The zero-threshold problem every premium sliding door installer knows

Why the tolerance for Alumil Eclipse, Reynaers Hi-Finity, and Sky-Frame installations is essentially impossible to meet with traditional methods — and how KORSYS solves it.

The most demanding sliding door installations in premium residential construction share one design intent: the threshold disappears. Interior flooring flows continuously across the door opening. There is no visible transition.

Systems like Alumil SUPREME S650 PHOS Eclipse, Reynaers Hi-Finity, Schuco ASE HI, Sky-Frame, Solarlux Cero, and Keller Minimal Windows Cero achieve this by recessing the aluminum track into the floor and covering it with the same flooring material used throughout the space. The architectural effect is unmatched.

The installation tolerance is punishing. And most installations fail to meet it.

The Problem Every Premium Installer Has Faced

An architect quoted in an industry publication put it simply: "Zero-threshold doors are very difficult to do right. The floor has to be dead flat and level for these systems to work properly." His firm learned this on their first lift-slide project — they had to rip out the doors completely and start over from scratch.

The story is more common than most builders will admit publicly. The reason it keeps happening is structural to the installation sequence itself.

In a typical construction sequence, the concrete pour happens early. The imported European door arrives weeks or months later. The interior flooring is installed after the door, often months later.

This means the door installer is setting the threshold to an elevation that doesn't yet exist.

The correct elevation must be calculated from the as-built concrete slab, the specified finished floor thickness, the underlayment or adhesive, and the manufacturer's threshold height. Every trade upstream of the door installation must have communicated their exact final elevations, and every trade must execute to them within a few millimeters.

That coordination rarely happens perfectly. The specifications are approximate. The as-built conditions vary. And when the finished floor arrives at a different elevation than specified — which is nearly always — the mistake is discovered only after the door is permanently in place.

Why Traditional Threshold Methods Fail Here

Wood shims, site-mixed grout, wet-set metal plates, mortar bedding — all common threshold methods share a critical limitation: once the door is set, the threshold height is fixed.

Changing the height means removing the door.

For a zero-threshold system where the visible result depends on 1mm accuracy to a floor that doesn't yet exist, permanent commitment on install day is the wrong approach.

When the gamble loses, the choices are:

• Accept a visible imperfection in an installation specified to be invisible

• Modify the flooring transition, destroying the design intent

• Cut and reshape the finished flooring (sometimes impossible with tile)

• Remove the entire door assembly and reinstall

None of these are acceptable outcomes on a project where the door alone often costs between $40,000 and $150,000.

Traditional methods also degrade over time. Wood shims absorb moisture and rot. Grout cracks. Wet-set metal plates corrode in the alkaline concrete environment and cause millimeter-scale settlement — enough to defeat a zero-threshold aesthetic.

KORSYS: The Assembly That Actually Works

KORSYS is a threshold assembly by WINDO built on one core principle: enable a phased installation workflow that separates door installation from final elevation commitment. Structure is carried by stainless steel and a purpose-engineered GFRP composite block. Thermal performance is delivered by rigid EPS foam in the load-free zone beneath the block, and by polyurethane expansion foam injected inside the block itself. Precision leveling happens in a scheduled second visit after finished floor installation. And every project is coordinated by custom KORSYS shop drawings.

The name reflects the product's dual identity: a structural CORE for the threshold, delivered as an engineered SYSTEM — not just a component.

KORSYS Block: Engineered Specifications

The KORSYS block is a precision-manufactured GFRP composite, purpose-designed for permanent embedment in concrete beneath sliding door thresholds:

At typical service loads, the block operates at 2-3% of design capacity — providing a 38× safety margin. The GFRP block capacity is closely matched to the stainless steel rod capacity (~21,300 lbf ultimate combined), creating a balanced structural assembly.

Why ECR-Glass and Vinyl Ester Specifically

The material specification for the KORSYS block was driven by one requirement: permanent embedment in concrete at pH 12.5+ for 50+ years without measurable degradation.

Standard "E-glass" fiberglass — the most common fiberglass in the world — contains 5-10% boron oxide that leaches out in alkaline concrete pore water, causing progressive strength loss over 20-40 years. This is why ACI 440 (the industry standard for GFRP in concrete) specifically recommends against E-glass for permanent embedment.

ECR-glass is a boron-free fiberglass chemistry specifically developed for alkaline environments. In accelerated aging tests, ECR-glass retains 85-92% of tensile strength after six months of concrete pore solution exposure, versus 45-60% for E-glass.

