Why profiles on the back of molding?

[Greg Crawford]

I've been doing a remodeling job, and have been working with various moldings. I've noticed that the backs of many styles have profiles cut into them. I've seen this for years, but just recently started to wonder why. It takes extra tooling and possibly an extra pass to make it, so I know it costs more, and I can only guess at the benefit. Does anyone know the true reason why the back of molding is sometimes profiled, back-cut, relieved, etc.?

[Steve Clardy]

Theres usually a backcut there to help with reducing warpage of the molding.

[Todd Bin]

By back profile I assume you are talking about the wide groove that is cut in the back. It is usually 1/16 to 1/8 inch deep and about 3/4 of the width of the molding.

This is done so that if there is any cup in the molding the edges are still snug against the wall. If it were "flat" across the back and there was any bow in the middle then you would not be able to get the molding flat against the wall.

[Greg Crawford]

Steve and Todd,

I've thought about both of those reasons, but the molding still warps like crazy, and walls are so crooked that caulk is always necessary. While I prefer to have something that fits well, and these reasons make sense from the installer's point of view, the way big companies are going down hill and not caring about quality anymore, I thought this would be one expense they would eliminate. I guess that's why I'm not the owner of any large business, I don't have the right mindset.

Thanks.

[Jason Roehl]

Without that backcut, Greg, it would be much worse, as the trim could cup away from the wall at the top and bottom. The backcut weakens the center portion of the trim enough that this doesn't happen.

I've actually painted a few houses where the walls were straight and the drywall was good enough that our caulk lines were small (on painted trim packages we caulk EVERYTHING).

[Paul Girouard]

Steve and Todd,

I've thought about both of those reasons, but the molding still warps like crazy, and walls are so crooked that caulk is always necessary. While I prefer to have something that fits well, and these reasons make sense from the installer's point of view, the way big companies are going down hill and not caring about quality anymore, I thought this would be one expense they would eliminate.



The reasons the cuts are there have been covered so lets look at the material they are using. That's the bigger cause of the issues , all the "good " wood is either protected from cutting OR is used in something other than stock molding.

The Sheetrock issues well that just bad rockers.

Similar issues of poor wood and workmanship can be seen the door industry here in the PNW. The doors themselves are of such crap wood and the jambs , mostly finger jointed pine , some times a ply type core , one company is using plastic , Codel doors.

The Jambs the same thing finger jointed pine , the workmanship of hanging them is a joke it so bad. And the hinges are so flimsy , well that really doesn't matter the door and or jamb will rot out before the hinges wear out , on exterior doors , before the crappy hinge becomes to much of a issue.

Other than that , whats the problem :(

[Chris Padilla]

I've actually painted a few houses where the walls were straight and the drywall was good enough that our caulk lines were small (on painted trim packages we caulk EVERYTHING).

Roger that...I've learned to lay a nice bead on all trim. Sorry for the hijack, but what kind of caulk do you like, Jason? I've been using the pricey stuff from GE: Silicone II XST but then again I don't do this for a living. :)

[Prashun Patel]

The back beveling is done also to accomodate any unevenness in the wall - not just to alleviate molding warp. They backbevel mdf molding as well.

But it's not just for the consumer's benefit:

Cutting out that material allows strips of molding to be snugged together and lightens the load during shipment and hence the transportation costs.

BTW, that's why aluminum cans have the top bevel. It's not to accomodate your lip; it's to save on material costs and enable stacking...

[Jason Roehl]

Roger that...I've learned to lay a nice bead on all trim. Sorry for the hijack, but what kind of caulk do you like, Jason? I've been using the pricey stuff from GE: Silicone II XST but then again I don't do this for a living. :)

I like the Sherwin-Williams 850 Acrylic Latex Caulk. Good filling properties, not too runny or sticky. For exteriors, I use the 1100 Siliconized Acrylic Latex.

