Woven & felted plastic ground-cover
We use this 5 year woven & felted ground cloth in all raised beds that we install where root competition from preexisting trees in the yard may become an issue. While we consider sun exposure to be the number one consideration for placing your raised bed in your yard, root competition is a close number two, because tree and palm tree roots are very string and competitive and they can infest and ruin a garden in just one season. Adding a liner is a good option if you suspect root competition in your yard, as it will put up a good fight against invading tree roots from below, however it isnt a permanent solution. There isn't a permanent solution unfortunately (besides growing completely above ground in containers) but adding a very strong yet permeable ground cloth like this one will give you about 3 seasons worth of growing before you may have to dig up the soil, replace the barrier, and start again.
We cut our ground cloth to 6 x 10 foot to be used in a 4 x 8 foot raised bed with enough extra on all sides to be folded up the sides of the bed. We lay it out neatly and fold & staple the corners and put a few staples in each side just to keep it out of the way while we fill the bed with soil.
We cut our ground cloth to 6 x 6 for a 4 x 4 foot raised bed.