Setting Blocks to ASTM C864: Durometer, Sizing, Placement & Material Selection
ASTM C864 is the standard specification for dense elastomeric compression seal gaskets, setting blocks and spacers. A short reference to what it covers, what it leaves to glazing design, and how dense solid silicone, dense EPDM and dense neoprene compare when a drawing calls for a C864 block.
Built for: engineers writing or reading a setting-block callout, and buyers turning one into a part.
-
1Send drawingUpload a DXF, STEP, or PDF, or describe the system and the joint. Shop drawings, a spec section, and a sample extrusion profile work too.
-
2Material reviewEngineering reviews the callout against the vendor TDS: hardness band, chemistry, dense-versus-cellular class, block dimensions against the glass area, and sealant compatibility exposure.
-
3Prototype24 hours when material is in-house; otherwise 5–10 business days after drawing review, subject to tooling. Made-to-order; MOQ varies by material and part.
-
4ProductionStandard production runs ship about 2 weeks after drawing approval, including kiss-cut-on-liner and laminated configurations. Expedited service available. Ongoing parts run with material traceability and lot-code TDS records.
What does ASTM C864 require for a setting block? C864 specifies dense elastomeric compression seal gaskets, setting blocks and spacers; Option I is the option normally called for setting blocks, typically at 80–90 Shore A per ASTM D2240. It governs the material, not the geometry.
Count, length, width and position come from glazing practice: two blocks per lite at the quarter points, length at approximately 0.1″ per square foot of glass with a commonly quoted 4″ minimum, width to glass thickness plus edge clearance, minimum bite around 1/8″.
Chemistry — solid silicone, EPDM or neoprene — is chosen for compatibility per ASTM C1087.
ASTM C864 · ASTM D2240 (durometer) · GANA Glazing Manual · ASTM C509 (cellular) · ASTM C1115 (dense silicone) · ASTM C1087 (compatibility) · ASTM E1300 (glass load) · ASTM C920 (joint sealants).
- Dead-load block: C864 Option I, 80–90 Shore A
- Beside structural silicone: dense solid silicone
- Named-EPDM spec: dense EPDM, verified
- Legacy spec: dense neoprene, verified
- Anti-walk block: softer dense block, cut to the pocket
- Compression gasket: ASTM C509 cellular
- Cushion behind a stop: EPDM foam or silicone sponge
- Not for dead load: open-cell or low-density foam
Engineers who have hit “setting blocks per ASTM C864” in a specification, glazing contractors reconciling a callout against a system manual, and buyers turning a callout into a die-cut part. Specifying a whole glazing pocket? Start at the curtain wall glazing & perimeter page.
What ASTM C864 actually says
The most useful thing to know about C864 is where its authority stops.
C864 tells you what the block is made of. Glazing practice tells you how big it is and where it goes. A complete callout needs both. [1]
What it covers
C864 is the Standard Specification for Dense Elastomeric Compression Seal Gaskets, Setting Blocks, and Spacers. Those three part families share one property: they are dense — solid rubber with no engineered cell structure — so they do not creep away under sustained load. The standard classifies material by option, sets the properties each must meet, and references the test methods that prove them.
Option I is the option normally called out for setting blocks, and the hardness band conventionally written against it is 80–90 Shore A per ASTM D2240 [3]. Write the band, not a point value.
What it does not cover
It does not size the block. Nothing in a materials specification depends on the lite you are glazing; length, width and thickness come from the glazing design, and the practice most specifications adopt is in the GANA Glazing Manual [2]. It does not place the block — quarter-point placement is glazing practice, and a system manufacturer may specify otherwise. It does not cover cellular materials: sponge and closed-cell gaskets belong to ASTM C509 [4], dense silicone accessories to ASTM C1115 [5].
And it does not confer sealant compatibility — a block can meet C864 in every measured property and still soften an adjacent silicone joint, because compatibility is a pairing property tested under the ASTM C1087 [6] framework.
Sizing and placement, the part C864 leaves out
Two blocks per lite, at the quarter points. Length scales with glass area at approximately 0.1″ per square foot, minimum commonly quoted at 4″. Width covers the full glass thickness plus the specified edge clearance, so the block supports the whole glass edge including the insulating-glass secondary seal. Bite of at least about 1/8″ keeps the lite captured after the block takes its service compression. Thickness follows the pocket.
The sizing calculator lives on the curtain wall glazing & perimeter page; heavy, laminated and sloped lites get engineer review instead.
