Learn how Gentek 1700 Series twin and triple single-hung window assemblies are planned, reinforced, mulled, flashed, handled, and installed. This guide also covers performance ratings, grid alignment, egress, homeowner questions, and contractor sales tips.

Article 55 of 100 | Mulled Window Assembly Guide

Gentek 1700 Series Twin and Triple Single-Hung Window Assemblies

Placing two or three single-hung windows together can create a wider view, more ventilation, and a stronger architectural feature than a single window. The resulting assembly, however, is not merely a row of independent windows. Mull joints introduce structural, drainage, flashing, handling, rating, and installation requirements that must be resolved before the units are ordered.

Gentek’s historical 1700 Series size guide identifies Model 1720 as a twin single-hung assembly and Model 1730 as a triple single-hung assembly. Current model designations, sizes, mulling methods, reinforcement, and ratings must be confirmed for each project.

What Is a Mulled Window Assembly?

Mulling is the process of joining two or more window units into a combined assembly. The connection between units is the mull joint or mullion. Depending on the approved system, windows may be joined at the factory or assembled at the jobsite.

A twin single-hung assembly combines two windows. A triple single-hung assembly combines three. Each individual unit generally retains its own frame, glass, operating sash, balance system, lock, weatherstripping, and drainage features.

Contractors can review the product family through Suburban Wholesale & Supply’s Gentek 1700 Series new construction window page .

The critical distinction

A mull cover can conceal the joint, but appearance is not the same as structure. The approved mull system, reinforcement, fasteners, sealants, flashing, support, and tested limitations determine whether the assembly is suitable.

Twin Versus Triple Single-Hung Assemblies

Planning factor	Twin assembly	Triple assembly
Visual divisions	Two primary vertical sections	Three primary vertical sections
Potential ventilation	Two lower operating sashes when ordered in the standard twin arrangement	Three lower operating sashes when ordered in the standard triple arrangement
Overall width	Moderately wide assembly	Wider assembly with greater handling demands
Mull joints	One primary vertical mull	Two primary vertical mulls
Structural planning	Combined-unit verification required	Greater combined width can increase reinforcement and support requirements

A triple assembly is not automatically the better value because it contains an additional window. The wall area, view, ventilation, structural opening, exterior elevation, room layout, budget, and installation access should determine the configuration.

Sales tip

Show the homeowner a scaled elevation of both options. Three equal windows can make a narrow wall look crowded, while two oversized units may create proportions that do not coordinate with the rest of the home.

Gentek’s Metal-Reinforced Mulling System

Suburban Wholesale describes an optional metal-reinforced mulling system for qualifying Gentek 1700 Series window combinations. The system uses a metal channel to secure one window frame to another.

The reinforcement is intended to add rigidity during shipping and installation and increase strength at mull intersections. The described system also includes a drip cap at the head intended to direct water away from the mullion.

What reinforcement does not prove

The presence of metal does not prove that every window combination meets every structural requirement. Reinforcement must be evaluated with:

Individual window dimensions
Total assembly width and height
Number and location of mull joints
Building height and opening location
Required design pressures
Wind exposure and local code
Glass weight and type
Header, sill, jamb, and intermediate support
Fastener and anchoring requirements

Unsupported claim to avoid

Do not tell a customer that a metal mull makes the assembly “stronger than separate windows” or suitable for any wind condition. Use the tested or engineered data for the exact configuration.

Factory-Mulled Versus Field-Mulled Windows

A factory-mulled assembly is joined under controlled manufacturing conditions before delivery. A field-mulled assembly consists of separate units joined at the project using an approved system and procedure.

Factory-mulled considerations

Controlled assembly process
Fewer field-assembly steps
Larger and heavier shipping unit
Greater lifting and access demands
Potential shipping-size limitations

Field-mulled considerations

Smaller individual units for delivery
More jobsite assembly steps
Requires correct components and trained labor
Greater exposure to dirt, weather, and assembly errors
Must follow the approved field-mulling procedure

The project team should not decide casually to separate a factory-intended assembly or field mull units that were not ordered for that purpose. Confirm the approved process, reinforcement, sealing, fastening, and certification before changing the planned assembly method.

The Combined Rating Matters

A window’s structural, air, and water performance is established for identified test specimens and configurations. When units are joined, the mull can become a critical part of the assembly.

Two individually rated single-hung windows do not automatically create a twin assembly with the same performance grade. Three individually rated windows do not automatically produce a triple assembly with that rating.

Obtain documentation covering the complete combination, including the individual models, sizes, reinforcement, mull construction, glass, anchoring, and overall dimensions.

