Aluminum windows & drywall returns.

Separate the finish from the frame

With a drywall return, the wall finish turns into the opening and meets the window without conventional interior casing. The aluminum frame can extend behind that finish. The distance between drywall faces is therefore a finish-opening measurement until a product-specific detail establishes another use.

Look for tracks, interior legs, mull members, a sloped or stepped sill, an exterior flange and concealed installation fins. Record whether the existing metal frame will remain, be cut back under an approved retrofit method, or be completely removed.

Do not assume a standard drywall thickness, a standard frame leg or a standard gap behind the metal. Plaster, tile, shims and earlier repairs can change both the opening width and its depth.

Measure the cross-section, not just the rectangle

• Interior return opening: widths and heights between the actual finish faces; record out-of-square and localized protrusions.
• Proposed frame opening: the space available at the intended seating plane after the agreed removal and preparation.
• Exterior opening: the trim, brick or stucco throat and any flange-cover area.
• Depth chain: from the interior finish edge to the exterior finish, retained frame and proposed mounting position.

Keep a sketch with these dimensions. Name the plane beside each number. A single inside width cannot stand in for all four checks.

If the metal frame stays

A flush-fin or similar retrofit needs an approved retained-frame condition. Confirm the clear opening after only the authorized components are removed, the condition of the remaining frame, the contact surface for the new flange, and the new frame’s depth.

The Andersen 100 Series flush-fin guide 9198072 is explicitly limited to specified existing aluminum-frame applications. Its depth, clearance and exterior-contact requirements are not a generic permission to install any flush-fin window over any metal frame.

Check whether existing projections interfere with the new frame, whether drainage paths remain functional, and how the required anchorage reaches sound material. Do not cut a frame leg until the method establishes that it may be removed.

If the metal frame comes out

Under a full-frame method, the new unit may be placed against a retained drywall return or the finish may need modification. The relationship between the frame depth and the return is critical. Pella’s FF_BFBS detail explicitly considers both drywall-opening dimensions and depth to the exterior.

Draw the proposed new frame on that section. Check fit, finish overlap, installation access and where the fasteners land. If the frame is deeper, shallower, wider or narrower than the retained return, identify the approved trim, cutting or blocking solution before ordering.

Do not let “save the drywall” become an undocumented constraint discovered by the installer after the unit arrives. Record whether finish modification is included and who approved it.

A mismatch is a clue

If the hidden boundary cannot be verified without opening the finish, keep the order on hold and arrange that investigation. Do not “split the difference.”