11:29AM | 08/26/01
Member Since: 08/25/01
5 lifetime posts
I am having a concrete patio put in my back yard. I would like to build a retaning wall as well to keep the dirt out. Is it possible to build a 2 foot high wall (using the interlocking blocks) using the edge of the patio as a base, or do I have to excavate/lay gravel for the wall. If it is relevant, I live in the Boston area.

Any advice would be very welcome since this is a new experience for me.


12:15PM | 08/26/01
Member Since: 09/01/00
312 lifetime posts
I have done this and had no problem,however,I would check with whoever is doing the patio for you so that they can be aware.There will be a time lag before you can build the wall,and you need to confirm that your project will not ruin theirs.As far as doing your own base and building off the patio?Probably a better idea if you're paying alot for the patio or if the contractor has to consider your request for any length of time.

Jay J

05:12PM | 08/26/01
Member Since: 10/26/00
782 lifetime posts
Hi Briggsmo,

I vote no because of drainage issues.

If you were to put the wall on the patio, even if the 'base' was at the same depth as a gravel base would be, how is the water going to drain off of the base? A solid base will only collect the water. Go with the recommended gravel base and drainage system that's normally built for a retaining wall. Be SURE to backfill w/gravel and use Landscape or Filter Fabric (not weed block) in this job. Use black, perforated black pipe too to carry the water away from the base of the wall. Your dirt base should slope about 1" per foot away from the patio slab too, back to where the black pipe is so the water can collec there and rise into the pipe to be carried away.

Go to, then click the Products link, then click the Retaining Wall link, then click the Do-It-Yourself Info link. I strongly recommend you do the research first BEFORE you do any digging.

My best to ya and hope this helps.

Jay J -Moderator


07:12PM | 08/26/01
Member Since: 08/25/01
5 lifetime posts

I understand most of what you are saying. the only part I am a little iffy on is the 1" per foot from the back of the patio slab for drainage. I went to the website you recommended and I didn't see anything about the drainage pipe. We are planning on putting one in the patio to drain to the driveway, and I can see the need for putting one behind the wall, but I am not seeing the visual picture that you are describing. Is there anonther way to explain it?

I appreciate your responding to my previous post.

By the way I just moved to Boston from the Philly area. (Bucks County actually)

Jay J

09:07AM | 08/27/01
Member Since: 10/26/00
782 lifetime posts
Go here: Allan Block Technical Information

Click the url labeled: Retaining Walls Installation Manual

Go to Page 6 (the TOP of page 6), and read. It's ALL over the place.

Since you have a patio at the BASE of the wall, you DON'T want to 'promote' the water to flow under your patio. By sloping the base at an angle that's AWAY from the patio, then any water that gets into the base will NOT run under the patio. Believe me, you'd be surprised at the amount of water that is passed out of a hill that's held back by a retaining wall. By angling the base 1" per foot, then you'll have a couple of inches of water that can rise UP into the pipe to be carried off by the pipe. It would be 'nice' if you could slightly slope the pipe in a downward direction too (but you may end up keeping it FLAT.) If you end up keeping it flat, that's OK - just be sure you have the base angled TOWARDS the pipe (at the back of the wall) and AWAY from the patio.)

If you're still unclear, post back.

Jay J -Moderator

[This message has been edited by Jay J (edited August 27, 2001).]

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