Questions for the project file

What performance class and grade are required?
Does the rating apply to the complete twin or triple assembly?
What overall tested dimensions are listed?
Which mull reinforcement was used in the tested configuration?
Which fasteners and anchoring conditions are required?
Does the rating change with glass, color, grids, or other options?
Is project-specific engineering required?

Flashing and Water Management at Mull Joints

Each window must be integrated with the wall’s water-management system, and the combined unit must also manage water at the mull intersections. A decorative exterior mull cover is not a complete flashing system.

The head, jambs, sill, nail fins, mull joints, drip cap, water-resistive barrier, exterior cladding, and perimeter sealant must be coordinated using the current approved details.

Gentek’s historical literature describes a drip cap at the top of its reinforced mull system to direct water away from the mullion. The current component, installation sequence, end conditions, laps, and relationship with the head flashing must be verified.

Frequent water-management errors

Ending head flashing at the edge of one unit instead of protecting the complete assembly
Failing to integrate the mull drip cap with the water-resistive barrier
Creating reverse laps that direct water behind the window
Using sealant as the only water-management layer
Blocking individual sill drainage openings
Leaving an unsealed or unsupported intersection at the mull and sill
Installing exterior trim that traps water against the assembly

Rough Opening and Structural Support

The rough opening for a twin or triple assembly must be sized using current Gentek requirements for the complete unit. Do not estimate it by multiplying an individual window’s nominal width. Mull components and manufacturing dimensions affect the overall size.

The sill should be level and provide continuous support as required. A wide opening with a sagging sill can distort multiple frames and create several operating problems at once.

The header must be designed for the structural loads above the opening. The window assembly should not be expected to carry wall, roof, or floor loads unless a specifically engineered design says otherwise.

Contractor check

Measure the opening at multiple locations and check both diagonals. A wide opening can have correct end dimensions while still containing a bowed header, crowned sill, or inward-curving framing member.

Installation Alignment Affects Every Sash

Each Model 1700 single-hung unit contains an operating lower sash supported by a block-and-tackle balance system. Every sash must align with its own frame, weatherstripping, lock, keeper, and sill.

If the combined assembly is installed out of level, plumb, or square, one sash may operate acceptably while another binds or fails to lock. That inconsistency often points to frame, sill, anchoring, or mull alignment rather than three unrelated product defects.

Before installing interior trim

Operate each lower sash through its complete travel.
Verify that each sash remains supported in the open position.
Test each lock and keeper independently.
Check weatherstripping contact around each operating sash.
Verify tilt-latch engagement after testing the cleaning feature.
Inspect mull covers and interior sightlines.
Confirm that insulation has not bowed any frame member.
Verify that every drainage path remains open.

Handling Twin and Triple Assemblies

Factory-mulled twin and triple windows can be large, heavy, and vulnerable to racking if handled incorrectly. Plan the delivery route, storage orientation, number of handlers, lifting equipment, scaffolding, and opening access before the units arrive.

Do not lift an assembly by a nailing fin, mull cover, grid, screen, packaging band, or unsupported frame section unless Gentek specifically directs that method.

Temporary shipping reinforcement should remain in place for the period required by the manufacturer. Removing it too early can allow a large assembly to flex during handling. Leaving temporary bracing in place when instructions require removal can interfere with installation or operation.

Receiving inspection

Inspect mull intersections, frame corners, nail fins, glass, tracks, locks, and shipping reinforcement when the assembly arrives. Photograph labels and any damage before moving or altering the unit.

Grid Alignment Across Multiple Windows

Grids can turn a twin or triple assembly into a strong architectural feature, but errors become more visible when several windows are placed side by side.

Gentek’s historical 1700 Series guide lists Colonial, Single Prairie, Single Perimeter, and Craftsman patterns with several grid profiles. Current patterns and availability must be confirmed.

Document these details

Grid pattern for each individual window
Number of horizontal and vertical divisions
Grid profile and width
Interior and exterior grid colors
Alignment across adjacent sashes and fixed upper sections
Relationship between grids, mull covers, and meeting rails
Alignment with nearby doors and windows

Do not rely on a generic note such as “standard grids.” Prepare an approved elevation for the exact twin or triple assembly.

Egress Is Evaluated Per Operating Opening

A twin or triple window may create a large overall glazed area, but emergency escape compliance depends on the clear opening of an individual operating sash—not the combined width or area of the complete assembly.

The mullions, individual frames, fixed upper sashes, meeting rails, and lower-sash travel determine the usable opening. Openings from separate windows cannot ordinarily be added together to create one compliant clear opening.

Obtain current clear-opening data for the exact individual Model 1700 size and compare it with the locally adopted code. Also verify sill height, window-well requirements where applicable, and any opening-control devices.

