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I'm building a retaining wall for a circular fire pit area in my backyard, which has some slope to it. I've stacked the stones along the front part of the fire pit to make a kind of retaining wall, but I haven't put mortar in-between the stones just yet. The plan is to eventually build up the backside of the fire pit area where we've dug into the hill. We'll set flagstone down eventually as the base of the circle. And just so you have a sense of scale, the diameter of the circle is roughly 14.5 feet and the maximum height of both the "uphill" and "downhill" retaining walls are roughly 1.5 feet (see photo below).

I'm a little unsure what to do about drainage for this thing. Should the circle be slightly angled downhill to allow for runoff? Or do I need to bury some piping behind either retaining wall to allow for drainage? And if I'm going to mortar the stones together (on both the "uphill" retaining wall and the "downhill" retaining wall), should I plan to leave holes in the mortar so water is allowed to seep through the retaining wall?

Thanks so much for the help, I'm a bit out over my skis on this one.

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    That is the largest fire pit I've ever seen. Are you going to roast a whole cow on it?
    – Steve Wellens
    Commented Dec 2, 2020 at 5:35
  • Ha! A whole herd, if I can fit it...actually, @SteveWellens, it may be a bit hard to gauge scale from this photo. The "circle" that you see may be better described as a "patio" with room for a fire pit in the middle and 4-6 chairs around it. So no, it's not an altar or anything. Just a level space for a small group to enjoy a cozy fire.
    – Will
    Commented Dec 3, 2020 at 13:19

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Water will drain through mortar. Is the area inside the circle still going to be dirt (instead of an impervious material)? If so, I don't see any need to do anything special.

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  • The flagstone will be in the circle, set in some kind of a sand mixture that's packed down. Then we'll fill in the area around the flagstone with pea gravel. Would it be better to do an impervious surface instead inside the circle?
    – Will
    Commented Nov 2, 2020 at 1:14
  • I don't see any advantage to doing so. The water will drain through the voids between the sandstone, through the sand and through the dirt, and eventually through and under the retaining wall. Nothing is watertight, but nothing should be IMO... You can put a couple of small pipes in to aid drainage if you want, but it would probably be unnecessary.
    – Vette
    Commented Nov 2, 2020 at 3:20
  • I usually put landscape fabric on the wall side and make a trough and back fill with rock this creates a large drain area and keeps the dirt in place as water drains through the wall at weep holes after a few years you end up with low spots if no water permeable material is there, rock makes a French drain to prevent blowouts that can happen if enough water is trapped and the dirt turns into liquid mud.
    – Ed Beal
    Commented Nov 2, 2020 at 17:44
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Landscape fabric does not really work well. The idea is to block the dirt and to allow only wager to seep through to the stone. However, the fabric itself is easily clogged, negating all the hard work put into the design. Two pieces of 1" diameter corrugated pvc pipe (holes drilled at home, on the underside of the pipe) buried at the base of the wall (facing us), extending in about half way in, about 1/3 (5 feet) apart, and cleaned out (manually, with a 1/2" wire drain snake) once a year is a better idea. However, a good quality tarp to cover the top of the fire pit is your best option, considering the great work you've done on it. That may be the standard with fire pits anyway. The question is, "Who do you contact, your local fire pit expert?"

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  • I like the idea of a tarp a lot. Thanks for the insight, @Bluethunderbird!
    – Will
    Commented Dec 3, 2020 at 13:21

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