If I'm building a patio out of big flagstones, do I need to put down a layer of gravel or sand under the stones.

Absolutely. It will help with drainage and also make it easier to get them level.

Edit :: I should add that I helped my BIL do this a few times and he is a licensed contractor and electrician.

^ What he said.

Source: Hubby works in construction and I’m his personal secretary.

Yes, and compact it too. You can rent one for that purpose and if you get a vibratory one then once the flagstones are down and you put sand/fill in between them you can run over the finished surface and compact the fill between the flagstones. (unless you are using mortar between the stones in which case this last step is unnecessary.

^ ^ ^

I’m so glad we had this time together!

Not only will you want to put in a layer of compacted sand for the aforementioned reasons, but you will also want to install the sand over some kind of impermeable barrier to prevent (or at least delay) grass and weed growth later, and pitch that away from any structure or foundation so that water drains away.

That is, the barrier and good drainage is necessary unless you want to encourage grass between the stones, which is also a pleasant effect, if done and later maintained neatly. For a really first-class job you’ll also want to put some kind of border around (even if all of it is sunk below grade and invisible to later users) to prevent surface roots in the soil from undermining stones later.

On the other hand, if you’re installing in a gravel bed, then nearly none of that is necessary. I’ve seen it done both ways.

Well, since you put this in social I’m just going to put this out there. I think you should put a time capsule under it for future generations to randomly stumble on one day.

“impermeable barrier” “so that water drains away”?

In all seriousness, I saw on TV one time where they whirled moss and buttermilk in a blender and made a paint-on moss culture. That would be very cool between the flagstones, and inhibit weeds. If you have shade.

I’ll agree with @GloPro‘s time capsule. WTF?

Gravel and sand are permeable, or pervious, not impervious

Yeah, I could have worded that better, @ibstubro: I mean that the barrier should be sloped, and should allow drainage into some place away from the patio. In a northern climate it won’t be a good idea to allow the water to drain into / under the patio area (unless it can be pretty much guaranteed to always drain deeply), because of the likelihood of partial drainage and refreezing at those times in the year, which could buckle the surface permanently.

I would go with what @CWOTUS said, especially concerning the drainage and the weed barrier.

Concerning the time capsule: when I was working in construction, I used to leave messages for “archaeologists of the future”!

Yes.