Sales claim to avoid

“This triple window easily meets egress because the overall opening is huge” is not a valid conclusion. Each qualifying emergency opening must be evaluated using its own net clear dimensions.

Glass and Energy Performance

Gentek’s historical 1700 Series guide lists clear, Low-E, Low-E with argon, tinted, obscure, laminated, and tempered insulating-glass options. Current glass packages and ratings must be verified for the exact assembly.

A twin or triple group increases total glass area. Even when each window has a strong U-factor, the overall assembly can have a greater effect on room comfort and heating or cooling loads than a smaller wall opening.

Evaluate orientation, shading, Solar Heat Gain Coefficient, Visible Transmittance, room use, and total area. Do not promise a specific reduction in utility bills.

NFRC ratings should be confirmed for the exact window configurations. If the project documentation addresses the combined assembly, verify how the required values must be reported.

AAMA Gold Label Certification

The Fenestration and Glazing Industry Alliance administers the AAMA Gold Label Certification Program. The program provides a certification framework involving product testing, manufacturing-site inspections, and continuing program requirements.

For a twin or triple assembly, do not stop after confirming labels on the individual windows. Determine whether the required performance documentation applies to the completed mulled configuration.

Record relevant labels before the frame is covered by trim. Compare the manufacturer, product designation, size, performance class, performance grade, referenced standard, and certification information with the approved submittal.

The Gold Label does not certify field mulling or installation work unless the applicable certification documentation specifically encompasses the completed configuration and approved procedures.

InstallationMasters Principles for Mulled Assemblies

FGIA’s InstallationMasters program provides professional training and certification for new construction and replacement installers of windows and exterior glass doors. Training covers site inspection, installation practices, cleaning, and maintenance.

A wide mulled assembly demands disciplined opening preparation and water-management integration. Small errors can accumulate across several units.

Verify the complete assembly size and rough-opening requirements.
Confirm mull type, reinforcement, factory or field assembly, and rated limitations.
Check the sill continuously across the entire opening.
Plan safe handling, lifting, and temporary support.
Install the required sill pan and head, jamb, and mull flashing.
Integrate all nail fins and flashing with the water-resistive barrier.
Use the specified fasteners and anchoring locations.
Shim without creating isolated pressure or frame distortion.
Test every operating sash before covering the perimeter joint.
Document labels, mull components, installation details, and final operation.

InstallationMasters training supports industry-recognized knowledge, but Gentek’s current instructions, approved mull procedures, engineering, project specifications, and applicable codes remain controlling.

Contractor Ordering Checklist

☐ Confirm twin Model 1720, triple Model 1730, or current equivalent.
☐ Verify current individual and overall assembly dimensions.
☐ Confirm factory-mulled or approved field-mulled construction.
☐ Identify the required mull reinforcement and drip-cap components.
☐ Obtain performance documentation for the complete assembly.
☐ Verify rough-opening, header, sill, and jamb support.
☐ Confirm glass, grids, colors, locks, screens, and extension jambs.
☐ Verify safety-glazing and individual egress requirements.
☐ Plan delivery, storage, lifting, and installation access.
☐ Coordinate mull flashing with the full wall drainage system.
☐ Review current installation and warranty requirements.

Plan Gentek Window Combinations with Suburban Wholesale

Suburban Wholesale & Supply helps contractors, builders, remodelers, property professionals, and homeowners plan Gentek window packages throughout Davenport, Bettendorf, LeClaire, Eldridge, Moline, Rock Island, and the Quad Cities.

Bring the window schedule, elevations, wall sections, opening dimensions, required performance ratings, mull details, glass choices, grid patterns, colors, and installation requirements. The team can help identify missing information before a twin or triple assembly is ordered.

Explore Gentek 1700 Series Windows Professional Quoting Request a Window Quote

Sources and Technical References

Suburban Wholesale & Supply — Gentek New Construction 1700 Series
Gentek Building Products — 1700 Series Window Collection Size Guide
Fenestration and Glazing Industry Alliance — AAMA Gold Label Certification Program
Fenestration and Glazing Industry Alliance — InstallationMasters Certification
Current Gentek mull instructions, size charts, product specifications, certified-product records, NFRC labels, warranties, project engineering, and applicable building codes.

Technical note: The cited Gentek 1700 Series size guide is dated February 2018. Historical model numbers, mull systems, sizes, options, ratings, and availability may have changed. Verify the complete twin or triple assembly using current Gentek documentation. This article does not replace manufacturer instructions, structural engineering, certified mull data, code review, or project-specific installation requirements.

Gentek 1700 Twin and Triple Single-Hung Window Assemblies