Question on installing crown molding

[Jeff Monson]

I have a room and built-in's that I'm about to install 4 1/4" crown on. The crown that I purchased is MDF (I have not installed MDF crown before), I sprayed it and cleared it to match the built-in's prior to installation. Do those of you that work with MDF use angle blocks to attach the crown to? I was thinking of installing angle blocks every 24" or so, but I'm unsure if this is necessary? Looking for some tips and advice on properly attaching MDF crown.

[Garrett Ellis]

havent installed MDF crown, but i used angle blocks for just regular pine crown. just screwed them into the wall top plates.

i hate finding studs, and the ceiling joints are going to be parallel to 2 of your walls...

[Chris Padilla]

The blocks take extra time but really make the job easier if done carefully.

[Joseph Tarantino]

+1 on both preceding threads. that's hoe Nahm did it on several TOH episodes (although, with pine crown). while i've not used MDF crown, i do use a lot of MDF for painted trim. if i have any doubts about nails alone securing the trim, a little yellow glue goes a long way.

[Erik Christensen]

I have done several rooms in my house with MDF crown and helped a buddy do his entire house. Did not use angle blocks nor did we even think about where the studs are - we used a Grex 21 guage pinner with long pins shot into the sheet rock - each pin was shot as a pair and angled opposite the other pin in the pair. Crown edges were caulked and then finish painted. Never had any issues with it coming loose and some of it has been up > 4 years. to me crown is not like door/window/baseboard trim - it does not get banged by vacuum cleaners or kids or... at most a semi-annual feather duster swipe so I went with quick and easy and have yet to regret it.

[Peter Quinn]

I have done several rooms in my house with MDF crown and helped a buddy do his entire house. Did not use angle blocks nor did we even think about where the studs are - we used a Grex 21 guage pinner with long pins shot into the sheet rock - each pin was shot as a pair and angled opposite the other pin in the pair. Crown edges were caulked and then finish painted. Never had any issues with it coming loose and some of it has been up > 4 years. to me crown is not like door/window/baseboard trim - it does not get banged by vacuum cleaners or kids or... at most a semi-annual feather duster swipe so I went with quick and easy and have yet to regret it.


The nice thing about baseboard trim, and even door jambs or casing for that matter, is that it ultimately rests on the floor, so gravity has a harder time having its way with it. I haven't ever used MDF crown, but having some experience with MDF trim and MDF in general, I'd say it does't like to go unsupported for much of a span, and it can hardly hold its own weight. I'd back it up with blocks personally.

Jeff, has it been molded or was it pressed into its form? Molded MDF that has had one factory face removed during milling like regular hardwood trim is even more flexible than some of the stuff which is thermo pressed into shape and thus has a factory face. The factory face seems to add some rigidity, the milled stuff is like al dente spaghetti. My point is that it might sag its way from block to block even at 24" OC spacing of blocking, I'd be tempted to go 16" OC to avoid that, and keep the blocking pretty close to the back of the crown. I usually leave the angled blocks a light 1/4" less than the crown on hardwood crowns, I'd probably tighten that up a little to avoid the "waves" effect. Your average person might not even notice this, and in some rooms it would be hard to read, but I've seen crown installed where it sort of "waves" from nailer to nailer, like its tighter at the nailers then sags just a little in between. Best avoided IMO. Caulking the transitions shortly after install is probably a good way to insure it stays tight to the ceiling. I have installed some polyurethane foam moldings that are held with nothing but caulk, they never fall, course they don't weigh much either. Love to hear how this works out.

[Jeff Monson]

Jeff, has it been molded or was it pressed into its form? Molded MDF that has had one factory face removed during milling like regular hardwood trim is even more flexible than some of the stuff which is thermo pressed into shape and thus has a factory face. The factory face seems to add some rigidity, the milled stuff is like al dente spaghetti. My point is that it might sag its way from block to block even at 24" OC spacing of blocking, I'd be tempted to go 16" OC to avoid that, and keep the blocking pretty close to the back of the crown. I usually leave the angled blocks a light 1/4" less than the crown on hardwood crowns, I'd probably tighten that up a little to avoid the "waves" effect. Your average person might not even notice this, and in some rooms it would be hard to read, but I've seen crown installed where it sort of "waves" from nailer to nailer, like its tighter at the nailers then sags just a little in between. Best avoided IMO. Caulking the transitions shortly after install is probably a good way to insure it stays tight to the ceiling. I have installed some polyurethane foam moldings that are held with nothing but caulk, they never fall, course they don't weigh much either. Love to hear how this works out.

Great advice as usual Peter, thanks. I'd have to say it is not a pressed face?? It was primed as purchased, but 14' lengths are VERY flimsy, thus my concern. Just supporting them to get them finished required alot of supports. I think I will go with a 16" center to play it safe. I'll also try to come up with some blocks that are 1/8" or so clearance so I dont shoot right through the mdf face.

What is the best, and easiest way to attach angle blocks to the ceiling? Would a 2" pocket screw work?

[Jason Roehl]

Attach the (pre-drilled) blocks to the walls with long construction screws (NOT drywall screws). Most modern houses will have a double top plate, so you would have about 2.5" of wood you can screw into.

[Jeff Bartley]

Good advice for you Jeff, I've put up plenty of crown and can report that I always use nailing blocks. Usually I only install them parallel to the ceiling joists, on the perpendicular walls you should be able to find the 16" layout without too much trouble. Also, I just put up continuous blocks, before going out to a job I just rip some out of 2x stock.
I can add two more things to help make your job smoother: first, cope it, yes, you can cope MDF, you have to be careful but it's still easier and better than miters; and second, built a simple 'T' that will hold up one end of you crown while you're attaching the other. Disregard the 'T' if you have a helper. And one more: at the corner joints you'll have one piece that is cut square and run into the corner, the other piece will be coped......on the square piece clip the upper corner off that will be hidden after the coped piece goes in place. Clipping the corner allows you to get your finger in there to adjust the piece if necessary. Have fun with this! I love installing crown!

[Peter Quinn]

Ditto what Jason said, pre drill blocks, figure at lead 1" of screw into the studs, plus drywall thickness plus block thickness minus counter sink if any. If the house is modern double top plate life is easy, you can hit almost anywhere. If its older and balloon framed like mine, or something like a basement addition which may have irregular framing I go for the studs. If the spacing is 24" OC on exterior walls such as 2x6 framing could be, I'd run longer blocks that catch a few studs. I cut them on the BS last time. Much safer than the TS set up I had been using to hit a 38 degree spring angle.

[Larry Edgerton]

I use a block of sorts. If rip mine out of 3/4 scrap, and they are a continuation of the spring angles on the back of the trim minus about 1/4". I cut them into about 18" strips and screw them to the wall ceiling intersection. I use 2 1/2" sencos to fasten. At inside and outside corners I miter both ways. In the middle spans I skip 14" so that my strips are on nailers. Sounds complicated but it is actually pretty fast and I have good nailers so I can use a small pinner.

Larry