Vinyl ester resin, similarly, is the industry-standard matrix for FRP in concrete embedment. Its chemistry has far fewer hydrolyzable groups than cheaper polyester (which fails in 15-25 years) and is proven at scale in FRP rebar, chemical tanks, and marine infrastructure.

The KORSYS specification — vinyl ester + ECR-glass — is not a cost optimization. It is the material combination that industry testing and ACI 440 recommendations converge on for permanent concrete embedment. Verified at 88% strength retention in CMA/CNAS certified testing.

From Bottom to Top

**1. Chemical anchor plus stainless threaded rod. **Stainless 1/2 inch threaded rods are chemically anchored 4 inches (100mm) into the concrete curb using structural epoxy adhesive (Hilti HY-200, Simpson SET-XP, or equivalent). Rod locations are set per KORSYS shop drawings.

**2. Lower positioning nut and washer. **Establishes the initial reference height. Transfers load onto the washer face rather than directly to the anchor bond line.

**3. KORSYS block. **A precision-manufactured GFRP composite block (50 × 30 × 230 mm, 3mm walls) slides over the rods and rests on the lower washers. Manufactured from vinyl ester resin with ECR-glass fiber reinforcement — chemistry selected for permanent embedment in alkaline concrete. Corrosion-immune, thermally neutral, dimensionally stable across the temperature range the building will ever see.

**4. Interior nut and washer. **Accessed through a socket-clearance hole in the top of the KORSYS block. Clamps the block to the lower washer for structural stability.

**5. Upper leveling nut, washer, and locking nut. **This is the precision leveling zone. Turning the upper nuts raises or lowers each anchor point independently, with millimeter precision. A second nut above secures the position.

**6. Aluminum door threshold. **Installed per the manufacturer's specifications on top of the KORSYS blocks. Temporarily fastened during Visit 1; permanently sealed during Visit 2 after final leveling adjustment.

**7. Expansion foam infill (inside each block). **After the interior nut is fully torqued and final leveling is confirmed, polyurethane expansion foam is injected into the interior cavity of each KORSYS block through the top access holes or open ends. This creates a monolithic composite block with no air voids — eliminating any potential for moisture accumulation and providing internal lateral support to the GFRP walls.

**8. EPS thermal break infill (below the blocks). **The cavity between the KORSYS blocks and the concrete curb below is filled with rigid EPS foam. Because the structural assembly carries 100 percent of the load, the EPS never sees compression — it does one job: eliminating the concrete thermal bridge between exterior and interior. Installed only after final leveling is verified.

Block Spacing: Engineered Per Project, Not Per Rule

KORSYS block quantity and spacing depend on the specific door system, application, and installation context. Variables include:

• Which door system is installed (Alumil, Reynaers, Schuco, Sky-Frame, Solarlux — each has different threshold profiles and load characteristics)

• Whether the configuration is zero-threshold, low-threshold, or standard

• Door width, panel count, and panel weight

• Interlock locations, which typically require support directly on either side

• Multi-track configurations where threshold width exceeds single-block coverage (4-track, 6-track multisliders)

• Chemical anchor manufacturer's minimum edge distance and spacing requirements

• Any project-specific structural, seismic, or wind load requirements

As a general reference, block spacing typically ranges from 12 to 24 inches on center along the door span, with additional blocks at each interlock and near each jamb. Multi-track systems receive staggered or paired configurations to support each track directly. But the exact placement for any given installation is determined by engineering review — not by a universal rule.

Shop Drawings for Every KORSYS Installation

WINDO produces project-specific KORSYS shop drawings for every installation. These are not generic reference diagrams — they are coordination documents that show, for the exact door being installed, the precise placement of every anchor point, the finished floor elevation targets, the coordination requirements for concrete and flooring trades, and the adjustment points for the Visit 2 workflow.

The purpose is to eliminate coordination gaps between trades. On a premium door installation, the concrete subcontractor, framing subcontractor, door installer, flooring installer, and sealant subcontractor all touch the threshold zone across the construction sequence. Without a coordination document, each trade interprets the specification independently and mistakes compound. KORSYS shop drawings give every trade one accurate reference — the same reference — so upstream decisions do not create downstream problems.

This is what allows the two-visit installation workflow to succeed. Between visits, other trades continue working around the threshold zone. The shop drawings tell them exactly what elevations to work to and where the adjustment points are located. When Visit 2 arrives, coordination has been managed proactively.

The Two-Visit Workflow That Zero-Threshold Doors Actually Need

Visit 1: Door installation

The full KORSYS assembly is installed per shop drawings and the door is functional. However, the aluminum threshold is only temporarily fastened at this stage. The expansion foam has not yet been injected inside the blocks, and the EPS thermal break infill is not yet installed. The threshold is set to the specified elevation, but not permanently sealed.

