Some of us have used shade cloth for years.

I only use it during extreme heat, which in Marietta usually means parts of July and August of most years.

Shade cloth is a great garden tool, but you have to be careful you don't overdo it. Too much shade can slow down plant growth and bring production to a screeching halt.

I normally use it only for peppers and tomatoes, but I would use it to cool down the lettuce and extend the season if I was growing lettuce in full sun. I plant my lettuce in a place where it gets afternoon shade so don't have to put shadecloth over it.

Be sure you leave adequate space for good air flow between the top of your plants and the shade cloth or you may end up baking your plants.

You can buy shade cloth that allows set percentages of sunlight to penetrate the cloth. I've only used 30% and 40% and both have worked well.

I'm going to link a recent discussion on shade cloth.

If you are in an area that is exposed to the wind be sure you have a good support structure for the shade cloth so it doesn't blow loose or partially loose and whip around in the wind beating your plants to death.

Here is a link that might be useful: Previous Discussion of Shade Cloth