Expert: Dan Griffin - 1/19/2011

Question
Hi Mr. Griffin,

We have a 2 tier stone and mortar retaining wall that extends 23 feet from the house. 25 years old in good shape There is a 3 feet planting bed between the tiers.  Bottom tier is 4 feet high, top tier is 3 feet high.

We want to build a patio enclosure or conservatory-like structure and include the bottom retaining wall within the new "room". So ideally, the side wall would sit on top of the wall.

I know we can't just put it on the wall and thought we could attach a support beam of some sort to the house and dig down through the planting bed to install footer "piers" of some sort to level and support the support beam.  

The side wall of conservatory would be 30'.  How many support "piers" across the 30' span will be required?  What is strongest lightest material most cost-effective support beam can be made of? The conservatory structure will be made of aluminum framing and insulated glass. 20' x 30'.  

Is it possible to dig down into the 3' planting bed between the tiers deep enough to install proper footing piers without disturbing the integrity of the the retaining wall and having to rebuild? Would have to dig the depth of 4' for wall and then another 2-3 for frost line in Ohio.

Thank you!
Terri

Answer
Terri, sounds like a great plan.  I agree that the planter is NOT structural on its own.  I assume you are planning the new outside wall to look like it rests on the outside planter wall.  A bearing beam should work out.  The real answer must come from a structural engineer as no code authority will accept anything less.  I would think that a beam 30' long with 4 piers would get the span down to less than 8' which can be handled by steel easily.  Talk to your glass man and engineer about a structural aluminum tube.  Someone needs to verify whether there might already be a footing under the planter wall.  YOu might be able to use that footing and/or it will create complications in setting piers.  It may be possible to drill the piers inside the existing flower box with haunches to carry the beam.

Water proofing and wind proofing the junction between flower box and beam will be problematic.  It may be more simple to remove the flower box and replace it with a concrete stem wall.

Let me know what an engineer says.