Between visits

The flooring installer completes the finished floor at its actual elevation, which may vary from specification. The KORSYS shop drawings serve as the coordination reference for all trades working around the threshold zone.

Visit 2: Final coordination and sealing (60-120 minutes)

The installer returns after the finished floor is in place. The aluminum threshold is temporarily lifted (door frame stays fully in place, only the horizontal threshold section is raised). This exposes the upper leveling nuts. Each anchor point is adjusted to bring the threshold to the exact elevation of the finished floor.

Once final elevation is confirmed and the upper locking nuts are secured, polyurethane expansion foam is injected into the interior cavity of each KORSYS block. This can be done through the top access holes or through the open ends of the blocks. The foam cures within 15-30 minutes to a tack-free state, fully sealing the interior of each block and creating a monolithic composite section.

EPS foam infill is then installed in the cavity below the KORSYS blocks. The threshold is re-seated, permanently fastened, and sealed. Flooring transitions are completed.

This workflow eliminates the elevation-guessing that causes traditional installations to fail. The threshold is set with precision to a finished floor that has actually been installed, not to a specification that may not match as-built conditions. And the finished assembly is fully sealed and monolithic — no cavities, no air voids, no long-term moisture concerns.

No other threshold method offers this capability. Wood shims, grout, wet-set plates, mortar bedding — none of them remain adjustable after installation, and none provide the sealed, thermally-broken assembly that KORSYS delivers post-installation.

Secondary Benefits

**Long-term dimensional stability, backed by test data. **Stainless hardware, the KORSYS GFRP block, and EPS are all corrosion-immune in the buried threshold environment. The KORSYS block has been tested at 88% bending strength retention after 60 days of alkaline aging (CMA/CNAS certified, Report LH250612050101E). The expansion foam infill provides an additional moisture barrier around the interior stainless nut.

**Complete thermal separation. **The EPS-filled cavity below the blocks eliminates the concrete thermal bridge, satisfying Passive House and high-performance envelope requirements. Combined with the foam-filled block interiors, the entire threshold zone becomes a fully insulated assembly.

**Structural overspec with balanced-system engineering. **Each KORSYS block provides ~5,650 lbf design allowable capacity against typical service loads of 100-150 lbf per block. The GFRP block and stainless steel rods are engineered as a balanced structural system with matched capacities.

**Compatibility with the ecosystem. **Every component except the KORSYS block is available at any construction supply house — including standard polyurethane expansion foam.

**Project-specific engineering. **Custom KORSYS shop drawings for every installation eliminate the coordination gaps that cause traditional installations to fail.

Applicable Beyond Sliding Doors

KORSYS works for any threshold where precision leveling and thermal performance matter: zero-threshold folding doors, wide entry doors, lift-and-slide systems, multi-track sliding systems, curtain wall base connections. The engineering logic is universal, and every application receives its own project-specific shop drawings.

The One Component You Can't Easily Buy — and the Engineering That Makes It Work

Raw GFRP tubing is available on the open market, but the material lacks the precise dimensions, hole geometry, resin chemistry, and quality control required for a threshold assembly. On-site fabrication involves specifying the correct resin (most options are inappropriate for concrete embedment), sourcing the correct wall thickness, drilling precise holes at exact positions and diameters, and accepting significant labor time per unit.

For most installers on most projects, on-site GFRP fabrication is neither cost-effective nor reliable.

And even with the right block, an installation without engineered shop drawings will suffer the same coordination gaps that cause traditional methods to fail. The block and the shop drawings work together — neither is fully effective without the other. This is why KORSYS is delivered as a system, not just a product.

The KORSYS block is manufactured specifically for threshold installations by WINDO. Every project engagement also includes custom shop drawings coordinated to the specific door system and application. Every other component in the assembly — stainless hardware, chemical anchor epoxy, expansion foam, EPS insulation — can be sourced locally at any construction supply house.

Why We Built KORSYS

WINDO is a specialty window and door dealer offering the Alumil SUPREME S650 PHOS Eclipse and other zero-threshold systems from leading European manufacturers. KORSYS was developed for our own high-performance installations, after seeing enough traditional installations fail on the exact points described above.

We now make KORSYS available to other builders, architects, and installers who want to specify a purpose-designed threshold assembly on their own projects — regardless of which door system they're installing. Every project includes custom shop drawings.

For technical documentation, specifications, or to request a KORSYS sample and project quote, contact WINDO.