Compare/Contrast: Morter Beds & Plaster
RichBeckman
| Posted in General Discussion on
It occurs to me that I frequently see posts that mention the dead flat surface of a morter bed. I have also seen several posts that talk about drywall looking out of place in an older home because it is flatter than the plaster.
Why, in general, is one dead flat but not the other? Or are the above mentioned posts misleading?
Thanks,
Rich Beckman
Replies
Good question.
Dry pack mortar beds for tile, must be dead flat because tile is dead flat, especially the new stone tiles, which have no shear strength.
Plaster does not have to be flat, because nothing other than paint is placed on it. Indeed, that is why guys skim coat blue board to make it look like an old time plaster job, with slight bumps and valleys. It is a sign of quality.
You will note the difference if you have ever tried to lay tile over a plaster wall. It is almost impossible, and looks really funky.
Boris
"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1927
You will note the difference if you have ever tried to lay tile over a plaster wall. It is almost impossible, and looks really funky.
Boris:
Thank you. The above is a great project, that I would never have thought of, and whose finished look will befit my screen name. My 1860's farmhouse displays nice hills and valleys on its plaster and lath walls. Think I'll hit the master bedroom this afternoon and throw up some 12" green tiles. Should kinda look like a topo of the similarly named mountains in Vermont, and I'm sure my wife will enjoy the "really funky" look...
Regards,
Rework
>> ... stone tiles, which have no shear strength.
Do you mean bending strength? I can't see how tile could be subject to shear loading.
Boris,
So you're saying the only reason plaster isn't flat is because it doesn't have to be. If someone wanted it to be dead flat there isn't any reason other than the competence (or lack thereof) of the installer to prevent that.
Is this correct?
Rich Beckman
If someone skilled wants plaster--even ol' tyme 3-coat plaster--to be flat, then they can get it as flat and slick as any concrete I've seen anywhere. I can't answer your original question, but slicked plaster is really, really smooth.
Take a tile and place it over a hollow area and step on it.
Take a piece of natural stone, like marble or granite and do the same.
Believe it or not, the natural stone will break easier than hardened and fired ceramic tile. Stone has no shear strength.
And yes, plaster can be dead flat but rarely is. It is not applied with the same methods, so it can not be.
Wall mud is applied to the wall over diamond lathe using float strips, which are made plumb to gravity and hopefully straight to eachother, and even more hopefully parallel to the opposite wall, and finally at a right angle to the adjacent wall. Once the floats are installed, the tile setter fills in the gaps with mud and a screed, and the wall is flat, plumb and square. The setter waits about 30 minutes and using a flat steel straight edge scrapes the highs and lows one more time, because the mud may have slumped. Wall mud must be dead flat so dead flat tiles are installed over it.
Old time plaster was set over wavey studs and only the scratch coat was screed in a similar fashion as described above. Neither the brown coat, nor the finish coat were checked for flatness. It only has to look flat, not be flat, because nothing goes on to the wall except paint. For fun, take a 6 foot straight edge or level and and place it on a wall and you will see.
Presumably, the plasterer could also screed the brown coat. The finish coat being so thin, might not have to be screed.
On blue board jobs, only a veneer coat of plaster is added to the gypsun, and assuming the gyp is installed correctly, the finish coat is very flat. Many plasters I have seen intentionally make dips and waves to make it appear as though the plaster is not a veneer product.
Regards,
Boris
"Sir, I may be drunk, but you're crazy, and I'll be sober tomorrow" -- WC Fields, "Its a Gift" 1927
In the couple of old house that I have owned the plaster is not flat (three coat on wood lath). The plaster does hide the imperfection in the framing and is flat to the grounds at the door and window jambs/heads. Plaster is much harder that drywall. I believe that this is the major benefit along with the issue that you will not see the joints on long or tall walls painted with semi-gloss or walls with raking light since it is a continuos surface. You can also skim coat drywall and get a relatively smooth surface, but plaster is harder.
I owned a circa 1901 house with lathe and horse-hair plaster (some over a brick wall, too) and that was so flat and smooth the (original?) occupants had wallpapered it and you couldn't see a single imperfection. Real old wallpaper. Not a peak or valley or wobble in the wall.
My plaster had horse hair too, but the walls were not flat. 1/8" - 1/4" devation in 8'. During demo found a piece of framing that the carp. had written "got drunk last night and am getting drunk tonight". I still have that piece of wood. My walls were also covered with canvas , a common method here to help prevent minor cracks.
Was the canvas on top of the plaster, like wallpaper, or what?
Yes, on top just like wall paper. I have seen it on all three of the houses that I have bought (1929-1931 era). You can still buy it and HD carries it. It's kind of cool to rip the canvas off. The paint flies off and leaves you holding a piece of clean canvas (watch out for lead of coarse). If there was movement in the house the it shows up in the inside corners. The canvas pulls away from the plaster and it looks like a rope was taped into the corners.
Thanks to all, and especially to Boris. I appreciate the time and effort to answer.
Rich Beckman