Silicone, EPDM or neoprene
C864 accepts more than one chemistry in the same band, so the decision is what the block lives next to. Dense solid silicone is the compatible choice adjacent to a structural silicone sealant. Dense EPDM brings weathering and ozone headroom where a spec names it; dense neoprene persists in legacy specs.
The risk with the non-silicone chemistries is migration, not strength: plasticizers and process oils can move into an adjacent silicone joint and soften it or stain the sightline, months after the wall passed its mockup. Get compatibility letters for the exact pairing. [6]
A block is not a foam
A setting block carries dead load for the service life of the wall, without relief and without inspection. Cellular materials are built to compress and recover; under permanent load they take a compression set instead. The block sets, the lite settles, bite is lost at the head, and the first visible symptom is a fogged unit or a leak that looks like a gasket problem. Cushioning duty is a different part, usually C509 [4]: closed-cell EPDM foam or silicone sponge behind a stop.
The setting-block parameter table
Every parameter a complete callout needs, and the document that sets it. Note how few come from C864 itself.
| Parameter | Typical value or requirement | Set by |
|---|---|---|
| Material — governed by the standard | ||
| Scope | Dense elastomeric compression seal gaskets, setting blocks, spacers | ASTM C864 |
| Option normally cited | Option I | ASTM C864 |
| Hardness band | Typically 80–90 Shore A | ASTM D2240 |
| Cell structure | Dense / non-cellular; foam and sponge out of scope | ASTM C864 |
| Chemistries in use | Solid silicone, dense EPDM, dense neoprene | C864 / project spec |
| Geometry — governed by glazing practice | ||
| Blocks per lite | Two, unless the system manual specifies otherwise | GANA Glazing Manual |
| Position | Quarter points — a quarter of the lite width in from each edge | GANA Glazing Manual |
| Length | ≈ 0.1″ per ft² of glass; minimum commonly quoted at 4″ | GANA Glazing Manual |
| Width | Full glass thickness plus specified edge clearance | Glazing design |
| Minimum bite | About 1/8″ of glass edge engaged | Glazing practice |
| Thickness | Pocket depth and required clearances | System manual |
| Design load | Lite weight from the project glass build | ASTM E1300 |
| Compatibility and adjacent specifications | ||
| Sealant compatibility | Verify the block–sealant pairing; keep the letter in the submittal | ASTM C1087 |
| Adjacent wet sealant | Classified separately; not supplied by H-O | ASTM C920 |
| Cellular alternative | Closed-cell preformed gasket — sealing, not dead load | ASTM C509 |
| Dense silicone alternative | Dense silicone gaskets and accessories | ASTM C1115 |
Conventions in general glazing practice and the cited standards, not a substitute for the project specification. Editions current as of July 2026.
How H-O converts to a C864 callout
What a die-cut converter can and cannot do with a materials specification.
H-O Products is a converter. H-O buys dense elastomer in sheet, roll and bun form and die-cuts, slits, laminates and kits it to a customer drawing in Winsted, Connecticut, under an ISO 9001:2015 certified quality management system.
H-O does not extrude or mold profiles, does not supply wet sealants or structural silicone, and does not perform the glazing design. A converter does not certify a material against C864 — hardness, tensile, compression set and aging belong to the compound as manufactured and are published by the material manufacturer on a technical data sheet.
Conversion adds geometry, repeatability and traceability: a block cut to your drawing, run after run, lot-coded back to that data sheet.
What to send: block dimensions (or lite size and glass build), the hardness band and any designation from the spec, the chemistry if one is named, and the sealant package. What comes back: a material recommendation traced to a published data sheet, a note on where the callout is under-specified, a flag on any pairing needing a compatibility letter, and a quote.
Typical prototype lead time is 5–10 business days after drawing review, depending on material availability, tooling, and inspection requirements. Most materials are commonly stocked or sourced on demand by H-O, subject to availability.
A block named inside a tested assembly is not a free substitution. Where a setting block appears in an impact, blast or fire-rated listing, the listing governs: the tested configuration fixes material, grade and dimension. H-O supplies parts to a listing and lot-codes them; it does not confer a rating. See the impact, blast & acoustic glazing page.
ASTM C864 setting blocks: engineer-grade FAQ
The eight questions engineers and buyers ask most once a C864 callout lands on a drawing.
What is ASTM C864?
ASTM C864 is the standard specification for dense elastomeric compression seal gaskets, setting blocks and spacers. It classifies the elastomer and sets the physical properties the compound must meet; Option I is the option normally called out for setting blocks. It does not say how long a block is, how many a lite gets, or where they sit — that is glazing design, from the GANA Glazing Manual [2], the system manual and the project spec. [1]
What durometer should a setting block be?
