Curved Crown moulding/Radii calculation
I have the unique challenge of installing crown mouldings on some curved walls I built for a client last year, the challenge lies in calculating the radius of the walls…I’m too old to remember how and too embarrassed to ask my 10 year old. The other thing (more importantly)is I will need a source for curved crown mouldings just a fairly standard profile which has the wood lenghts most of us are used to to match the other straight walls.Ideally a 4-1/4″ profile and any hints from those who have been there would help. As a last resort I may look at making my own profile blank and building up with plaster(as seen in an old issue) as I suspect the cost and necessary accuracy may be a deterent to all involved. Any help/advice would be great. thanks. Larry Sivell
Replies
This usually gets asked in terms of an arch, so I'll use the terms I'm familiar with. W = the width of the arch, H = the height of the arch at the midpoint.
R = radius = ((W / 2) squared - H squared) / 2H
The derivation follows, if you're interested. I can't ever remember the formula, but I do remember how to derive it, so that's just as good.
A radius from the center to one end of the arch is the hypotenuse of a right triangle. One of the other two sides is half the width of the arch. The third side is part of a radius that passes through the midpoint of the arch, specifically that part between the center and the chord, or R - H. By the Pythagorean theorem,
R squared = (W / 2) squared + (R - H) squared
Multiply (R - H) times (R - H)
R squared = (W / 2) squared + R squared - 2RH + H squared
Subtract R squared from both sides
0 = (W / 2) squared - 2RH + H squared
Add 2RH to both sides
2RH = (W / 2) squared + H squared
Divide both sides by 2H
R = ((W / 2) squared + H squared) / 2H
Edited 1/10/2003 10:39:20 PM ET by Uncle Dunc
thanks uncle dunc, that's the formula I was trying to figure out...now from what I'm seeing and hearing the cost will be the next biggest problem...so far $19.00 a running foot and all preformed radii... I'll need several different arcs and all totalling 60 lineal ft...At $1200.oo for crown moulding I may attempt the plaster routine, messy, and slow but very interesting to try. thanks again. Larry sivell
You ain't doing radiused crown for only 19/ft unless you love working for pleasure.
Excellence is its own reward!
Just for the record it's $19.95. a foot.plus taxes and delivery, that doesn't include my time or materials, I should also add this is a paint grade crown not a stain grade material. I get the impression that I should be sharpening ny pencil but I see it as a little steep in cost. Thank you all for your insights. Larry Sivell
Sorry, I thopught you were trying to call that an installed price.
Excellence is its own reward!
I don't know if it is a cardinal sin to refer to something Norm and Tommy did on TOH but they used a bendable plastic moulding or maybe it was PVC. Bottom line Tommy said it was near impossible to wrap a crown moulding around a curve. Even cutting relief cuts on the back won't work because of the angle of the crown.
Bill
I would suggest getting the curved (flexible) crown and also enough straight (flexible) to finish a complete wall. Tie into standard (wooden) crown at an inside or outside corner, the profiles can be made to look right there.
If you have a wall that runs straight for 4', curves for 9', then runs straight for 7' get 10' curved for your radius and 12' for your straight walls. other wise the joints can make the job look terrible.
I think this is the same answer as Unc's, but there's a graphic to click on, which might help.
Can't help you with the curved crown profile supplier----- unless you're willing to dig deep into your wallet and hand over lots of folding green ones for me to custom make it, ha, ha. Slainte.
try that post on knots
http://forums.taunton.com/tp-knots/messages/?start=Start+Reading+%3E%3E
recently trimmed a high end home with all oak, which had 3barrell ceilings and 3 curved walls. customer insisted on having crown thru-out. we made templates out 1/2" plywood to get radius. traced radius onto floor and screwed blocks to conform to scribe line on floor. we than took a section of existing crown and broke into 1/4" sections as it would appear in profile ie: (section), on a piece of paper. measured the vertical distance and added 3/4" to each measurment. ripped oak to 1/4" thick and measurments determined by sections. used small paint roller and rolled glue onto slats and laminated together clamped to blocks on floor. allowed 2 days for drying in form. used angle grinder to rough out stepped laminate and then proceed with card scrapers to get final shape. allow for extra length on each end. end result was very accurate and job came out beautiful, however it was also very time consuming. fortunately it was a T&M.
try this link http://www.flexmoulding.com/catalog/products.pl?area=east&catcode=12&subcatcode=30 flexable and can be faux to look like wood.
Try
flexiblemoulding.com
resinart.com
flextrim.com
They all manufacture flexible polyester resin moulding products. Crown is a special order based on your particular pattern, inside radius, outside radius etc. Look for an article in the Jourrnal of Light Construction this spring.
Ross
Thanks Ross..i'll have a look...and keep my eyes open. L Sivell