Expert: Richard Taylor, AIA - 10/2/2007

Question
Hi Richard. I need to rebuild the gate between my house and the neighbor's. I live in San Diego and want to put in a Southwestern/Santa Fe/Pueblo-style archway and gate. I have never built something like this and realize it's not an easy thing to do, but I spent many years helping my father build his house. He put an archway into adobe by building a plywood form and filling with concrete, then chicken wire and plaster over that to finish. What I was thinking of doing was this: build the main structure with cinderblocks (and mortar) but then make a plywood form for the top, but concrete seems unsafe to me and would need rebar for support right? Should I just make it out of plywood and leave it hollow, then plaster over that? Or can I use sheet metal as a form? Thanks for any input you can give me. Valerie

Answer
Hi Valerie - thanks for your question!

The steel you might put in the concrete arch would be much more for helping keep cracking under control rather than serving any structural role.  Unless you're planning on standing on top of the arch, there's really no load on it.  A concrete arch is an inherently stable structure.

A more important problem is the difference in expansion rates between different materials.  If you build part of it in block and part in concrete (or plywood, or whatever) then you'll have cracking between the two materials.  That's why your Dad built the whole thing out of concrete.

Since you're going to cover the whole thing with plaster and lath you'll be allowing for some expansion and contraction, and if done properly, you might be OK with the combination of block and concrete.

But rather than building a plywood form and filling it with 500 pounds of concrete, you could build a "centering" arch between the columns and rough out the arch shape with concrete brick mortared together.  Then remove the centering and cover the whole thing with the chix wire and plaster/stucco.  Now the whole thing's made with block and you don't have to lift a heavy arch or pour concrete up in the air.

I certainly wouldn't build any part of it with plywood - it's just not going to hold up very long and will take you just as much time to build.

Hope this helps!

Richard Taylor, AIA
www.rtastudio.com
www.rtaplans.com