Help Sizing Mulled Window Unit to Match Frame Size

[DefinedDesign]

Hi all, I could really use some help dialing in the sizing for a mulled window unit in Chief Architect Premier X16.

In the plan, are two standard 36” x 60” double-hung windows next to each other and mulled. After sending the schedule to our window supplier, they returned a spec showing the actual frame size is 71.5” x 61.5” (see attached).

I’m struggling to recreate that exact frame size in Chief so the schedule and elevations match what was actually ordered, and ensure the framing is correct for Rough Opening.  I've tried adjusting each individual window size and then mull them back together - it seems like this should work but it doesn't.  Unsure of what I'm doing wrong.

 

[PitMan71]

A couple thoughts.....

1. Post your plan so we can look at.

2. It is my experience that the builder will use the rough opening size for the windows ordered which is contained in the document you posted. In a lot of cases the window folks will change the window size slightly to order a stock window rather than something custom which cost more. I used to work for a builder and that was common practice. You may even consider remove the RO information from the schedule to avoid the headache of changing them all. Juice ain't worth the squeeze, especially when it's just a couple inches here and there.

The only time I would concern myself with this kind of thing is if the windows lined up in a certain way and changing the size affected the look I wanted on a particular elevation, if that makes sense.

[tundra_dweller]

I agree with everything Shane said, but if you want to recreate the window as spec'd by the supplier:

 

1. Change the frame size on your two windows to 35.5 x 61.5.

2. Using your temp dimensions (if set to pick up opening "sides") or some other method to set a 1/2" space between your two window frames.

3. Mull the two windows.

4. Open the mulled unit and make sure your rough opening settings are at 1/2" total width clearance and 1/2" total height clearance. It shouldn't matter whether you use the "Additional Space" or "Clearance Gap" method in the rough opening setting, just that the overall clearances are correct.

 

Note: Your mulled units will not take on the rough opening settings from the windows you are mulling, for me they only seem to come in with a 1" clearance gap for width & height. If there's a default setting to change the R.O. setting for mulled units, I'm not finding it. 

 

[MPDesign]

In my experience, you need to make sure your default setting for minimum separation is set to 0. 

Then your rough opening settings are set to the Manufacturer's offset. 

 

 

 

test plan for review attached.

 

 

mulled window test.plan

[GeneDavis]

You need to know the mullion spec for that manufacturer's factory-mulled windows in that particular product line.  It might be zero, or maybe a half inch, but whatever it is it is used by you in placing and joining the windows.  Those manufacurer's sizing charts are the resource, or a quote can be used as you show.

 

If you really want to get into the weeds, you can go to the maker's site and bore in to get .pdf views or CAD downloads of mull sections.

[Doug_N]

2 hours ago, GeneDavis said:

You need to know the mullion spec for that manufacturer's factory-mulled windows in that particular product line.  It might be zero, or maybe a half inch, but whatever it is it is used by you in placing and joining the windows.  Those manufacurer's sizing charts are the resource, or a quote can be used as you show.

 

If you really want to get into the weeds, you can go to the maker's site and bore in to get .pdf views or CAD downloads of mull sections.

Hit the nail on the head Gene.  Let's hope that the manufacturer isn't a hole in the wall company that only has pictures or a gallery of their wonderful product and actually has that type of data on their website.  A lot of retrofit manufactures (cheap cheap cheap) have scant information about what they do and how it's made.  

[GeneDavis]

Looks like the @DefinedDesignpost is about Ply Gem windows.  I'd never get into the specifics of window or door opening sizes unless the job's being panelized, and I size windows (not their ROs) in whole inches, always dimensioning to centers.

[Doug_N]

1 hour ago, GeneDavis said:

Looks like the @DefinedDesignpost is about Ply Gem windows.  I'd never get into the specifics of window or door opening sizes unless the job's being panelized, and I size windows (not their ROs) in whole inches, always dimensioning to centers.

Really good advice!

[ValleyGuy]

On 7/16/2025 at 2:48 PM, DefinedDesign said:

Hi all, I could really use some help dialing in the sizing for a mulled window unit in Chief Architect Premier X16.

In the plan, are two standard 36” x 60” double-hung windows next to each other and mulled. After sending the schedule to our window supplier, they returned a spec showing the actual frame size is 71.5” x 61.5” (see attached).

I’m struggling to recreate that exact frame size in Chief so the schedule and elevations match what was actually ordered, and ensure the framing is correct for Rough Opening.  I've tried adjusting each individual window size and then mull them back together - it seems like this should work but it doesn't.  Unsure of what I'm doing wrong.

 

Lisa, it appears that you are trying to reinvent the wheel. I wouldn't spend the time to try and mess with the windows to make the exact framing. Carpenters have been using the window manufacture's supplied information to cut appropriate sized lumber for RSO's since the beginning of time. If the exact window information had to be in the plans, and I had the window manufacture's information that you are showing in the lower picture, I would put it onto a CAD sheet and mask off the info (headers, pricing, ...) that I didn't want to show and send that compiled CAD page to the layout.  You have all the exact information, including your W-04 call out, already listed. As stated above, there are many different RSO sizes from the many different suppliers, you really are flirting with danger trying to make your windows that exact. You may also find that adding in all the fractions to your dimension lines and labels may start to clutter up your plan too.