undercutting stone fireplace

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  #1  
Old 03-29-11, 07:54 AM
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undercutting stone fireplace

I have a stone fireplace that I'm going to be undercutting for a solid hardwood floor installation. The fireplace is pretty jagged, as you can see in the pictures. I'm curious what sort of problem I will have doing this. I was planning on renting an electric jamb saw with a mason blade or diamond blade (I forget which one is best). However, I'm not sure the blade will be able to cut "far enough" into the mortar that is set back from the front of the fireplace. Any ideas on whether the jamb saw will even be able to handle this, and if not what I can do about it?





It's a bit hard to see the amount the mortar is set back from the pictures -- hopefull the carpet tackless strip will give you an idea of the scale.
 
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Old 03-29-11, 02:24 PM
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If all you have at the bottom is mortar, your idea may work, although I have never tried it. I always scribe my hardwood to the aberrations of the stone. If you have stone at the bottom, you may find it difficult to do with a jamb saw. A grinder may help, but those boogers are tough to grind
 
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Old 03-31-11, 05:00 PM
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My installers have done this and it worked. I am pretty sure they used an undercut saw with a masonry blade as you described and then chiseled out a channel. You will still have to scribe your planks though. What you are looking to achieve is an undercut about 3/8-1/2" deep that follows the contour of the stone. That way, your scribed pieces will fit very slightly under the stone while still maintaining room for expansion without undermining the strength of the masonry. It's tedious work, but gives a much more seamless look.
 
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Old 04-01-11, 04:21 AM
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Originally Posted by staabc View Post
My installers have done this and it worked. I am pretty sure they used an undercut saw with a masonry blade as you described and then chiseled out a channel. You will still have to scribe your planks though. What you are looking to achieve is an undercut about 3/8-1/2" deep that follows the contour of the stone. That way, your scribed pieces will fit very slightly under the stone while still maintaining room for expansion without undermining the strength of the masonry. It's tedious work, but gives a much more seamless look.
I'm curious why you're scribing your planks still? The ends aren't going to be visible -- just slipped underneath the fireplace. Am I misunderstanding you? Or do you mean that since the fireplace is so irregular, that the undercutting is going to be irregular, so I'll have to scribe the planks to "conform" to that irregularity?
 
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Old 04-01-11, 11:23 AM
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Yeah, your fireplace looks like it varies by up to 1-11/2 inches. In order to lay straight planks, the farthest point would have to be undercut by maybe 2 inches. This would be a serious pain, especially if you have to cut some of the stone as opposed to just the mortar. Not to mention the fact that you could end up compromising the strength of the stone. Since wood is easier to cut than stone or mortar, minimize the amount of stone or mortar you need to cut and shape your wood pieces to fit.
 
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Old 04-01-11, 12:37 PM
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The fireplace is what it is. I also agree with staabc that you can end up with a very good result just by scribing and fitting the wood. I have done this a few times, and it is surprising how good it can turn out with a little patience.
 
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Old 04-01-11, 05:49 PM
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Easiest way to scribe if you are working toward the fireplace is to lay but not nail your next to last row of hardwood across the front of the fireplace, just as if it was to be installed. Then you can scribe the irregularities taking into consideration the total width you will need to go under the rock. Remove them, cut them, install more wood in the space you just vacated , then install the scribed pieces. You may have to angle up the next to last piece in order to get the last piece in, so you will face nail both of them.
 
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Old 04-10-11, 04:38 AM
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Thanks for all the replies everyone. In the event that I decide to scribe the planks (without any sort of undercutting), do people still use some amount of filler between the plank and the stone, or can scribing get it perfect enough that there's no need for any filler. Obviously, scribing is tricky, but is there a good way of finishing the imperfections to make it look nice. (Of course, if scribing is way off, then I'd just grab another plank and try again -- I'm just talking about a small amount of imperfection.)

I'm aware of Timermate wood filler, which can be stained, etc. Apparently Minwax makes some sort of filler in a pencil form, but I'm not sure about how well that works for this type of application.

Oh, here's one more question: the direction of the planks would butt into the fireplace face. In that situation, most wood expansion would be across the grain, and little would be in the direction of the fireplace, so there's not very much concern about there being no expansion space. If I wanted to have a border around the fireplace, 2 planks for example, with the rest of the room just butting into these two planks (and running parrellel on the sides of the fireplace), now I have the expansion taking place across the grain and into the face of the fireplace (where there's no space for expansion). Is that going to be a problem with scribing?
 
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Old 04-10-11, 01:23 PM
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I think if the main body of the floor is running perpendicular to the face of the fireplace, you probably won't have a problem with just scribing your border to the fireplace. Don't worry about cross grain expansion on the border if you are only using two boards. Of course this depends on how large the room is. Even though most of the expansion in hardwood occurs cross grain, if you have a very long run up to the fireplace, say 20-30 feet, you may still experience some expansion along the grain. Now that I've covered my butt, if this is a normal sized room, 12-18 feet wide, I don't think expansion along the grain will be enough to cause you problems. Last word about undercutting: You undercut for two reasons; to eliminate or minimize your expansion gap, and to make scribing easier. Every part of the face you can undercut, or at least chisel out a channel, will make allow you to be less exact with your scribing. This both makes scribing less tedious and limits the amount of visible gap. Regarding filling the gap: If you're bumping direct to the face, I would try to leave an 1/8" or so even though we've determined that expansion shouldn't be a problem. Better safe than sorry. I would try filling this with caulk that matches the color of the mortar, not the wood. Get a grout color chart and pick the color that's closest. Grout makers also supply caulk in matching colors.
 
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Old 04-11-11, 06:55 AM
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Make sure also that your hardwood comes into the house 2 or 3 days before you drop it down to ensure that it has time to adjust to the humidity/moisture level of that in your house. This will help in reducing future contraction and expansion early on at least. If you really feel you need to fill any gaps after scribing and installing a colored flexibile caulking is your best bet in reducing any likelihood of it falling out later. I don't know if the idea of using a parging cement mixture with a modifying bonding agent in it, like a 'top n bond' mix, then masking the floor and extending the fireplace mortor within 1/8 inch would work but it might also. Just another way of thinking about things. It might be possible to extend the mortor where you need to and point any excess with a damp sponge with the masking tape on to protect your new floor.
 
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