How thick can mortar be?


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Old 02-16-22, 08:34 PM
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How thick can mortar be?

I will be installing a mantel (essentially a piece of wood 2.5" thick X 12" wide) over a fireplace made of stone/mortar. The top of the stone/mortar is not level such that the right side slopes down (evenly) to about 1" lower than he left. In addition the mantel due to planing (sp?) to fix warpage changes in thickness from 2.5" on the left to 2" on the right in the back. The front of the mantel thickness remains a constant 2.5". This results in the thickness of a mortar bed being 1.5" thicker on the right than on the left.

Which would be better; adding a filler piece of wood that takes up some of the thickness on the right (inset so it would be completely surrounded by mortar for aesthetics) or just having my bed of mortar go from .5" on the left to 2" on the right?

 
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Old 02-17-22, 01:18 AM
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Concrete (mortar) is routinely poured much thicker, think driveways, walkways, shower pans, foundations so that is not an issue.

A picture of the project would help but based on your description either would work.
 
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Old 02-17-22, 12:37 PM
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The bottom of the tape line is level and would be even with the top of the mantle. At this position it would allow 1/2" of mortar on the left hand side.

Good point about concrete being thick, for some reason that didn't occur to me. I'm just so used to seeing "mortar" being around 1/2" thick I was concerned about it cracking if it wound up being 1"+ thick. I guess mortar would have the same adequate compressive strength? In addition, the mantle weighs at most 25lbs.



 
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Old 02-17-22, 01:19 PM
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You might want to add in some stone/concrete pieces to shim the mantel so it doesn't sink on the low side.
 
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Old 02-17-22, 02:00 PM
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Are you talking about when the mortar is wet, under gravity? I do have some nails on the underside of the wood that are set at a depth so that they will assist it setting the mantel to the proper level. Is this what you are referring to?
 
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Old 02-17-22, 03:52 PM
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I'm just so used to seeing "mortar" being around 1/2" thick
And that is the norm because that is what most use it for!

Only question, are you "forming" the mortar bed, IE what will you do to hold it in place?
 
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Old 02-17-22, 05:53 PM
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what will you do to hold it in place?
I'm hoping to make the mortar a bit dryer than normal, apply it, then settle the mantle down on the mortar kinda squeezing it out. Then with the trowel scrape off the excess. If necessary, I can kind of tool it with a mortar joint tool?

Does this sound reasonable? I've done concrete in the past, this will be my first attempt w/mortar.
 
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Old 02-18-22, 12:48 AM
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Does this sound reasonable?
Yep, will the stone be sealed?

Only though is that as your working the mortar bed you might get some on the stone face, it unsealed cleanup is going to be a pain especially if the material is pours. At a min cover with some blue tape to keep as much off!
 
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Old 02-18-22, 09:14 AM
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How much if any will the mantel be cantilevered beyond the face of the stone? In addition to making the mortar pretty stiff I would tape plastic reaching the bottom. Tape it just on the front edge over the top just enough to hold the plastic in place. Then when the mortar has set cut the plastic and tape. You can never protect the stone too much.
One more thing. Spread a tight coat of mortar onto the stone top to bond the mortar to the top of the stone then lay the full thickness of mortar and set he mantel while that bond coat is still wet.
 
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Old 02-18-22, 12:57 PM
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RE: Cantilevered : The fireplace has 3 stone sort of corbels which support the cantilever. The mantel extends maybe 2" beyond them You can kinda see them in the pics I posted.

will the stone be sealed? / You can never protect the stone too much.
I planned on doing what you suggested. I'll run a 2" line of painters tape around the top edge to really protect where the mortar meets the stone. Then use some of my masking plastic (tape/w 4' pull down plastic sheeting) to cover the face from that painters tape on down. In addition, I'll have plastic covering the hearth and floor area.

tight coat / bond coat
If I understand what you mean here: I should apply a "thin" coat of mortar first to ensure there is a good mechanical bond between the mortar and the stone, then add the additional mortar to the needed thickness?

I was also planning on "spritzing" a light coat of water from a handheld sprayer onto the top to prevent the mortar moisture from being sucked out too quick?
 
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Old 02-18-22, 01:45 PM
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I was also planning on "spritzing" a light coat of water
Only needed when it's really hot and stone sucks up moisture but not really going to hurt anything.

Spritzing, yes a good choice vs spraying, HAHAHAHA
 
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Old 02-23-22, 05:29 PM
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Since i am a portland guy, i would suggest you use atleast type N mortar. Even if it is indoors. Good luck.
 
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Old 02-23-22, 05:30 PM
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Spritzing, yes a good choice vs spraying, HAHAHAHA
I do that when i am smoking. LOL.
 
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Old 02-23-22, 05:49 PM
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If I understand what you mean here: I should apply a "thin" coat of mortar first to ensure there is a good mechanical bond between the mortar and the stone, then add the additional mortar to the needed thickness?
You want your new mortar to stick to the stone? Easy. The best and oldest method is to mix portland cement into a paint. Wet the stone first. When wet, apply mixed portland onto the stone. Again, wit the wet portland, place mortar on top of stone. The new mortar will bond to the stone. This is the oldest method. I do it. It works. And i do it outdoors. It's holding up well in freezing temp. Good luck.
 
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Old 09-09-22, 08:15 PM
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Closing this out, I'm pretty happy with the result. Thanks everyone.



 
 

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