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How to Mull a Picture Window Between Two Casements (DIY Guide)

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How to Mull a Picture Window Between Two Casements (DIY Guide)

Field-mulling three vinyl units—Left Casement + Center Picture + Right Casement—doesn’t have to be guesswork. This guide shows exactly which flanges to remove, where the screws go, and how to seal the joints for a clean, durable install.

Overhead photoreal bench scene: left casement, center picture, right casement staged with mull strips, flange cuts, sealant bead, and screw direction tags.
Field mulling staged on the bench. Note the flange cuts, gray plastic mull strips, sealant beads, and screw direction.

What You’ll Need

  • Drill/driver with #2 bit (Phillips or square)
  • Utility knife or tin snips (trim vinyl flanges)
  • Soft blocks/padded surface, clamps
  • Plastic mull strips/spacers (usually included in box)
  • Exterior-grade sealant (OSI QUAD Max / SikaFlex), backer rod
  • Low-expansion window & door foam
  • Shims, level, tape measure
  • Gloves & eye protection
Label first: “L” = left casement, “C” = center picture, “R” = right casement (viewed from exterior).

Part 1 — Prep & De-Flange

  1. Dry-fit & mark. Lay L, C, R face-up, bottoms aligned. Confirm rough opening (RO) fits the combined width plus shim/foam tolerance.
  2. Expose the mull screw path. Open both casements and remove the latch/keeper bars and any trim covering the side stiles. Bag and label hardware for re-install.
  3. Remove the correct flanges.
    • Picture (C): cut off both side nailing flanges; keep head & sill flanges.
    • Casements (L & R): cut off only the inside flanges (faces toward the picture); keep the outer, head & sill flanges.
    This exposes the casement pre-drilled holes you’ll screw through.

Part 2 — Mull on the Bench (Recommended)

  1. Stage with spacers. Cut plastic mull strips to full frame height. Run a continuous bead of exterior sealant on both mating faces. Place strips between L↔C and C↔R.
  2. Screw the units together. From the bottom up, drive screws through the casement’s pre-drilled holes → mull strip → into the picture frame, spacing 8–12″ OC, first/last screw ~2″ from ends. Keep bottoms flush and faces co-planar.
  3. Seal the mull joints. Tool a neat exterior bead along both vertical seams. Add backer rod if the gap > ¼″. Install mull caps if included.
  4. Re-install latch hardware. Verify both casements open/close and locks clear.

Part 3 — Set the Assembly in the Opening

  1. Sill pan & flashing. Install a pan or self-adhesive flashing with an upturned interior leg. Shim sill to dead level.
  2. Place & plumb. Lift assembly to RO (use a helper). Center, level, plumb, and confirm square (equal diagonals/even reveals).
  3. Fasten per manufacturer. Use remaining outer flanges plus head/sill flanges and/or approved frame screws. Shim at hinge/lock points.
  4. Insulate & weatherproof. Lightly foam gaps with window & door low-expansion foam. Tape and flash flanges into WRB—head flashing last. Caulk inside after trims.
  5. Final checks. Operate both sashes, confirm locks, reinstall screens, clean excess sealant.
Technical diagram labeling which flanges to remove, mull strip location, screw direction, and spacing 8–12 inches on-center.
At-a-glance diagram: flange removal, mull strip placement, and screw path/spacing.

At-a-Glance Checklist

  • Remove latch bars on casements
  • Cut flanges: both sides on picture (C); inside only on casements (L/R)
  • Sealant on both mating faces; install plastic mull strips
  • Screws 8–12″ OC, first/last ~2″ from ends, bottom → top
  • Cap & seal mull joints
  • Set assembly, level/plumb/square, fasten & flash
  • Foam lightly; verify smooth operation & lockup

Alternate: Mull In the Opening

  1. Prep RO (sill pan, shims).
  2. Set the picture window (C) first, perfect level & plumb.
  3. Sealant + mull strip on one side; bring in L; screw through L’s pre-drilled holes into C (bottom→top). Shim/fasten L’s outer flange.
  4. Repeat for R. Cap & seal, then finish flashing/foam/trim.
Safety & Warranty: Field mulling and flange cuts may affect warranty—check your brand’s instructions. Use low-expansion foam only to avoid bowing frames.

FAQ

Do I really cut off flanges?
Yes—both side flanges on the picture window and the inside flanges on each casement. Leave the outer, head, and sill flanges intact.
Where do the screws go?
Through the casement’s pre-drilled holes → across the mull strip → into the picture frame, spaced 8–12″ OC, first/last ~2″ from ends.
Ground vs. in-opening assembly?
Ground assembly is straighter and faster. In-opening is feasible when lifting one big unit isn’t practical.
No plastic strips in the box?
Ask the seller for the proper mull kit. As a fallback, use a continuous PVC/composite spacer of matching thickness plus exterior-rated sealant.

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