Storefront vs. Curtain Wall: How the Systems Carry Load and Shed Water
Welcome
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Alright, Storefront vs. Curtain Wall: How the Systems Carry Load and Shed Water. Don't let the plain title fool you. Here's the part that actually matters on the job: Storefront bears on the slab and drains through its sill, while curtain wall hangs from the structure and drains at every lite — choose the system for the span and the exposure, and build it the way its drainage was designed. Learn it well and you will stop asking a system to do a job it was never tested for, which is where most leaks and cracked glass begin.
The intro course named the systems. This lesson is about why they behave nothing alike in a storm.
What storefront actually is
Storefront is a slab-supported system. The weight of glass and aluminum travels down the verticals into the sill and into the floor. The sections are shallow — commonly about 2 inches wide by 4-1/2 inches deep, glazed with a 1-inch insulated unit set at the center or front of the frame. It is stick-built on site: members cut to length, joined with screw-spline or shear-block corners, and screwed through shims to the rough opening. It is a single-span product, tested as a one-story frame under modest pressures. Thermally broken versions add a polymer strut between inside and outside metal so the frame does not sweat.
What curtain wall actually is
Curtain wall is a hung system. The mullions pass in front of the slab edges and hang from anchors at each floor, so the weight goes into the structure at the slab, not into a sill. The sections are deeper — 6, 7-1/2, and 10 inch depths are common — because the mullion spans floor to floor. The glass sits between an interior gasket and an exterior gasket compressed by a pressure plate bolted to the mullion, then hidden by a snap-on cover. Where the architect wants no exterior metal, structural silicone glazing bonds the glass to the frame instead. Curtain wall is a multi-span, pressure-equalized product tested to higher pressures and to the movement of a tall building.
Stick, unitized, and window wall compared
- Stick curtain wall ships as loose extrusions. Mullions go up first, usually in two-story lengths with a splice sleeve, then horizontals, glass, pressure plates, and covers. Nearly all the sealant work happens in the field, so field quality is the system's quality. It needs the least hoisting and tolerates late changes.
- Unitized curtain wall is built and glazed in a factory as story-tall units one or two mullion spacings wide. Units ship on racks, are set from the floor with a crane, roof-mounted monorail hoist, or mini-crane, and hook onto surveyed anchors. Adjacent units interlock at stack joints whose gaskets form the weather seal. It encloses a floor in days but demands early engineering, a precise slab-edge survey, and a hoisting plan.
- Window wall sits on each slab between slab edges, with a slab-edge cover outside. It costs less, bears on the floor like storefront, and adds a flashed joint at every floor.
Loads, spans, and deflection
Design wind pressure comes from the structural engineer under ASCE 7, and it is higher at corners and parapets than in the field of the wall. Mullions are checked for strength and for deflection, because a mullion that bends too far lets the glass edge leave its pocket and tears the seals. A commonly specified limit is span divided by 175 for shorter spans and a tighter limit for long spans; the project specification controls. This is why storefront verticals get steel reinforcement as openings grow tall, and why past a certain height the honest answer is curtain wall.
Aluminum grows about 13 millionths of an inch per inch per degree Fahrenheit, so a 20-foot run through a 100-degree swing moves about 5/16 inch. Every splice, anchor, and perimeter joint is sized for that.
How each system handles water
Storefront is a barrier-plus-drainage system with one path out. The exterior gasket or cap bead keeps most water out, and whatever gets past collects in the sill. The sill sits on a subsill flashing with end dams and a back leg, each bay is zone-dammed with sealant at the verticals so one bay cannot flood the next, and weep holes let water out the front. Horizontals above the sill are generally not drained, so every horizontal-to-vertical joint and gasket corner is sealed.
Curtain wall treats each lite as its own compartment. The exterior gasket is the first line, the interior gasket is the air seal, and the space between is vented and drained through weep slots in the pressure plate and cover at each horizontal. Because that space is pressure-equalized with the outside, little force pushes water inward. Mullions act as gutters; unitized systems drain at the stack joint; structural silicone systems still drain internally.
Going Deeper (Intermediate)
Storefront belongs on single-story openings with moderate pressures where the sill can drain outward. Curtain wall belongs on multi-story elevations, exposed sites, and anywhere the specification calls for more than a single-span frame can deliver. The test report shows what the system can do: the AAMA 501 mock-up sequence runs air infiltration under ASTM E283, static water under ASTM E331, dynamic water under AAMA 501.1 with a wind generator, structural load under ASTM E330, interstory drift under AAMA 501.4, and thermal cycling under AAMA 501.5. Check those results against the project's numbers before you bid.
Advanced / Pro-Level
Pros see the whole wall, not one frame. They treat transitions between systems — storefront meeting curtain wall at a canopy, window wall meeting a precast spandrel — as the highest-risk details on the job and demand a drawn detail for each. They know four-sided structural silicone glazing is shop-glazed, and that the sealant's allowable design stress, commonly held to about 20 pounds per square inch, sets the bite on the glass. They model the system U-factor under NFRC 100 procedures and watch condensation resistance on humidified buildings, because a cold frame in a natatorium sweats long before it leaks.
Practice Challenge
An architect wants a two-story lobby with a 24-foot-tall glazed opening and asks for the project's standard 4-1/2-inch storefront with 1-inch insulated units to hold the budget. What should the glazing contractor say? (Answer: a 24-foot storefront vertical cannot meet the deflection limit even with steel inside, the sill was never tested for two stories of dead load, and sill drainage cannot handle two stories of water — propose curtain wall, or storefront on an engineered intermediate steel support with a drained horizontal, and get the system engineer's calculation before pricing.)
In Practice
A glazier who knows storefront drains through its sill zone-dams every vertical and never leaves a horizontal end unsealed. One who treats storefront like a small curtain wall assumes it drains itself and gets the leak call at the first bay that fills up.
Common Mistakes to Avoid
- Bidding storefront for multi-story spans because the section looks similar
- Building system transitions without a drawn, reviewed detail
- Ignoring the test report pressures when the project's design pressure is higher
- Skipping zone dams and horizontal end seals on storefront because the covers hide them
Takeaway: Storefront bears on the slab and drains through its sill, while curtain wall hangs from the structure and drains at every lite — choose the system for the span and the exposure, and build it the way its drainage was designed.
Educational content only — not design, engineering, or a safety program, and not a substitute for training under a qualified glazier or review by a licensed design professional. Follow the building code and energy code as adopted in your jurisdiction, the project documents, the manufacturer's installation instructions, OSHA 29 CFR 1926, and your employer's procedures.