out for.
Weed fabric is a very closely knit fabric made from polypropylene which will not rot in the ground. It is coloured black. Being so closely knit it will not allow light to penetrate beyond it. This will stop weeds growing through. It is however, dual purpose when used in construction as you will see from reading on.
Admittedly, black polythene will do the same job (as far as light is concerned) but polythene collects and retains water which may get through to it. This water will sit on the polythene, under your path or base. When the water freezes, it expands. This expansion cracks the concrete or mortar you have used for the path or base. These cracks attract more water when it rains and the cycle begins all over again.
The image on the left shows a slab path being laid on weed fabric.
See our project on Freeze-Thaw action. Freeze thaw action is the reason you see the faces blown off the bricks in a wall.
Weed fabric allows water to pass through into the soil. This means it cannot freeze in, or under the base.
When laying weed fabric to a path or base it is a good idea to level the area first and to apply some very strong weed killer. Some weeds are very hard indeed and will search for light by growing horizontally through the ground, under the weed fabric until they can see some light. They will then change direction and grow vertically towards the light. This will of course be directly up the side of your path of the base so be careful to allow additional weed fabric to extend beyond the area you intend to work on.
The weed fabric can go directly onto the ground or on top of any sub base you may use to provide a stable foundation for your path or slab. The important thing is that the weed fabric is laid on a surface which will not puncture the fabric when the top level is applied to it.