[Mike Vermeil]

The Sheetrock issues well that just bad rockers.:(

Or bad framers. A good rock job begins with framing.

[Jason Roehl]

Or bad framers. A good rock job begins with framing.

Exactly. I've seen plenty of drywall work that is not good, but it generally is just exacerbated by the bad framing.

[Chris Padilla]

And good framing begins with a good foundation!! :D

[Steve Clardy]

And good framing begins with a good foundation!! :D

Blame it all on the concrete guy. :eek:;):D

[Dan Gill]

I think it goes back to the guy who levels the site!

[Amy Leigh Baker]

Personally, I've found that architects have no idea what they're doing! :eek:

[Bob Wingard]

I once knew a guy who milled hardwood flooring .. .. .. he said the only reason he did it was to reduce weight. Their semi-trailers went up to the weight limit before being full to capacity without the back-relief. With the relief, the truck was full to capacity just before hitting the weight limit.

simple huh ?? ?? ??

[Peter Quinn]

Ditto Amy...I've seen architects design some complicated moldings on curved trim that while nice on paper were technically impossible to produce... like an Escher painting...but I digress.

The back cuts on moldings and casings (and flooring) minimize the point of contact with the substrate (wall/floor) which reduces binding and allows for seasonal wood movement. Think mini table top! Even lowly millwork manufacturers and trim carpenters understand that wood moves. Framing lumber moves considerably from dry in to finished house, and most moldings are nailed to that framing. Back cuts also allow wide casings to lay flat over less than perfect substrates. Some back bands are actually milled with a back bevel that allows them to be scribed to the finished wall like a cabinet. Nobody can build a house to the tolerances cabinetry and millwork is and was made..not now and not 100+ years ago. And even if they could...if its made of wood its going to move.

The back cuts don't stop warping. Except on very deep crown moldings with 'kerfs' cut into them the back cut is not substantial enough to change wood's inherent tendency to warp. The wood will warp if it is not properly seasoned or if the relative humidity of the environment in which it is installed is significantly different from that in which it was produced. The sapwood side of a board ALWAYS shrinks/grows more than heartwood side when exposed to changes in relative humidity. Don't believe me? Try installing moldings made in Florida in a house in Arizona!

Back cuts don't require any extra labor or tooling to produce. A moulder, be it a modern 7 head Weinig or an antique steam powered type produces a finished cut on all four surfaces in one pass, so cutting the back is a simple affair. Not back cutting is simply a mistake.

[Tom Godley]

i was told that the back bevel was done to reduce the contact area so the mld would lay flat -- or look to lay flat. This way you can "bend" the top and bottom for the edges to contact if need be. Also In a wall you often get a mismatch at the edge -- the back cut allows this to fit in the space.

If you have ever made your own mld - or used flat stock to build up a mld profile -you will quickly discover the need for the cut.

On a can you must have that fold for strength - like the fold in the roof of a many cars. I have a good friend in the beverage business-they make those cans at a rate that is unbelievable.

I use phenoseal (sp) -- I love the stuff for filling gaps before painting. Very easy to work with!

[Matthew Voss]

Even lowly...trim carpenters understand that wood moves.

Whoa...Ouch...

[Jim Becker]

That was always my understanding, Tom. The recesses help with many things including making it easire to conform the moldings to the wall...which is invariably always "not flat"...

[Paul B. Cresti]

Ditto Amy...I've seen architects design some complicated moldings on curved trim that while nice on paper were technically impossible to produce... like an Escher painting...but I digress.

.

Gee guys & gals......I take offense to the comments that us architects have no idea what we are doing....some of us architects not only design but build as well....hey Mark Singer where are you, back me up on this???
The real problem is NO ONE wants to pay for design and good workmanship! People want quantity and they say quality but at bargain basement prices!! It is a viscious cycle.

[Matthew Voss]

One addition - The relief cut on the back side was to reduce cupping when machining, not after it was hung. Removing material from the back offset the casehardening stress from machining the face.