Replacement work is a different dimensional problem from new construction.

Full-frame replacement

The existing window and frame are removed. The structural rough opening is exposed and reused, repaired or modified, and the new unit is installed like a new-construction window.

Insert (pocket) replacement

The new unit fits inside the existing retained frame. Measure the opening that the manufacturer’s procedure calls for, not the original framing RO.

Do not size an insert replacement from the structural RO unless the manufacturer’s measuring instructions say to. Marvin refers to ASTM E2112 for replacement and recessed-opening installations.

Use an official product series. Andersen’s 100 Series double-hung guide gives the window dimension (outside frame-to-frame) and the minimum rough opening, which is the window dimension plus 1/2 in. in each direction.

Clear opening = width – 3.470 in. and (height / 2) – 3.790 in., per Andersen’s formula for this series. Andersen marks 3050, 3650 and 4050 as meeting 5.7 sq. ft. Window dimensions are derived from the guide’s width and height columns. Do not assume Andersen conventions match Marvin’s.

Marvin’s Essential Single Hung manual keeps four measurements separate: nominal size, rough opening, actual frame and daylight opening. The clear opening is a fifth, operation-dependent value.

The (E) flag is Marvin’s mark that the window in equal sash meets national egress codes; local codes may differ. Daylight width is the visible glass of one sash.

The single-hung top sash is fixed, so the usable opening is only the lower part of the frame. Compare the 3040 and 3050 rows: same width, different clear height and area.

Casement windows differ from double-hung windows in frame construction, operation, sash clearance and net clear opening. Use the manufacturer’s casement table, not a double-hung table.

The (E) flag is Marvin’s mark for meeting national egress codes; local codes may differ. Casement operating geometry means the clear opening is not the full frame.

Andersen publishes net clear opening dimensions for its 100 Series casements separately from catalog sizes, which is why you should not infer egress from the frame size.

A sliding window has an overall frame, an operating sash and a fixed sash. Installation allowances and sizing come from the manufacturer.

• Do not assume the usable opening equals the full frame width.
• Only part of the width opens, so the clear opening is much smaller than the RO.
• Keep drain weeps clear of sealant, as Marvin warns for its sliding products.

Use the selected product’s sizing table for the RO.

A picture window is a fixed unit. Its RO follows the same logic as other windows: actual frame size plus the manufacturer’s allowance. Marvin’s Essential casement picture units, for example, have a 5’0″ RO and a 4’11-1/2″ frame.

• Daylight opening (visible glass) is smaller than the frame.
• Larger units are heavy and need careful handling and support.
• Structural support depends on the header and framing design.

A fixed picture window does not provide an operable emergency escape opening.

Bay, bow and mulled assemblies need a complete assembly specification.

• Individual unit dimensions and mullion dimensions.
• Factory versus field mulling.
• Support requirements and structural mullion reinforcement.
• Overall assembly RO, projection and loads.

Do not add individual nominal widths and assume the sum is the assembly RO. In Marvin’s Essential tables, a two-wide ribbon of 2’0″ casements has a 4’0″ RO, but that is a published assembly value for that product. Marvin also limits mulled assemblies, for example a single-hung ribbon to 114 x 78 in. maximum RO.

A masonry opening (MO) is for brick, stone or other masonry construction. The framed RO is for wood, vinyl or metal siding. They are different dimensions for the same window.

• Masonry buck: a wood or other frame set in the masonry that receives the window; follow the product’s mounting system.
• Brick veneer over wood framing: keep at least 1/2 in. between the bottom of the window sill and the top of the brick to avoid brick bind.
• Sill support and sealant joint depth must match the joint width.

Marvin’s technical installation specification allows a masonry width 1/4 to 1/2 in. larger and a height 1/8 to 1/4 in. larger than the frame. Do not transfer these values to every masonry product.

The rough opening height and the sill height are separate measurements.

• RO height runs from the rough sill to the header underside.
• Sill location is the elevation of the rough sill above the floor.
• Frame height, sill-pan thickness and finished-floor elevation affect fit and finish.

A window can have a 48 in. RO height whether its rough sill is 24, 30 or 36 in. above the floor. Egress rules limit the elevation of the bottom of the clear opening, not the RO height.

Flashing and sill-pan systems can affect installation clearance. Marvin states that sill-panning products may require the opening to be adjusted and that sill pans decrease the rough-opening height clearance.

• Sill pan and flashing thickness reduce vertical clearance.
• The panning must drain water to the exterior of the cladding or the weather-resistive barrier.
• Shimming, drainage and integration with the water-resistive barrier follow the manufacturer and ASTM E2112.

Do not add a universal extra allowance for a sill pan. Use the thickness of your product.