[les digits]

My 1890 home has redwood porch columns about 13" diameter.
The wood column bases rest on raised limestone slabs that are 19" square & about 2 ½" thick.
Under the limestone square is a sheet of lead about 1/16 thick. It is trimmed about even with the 19" square, so it is not flashing.
That sheet of lead rests mostly on the sandstone porch foundation, but as the sandstone is only 12" wide, the assembly also rests 7" on the concrete porch slab, which is level with the sandstone.

What is the purpose of the lead gasket?
Any guess will do. I'm not planning any changes, but am just scratchin my noggin on the purpose of a flexible material in this location.
FYI this was, in its day a high end home, & the masonry work is generally of good quality.

thanks,
Les Digits

 

[reveivl]

Just a guess, I'm not a mason, perhaps to fill any voids between the two stone surfaces and so avoid cracking, much like thin set applied beneath cement board in a tile installation. Rich.

 

[6stringmason]

Very good guess. It could also be used as some sort of expansion joint. Where is the home located?

 

[mdshunk]

I'd guess it is to take up any imperfections to gaurd against cracking of that slab of limestone. Any high spots will "point load" great forces on that spot, and crack the limestone. You might note that the Washington Monument was similarly constructed for that same reason. The granite blocks were nearly perfectly milled, but they were still set with the thinnest amount of mortar to take up any imperfections to keep the granite blocks from cracking under any point loads.

 

[Leo G]

What is the purpose of the lead gasket?
Any guess will do. I'm not planning any changes, but am just scratchin my noggin on the purpose of a flexible material in this location.
FYI this was, in its day a high end home, & the masonry work is generally of good quality.

thanks,
Les Digits

The lead was most likely used to isolate the wood from the stone to help prevent premature rotting of the redwood columns. It also helps relieve the imperfect load distribution on the stone from the support that the columns are giving to whatever is on top of them.

 

[mdshunk]

The lead was most likely used to isolate the wood from the stone to help prevent premature rotting of the redwood columns.

That's what I thought at first, but he does say that the lead is under the limestone and not under the post.

(does redwood rot?)

 

[les digits]

6stringmason,
The home is located in Columbus Ohio

Well, the limestone slabs are each intact & still level, so whatever the reasoning of the installers, it worked out fine.

I would have thought that bedding the limestone slab into mortar would have been sufficient to evenly distribute the load. I guess that the lead, being a bit resilient, would be better if any components shifted.

Mdshunk,
While redwood is quite rot resistant, if wet, the carpenter ants will have a field day. One of the three redwood bases was almost hollow from their activity.

Thanks all for helping my scrach my noggin on this one.
les digits

 

[mdshunk]

Pleased to help, Les. I'm formerly from Columbus too (Bexley), and met my wife from there also (Reynoldsburg).

Go Buckeyes!

 

[les digits]

Mystery solved

To those that have interest in this mystery:
I went to a local architectural stone supplier to investigate the purchase of some replacement sandstone blocks.
While there I questioned the owner about the use of lead in the location stated in the initial post here.
He said that the lead installation was standard practice at the time this home was built (1890).
It served two purposes:
1)It is normally not a full sheet, but runs around the perimeter & serves as a gasket that produces a vacuum to hold the slab in place.
2)Shimming was done with lead strips to adjust the final installation towards perfectly level