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half round window framing

glsstep's picture

I am looking for some pictures of framing techniques used for framing in a half round window.  I know that the R.O. is framed  to a square or rectangle shape then filled in with plywood & filler pieces but I am looking for some pointers from those of you who have done this before and may have learned a couple tricks or have some recommendations. 

The window I will be installing will be a three window unit.  Center window is a single hung half round top with the flankers about 2/3 the height of the center window. 

Maybe I can post a picture if needed. 

Thanks in advance for the help.

(post #102876, reply #1 of 9)


cut 2 pieces of 1/2" plywood to fit the RO at the arch section 

cut the arch in the plywood to match the window

fasten the plywood flush with the wall on each side using nailers set back 1/2" against the header and arch portion of the jamb sides

cut short blocks and install them inside the plywood at the arch, with approximately 1" spacing between blocks

We recently covered several with stucco outside and drywall inside...otherwise I could send a pic 



(post #102876, reply #2 of 9)

I always raise my headers an extra inch and 3/4's to allow for the plywood arch like tx. described. I learned it to be easier to cut and assemble the arch then pop it in before setting the windows, and it leaves a better reveal when sheetrocked than the hack style add some blocks after the fact...


(post #102876, reply #3 of 9)

Heres a trick I taught myself:

Lay the window down on your sheet of Plywood and then trace around it with a pencil and a washer for foundation bolts.  They are the perfect size for a 1/2" bigger opening than the window.  Then cut out your pattern. 

Take your pattern and tack it to another sheet and cut it out with a router and a straight bit with a bearing.  

Then cut your filler blocks and nail them in.  Staple a piece of tyvek or insulmesh so insulation can be blown in and it won't fall out the bottom. 

Clear as mud?



Matt- Woods favorite carpenter. 

(post #102876, reply #4 of 9)

Your best bet is to install the window and then scribe the arc. Sometimes the window dimension will not match the actual window. 

(post #102876, reply #5 of 9)

One inside piece and one outside piece of 3/4" ply cut to fill between arch and opening.  Along the sides and top rip a 4" wide 2x for a ply nailer.

Nail the outer skin to the 2x nailers, add two layers of 2" rigid foam in the center and nail the inner ply.   You'll have about R-20, which is better than a stick in the eye.

Put a few clamps on and make sure everything is flat prior to screwing the inner and outter ply together. 

In the center of mass and along arch edge, run 6" deck screws through to hold the two sides together and firm it up, with the heads countersunk on the inside.  Slightly bend over the tip going through the outer ply a few times until it breaks off flush.

If the whole works is slightly thicker than the studs an electric hand plane quickly and easily takes a little off the foam.



Beer was created so carpenters wouldn't rule the world.

If I could edit my location it would say I'm now in Reno :-)

(post #102876, reply #6 of 9)

Keep the ideas coming.  I like to hear the different approaches.  I think I may combine a couple of them and then plan my attack.  Anyone with pictures?  Thanks again.   

(post #102876, reply #7 of 9)

I have a picture of a non structural wall. The headers are smaller than you may need. I didn't frame this, I'm the finish carpenter. The framer used some 2x material and cut it to match the arch of the window. This isn't necessary. The 2x could have just been left straight. Due to the size of these windows, a couple of 2x's could have been cut and placed on angles rather than one on a 45. You need nailing for the sheetrock and the framing should be flush. You don't need a lot of continuous nailing for the trim. For the finish installer, keeping things flush is important. The top arched window was installed about 3/16" off from the one below and made the trim install a bit more difficult.

A couple of points of interest on this house. Boards were used as sheathing on the 2x6 walls. This meant that the factory extension jambs did not fit. They are made for 7/16" sheathing. Some of the corner blocking pieces in the framing had warped. Because the framers made the arched cut out of wide material, the ends were delicate and split. Many parts of those angled framing pieces didn't stay flush with the framing and they were very difficult to toe nail/screw back in. Cutting an arched shape from wide material left weak grain that would split. There were 16 arch top windows in this place in 8 different sizes. This, along with the extensions not fitting, and making custom casings, drove the labor costs up, unnecessarily. The arched windows also made for a lot of extra work on the exterior trim as well as fitting flashing and siding.

The framers went to a lot of trouble cutting the 2x12's on the arch. Unfortunately, this caused more problems than it solved. The ends just couldn't be fastened without splitting apart. You don't have to get fancy, just close the gap in a few key places, you are just looking for some nailing, not structure.

Beat it to fit / Paint it to match

Beat it to fit / Paint it to match

(post #102876, reply #8 of 9)

It sure looks like timber framers framed those windows! *chuckle*

Although they are cut a little rough for any timberframers I've known. 


Beer was created so carpenters wouldn't rule the world.

If I could edit my location it would say I'm now in Reno :-)

(post #102876, reply #9 of 9)

You're right, Don. It was the timberframers. Most of their work was in barns and a lot of that was repair. They were not used to sticks and nails. It was a different story with the big stuff and mortice and tenons.

Beat it to fit / Paint it to match

Beat it to fit / Paint it to match