Better Living Through Blankstock
By Jim Parodi
Jim Parodi is a second-generation paperhanger based in Cornwall-on-Hudson, NY. An NGPP member since 1987, Parodi is a member of The Bergen County Mastercraftsman Paint and Paper Association in the suburbs of New York City.
Have you ever looked up at the ceiling in a modern building with exposed wooden beams? Nowadays, more likely than not, you will see 1 foot wide beams which are actually made up of 1 or 2 inch wide wooden planks with seams offset from each other and laminated together by adhesive. Fat, laminated beams held together by glue are found in ski lodges, restaurants and malls. They accept tons of roof weight and are designed to handle the weight of additional tons of snow in northern climates.
Cheap modern construction you say? Actually, these beams are more trustworthy than the old "one piece" types which may have had hidden stress lines within them. With the newer beams, every cubic inch can now be examined before they are assembled. Even pre-fab truss beams are now made out of plywood-like material and these humble materials too, take on substantial weight and building stress.
Plywood itself is an amazing thing. Thin sheets of weak wood are mated together, with each successive sheet's grain direction counterpoised 90 degrees and adhered together so that this "weak" wood is now able to withstand heavy furniture as flooring material, even hurricane winds.
So what's this got to do with wallpapering?
Well, every time you use blankstock lining, in terms of strength, you
are taking advantage of this "multiplier effect" by laminating two
sheets of wallcovering together with an adhesive.
If you hang a piece of blankstock vertically, since it is made of paper, it will be subject to the drying and shrinking forces that any fiber product will be susceptible to, like cotton jeans after the first washing. By observation when seams pop, it looks like that the paper will try to shrink in from the seams towards the center of the sheet. This makes sense, when you take into account the manufacturing process of the paper. For years, papermakers and paperhangers have noted that the paper has a directional "grain" to it-just like wood. And just like wood, shrinkage occurs to a greater extent laterally across the grain rather than lengthwise vertically. But with both materials, it is understood that shrinkage occurs everywhere to different degrees and for this reason the observation that shrinkage starts at the seam and only travels in one direction is flawed.
To illustrate (without an illustration), imagine that the entire sheet of paper is a grid of 1" squares separated by a ½" space between them on all sides. Let's say that after drying, each ½" space wants to be 1/64th of an inch narrower (of course, the 1" squares , since they are made of paper too, also want to be shorter and narrower, but for the moment let's just say they remain a constant 1' square. Also for the moment let's leave out the shrinkage from top to bottom).
The vertical column of 1" squares directly down the center of the
sheet wants the column to the left and right of it 1/64th of an inch
closer to it. In fact, each vertical column of squares wants the
neighboring left and right vertical columns 1/64th of an inch closer to
it. No shrinkage movement can occur, however, due to the strength of the
wallpaper surrounding each square.
But when we get to the vertical columns running down the left and right seams there is an opportunity for weakness if the paint film there does not have the adhesive strength to the drywall which can withstand this force. If the paint lets go, the seam pops and travels toward the center.
However, if you took a blade and cut the same sheet of paper down the middle from floor to ceiling, you could start the paper shrinking in the other direction. The new weak seam in the center would start to shrink outward towards the left and right seam if the underlying paint let it do so.
Even after it is dry, the paper still exerts a tension on the painted surface and seems to maintain this "pull" over time for the duration of the installation. In other words, it seems that the paper never tires or becomes like a weak spring. But what happens when you apply another paper over the blankstock, engineering the room so that the pattern seams fall in the middle of the blankstock seams? The pattern paper wants to shrink all over, too. But instead of finding a weak toehold at its seam, it finds an incredibly strong substrate which cannot be laterally pulled apart. Now you have effectively achieved this "lamination effect" and have put the two paper's shrinkage tendencies into a dynamic tension. (If you have any doubt about the tremendous force required to laterally tear paper, take a sheet of type writer paper where you pull on one end and someone else pulls on the other tug of war style. If you put even pressure at each end you will see how incredibly strong this ordinary material can be.)
What this also means is that if the blankstock paper wants to shrink, it is effectively "nailed down" by the entire surface area of the adhered paper on top of it. It can't shrink at the seam or anywhere else* because the force required to pull apart the overlying paper would be tremendous, too. The overlying pattern sheet is "all-over" adhered and laminated to the blankstock so that the blankstock seaming throughout the room is now "capped" and immobile. This real benefit is that now there can be no more stress on the underlying paint or primer film at the seams because there is now zero tension on that paint film-the tension now being controlled by the overlying sheet. This is the key to understanding why someone would recommend blankstock where the painted surface is suspect.
* The asterix above is for the areas of the room that are not in dynamic tension. By mating the blankstock and pattern paper in this way you have actually manufactured a giant, seamless new piece of wallpaper. But this giant piece that covers the whole room still remains a paper product and still wants to shrink where there is no counter force to stop it. Like a regular piece of normal width wallpaper, it will find weakness at its edges, so that the cuts in this giant wallpaper piece at windows, doors, ceiling and baseboard are the places that could shrink if given the chance.
Since both overlying and underlying sheets have been lopped off in exactly the same place in these areas, they can both pull in tandem and shrink loose from the wall around these fixtures. The same thing could happen if, by bad room engineering, one of the overlying seams landed smack on top of an underlying blankstock seam.
Often paperhangers are taught to cut the blankstock a ¼ inch short of windows, doors, etc. and most times it is successful. This can't be due to dynamic tension-since there isn't any at these points-rather, I think it has more to do with the extra layers of better quality paint found around these fixtures where the painter went an inch or two onto the wall with trim paint (instead of garbage flat). Luck could have something to do with it, too. I don't know. But, if you are in the habit of painting the trim with this 1" or 2" wide band onto the wall using a VOC alkyd and then papering without prep-coating that band of paint, it may be time to re-think that knowing what forces are at work.
So how can a blankstock installation fail?
Gravity or adhesive failure.
If for some reason the blankstock was hung on chalky paint or powdery joint compound or plaster then the whole laminated mess can fall DOWNWARD like a lead balloon due to the force of gravity and not lateral stress. In this case everything would be brought down by it's own weight. I have only heard of this happening twice in 23 years of hanging-but there's always a next time. An X test should be performed before the liner goes up.
Besides gravity, failure could occur if the adhesive holding the liner to the wall is weak or the surface wasn't properly prepped, causing a garden variety adhesion failure due to water leaks, high humidity, etc.
And what about the recommendation to cross line, i.e. hanging blankstock horizontally or what for years has been called "railroading"?
Do it if it makes you feel better. But it seems there is a growing segment of the paperhanging world that lines vertically with proper room engineering. The whole appeal of cross lining is that you will never make the boo-boo of having a seam fall on top of another.
As far as per/square/inch tension is concerned, it doesn't really make a difference. However, getting back to the grain direction of paper, it would appear that railroading the underlying paper would turn the greatest amount of potential lateral shrinkage 90 degrees on it's side making the paper pull hardest downward towards the floor at the ceiling, and upwards towards the ceiling at the baseboard.
Also, be aware that with railroading, unless your underlying blankstock seams are excellent, there is a chance of seeing long horizontal lines in the wrong lighting. It's true that if you have a less than perfect vertical seam that that may show too, but my very subjective opinion is that a vertical underlying line isn't as bad, because it echoes the vertical lines of the pattern paper. Whatever your opinion, the trick is to get good invisible blankstock seams.
On a final note, this should go without saying, but let me say it again anyway, blankstock is blank paper and not the synthetic white spun poly used for bridging purposes.