The band normally specified is 80–90 Shore A per ASTM D2240 [3]. Below it the block deflects under sustained dead load and the lite settles; above it the block stops distributing load and behaves like a hard point. Durometer is a hardness, not a load rating, which is why length still scales with glass area. Write the band, not a point value.
Does ASTM C864 tell me how long the setting block should be?
No — C864 governs the material, length comes from practice. The GANA Glazing Manual [2] scales block length to glass area at roughly 0.1″ per square foot, with a minimum commonly quoted at 4″. A 24 ft² lite computes to 2.4″ and takes the 4″ minimum; an 80 ft² lite computes to 8″. The calculator on the curtain wall glazing page runs the arithmetic.
How many setting blocks per lite, and where do they go?
Two blocks per lite, at the quarter points — one quarter of the lite width in from each vertical edge. That balances glass bending across the supported spans against the load delivered into the frame. Corner-set blocks load the glass edge and the insulating-glass edge seal; center-set blocks increase the overhang. Where the system manual specifies otherwise, the system manual governs. [2]
Silicone, EPDM or neoprene setting blocks: which should I specify?
All three come dense and in the C864 band, so the decision is compatibility, not load capacity. dense solid silicone is the compatible choice next to a structural silicone sealant. dense EPDM brings weathering headroom; dense neoprene carries the longest service history. Both can stain a sightline or soften a silicone joint through plasticizer and process-oil migration, so verify the pairing under the ASTM C1087 [6] framework and keep the letter in the submittal.
What is the difference between ASTM C864, C509 and C1115?
C864 is dense — compression seal gaskets, setting blocks and spacers, the parts that carry load or hold a dimension. C509 [4] is closed-cell cellular — preformed sponge that seals by compressing across a joint. C1115 [5] is dense silicone specifically. Writing C509 on a setting-block callout is the most common mix-up on incoming drawings.
Is a setting block a foam?
No. A setting block carries the dead load of the glass for the life of the wall. Cellular materials are built to compress and recover, which is the wrong property under constant load: the block takes a permanent set, the lite settles, bite is lost at the head, and the insulating-glass edge seal takes stress it was not designed for. Cushioning duty — closed-cell EPDM foam or silicone sponge behind a stop — is a different part, usually C509.
Can H-O supply setting blocks to ASTM C864?
H-O is a die-cut converter. H-O converts dense silicone, EPDM and neoprene stock — materials commonly evaluated against C864 by their manufacturers — into blocks, side blocks, spacers and shims cut to your drawing, with lot-code TDS records. H-O does not extrude or mold profiles, does not supply wet sealants, and does not itself certify a converted part against C864; the source data sheet and any project testing govern. Send dimensions, hardness band, chemistry and the sealant package.
Glossary: terms used on this page
Quick reference for the setting-block terminology used above.
Setting block
The dense elastomeric block that transfers the dead load of a glass lite into the frame. Material governed by ASTM C864 [1]; geometry by glazing practice.
Option I (ASTM C864)
The C864 option normally cited on setting-block callouts. It fixes material class and hardness band, and says nothing about block length, width or position.
Dense (non-cellular) elastomer
Solid rubber with no engineered cell structure, so it supports sustained load with minimal creep — the opposite of a cellular material per ASTM C509 [4].
Bite
The depth of glass edge engaged by the frame or stop. Practice holds a minimum around 1/8″ over the block, so a block that creeps or is undersized erodes bite as the lite settles.
Quarter points
Block positions a quarter of the lite width in from each vertical edge. Corner- or center-set blocks change the stress pattern the glass design assumed under ASTM E1300 [7].
Shore A durometer (ASTM D2240)
The indentation hardness scale for elastomers, per ASTM D2240 [3]. Setting blocks sit at 80–90 Shore A. A hardness, not a load rating.
Anti-walk (side) block
A block at the lite's vertical edge that keeps the glass centered when the wall racks. Typically softer than a setting block, and specified separately.
Last updated · Reviewed by H-O Products engineering (ISO 9001:2015 certified converter, Winsted, CT).
Standards, test methods & technical references
Standards cited on this page, with live links to the publishing bodies. Editions current as of July 2026; verify with the publisher before final spec.
ASTM C864 (Option I)
Dense Elastomeric Compression Seal Gaskets, Setting Blocks, and Spacers — the governing materials specification for the parts on this page. store.astm.org/c0864
GANA Glazing Manual
The industry reference for setting-block placement and sizing practice, published through the National Glass Association. glass.org
ASTM D2240
Rubber Property — Durometer Hardness. The method behind the Shore A number in a block callout. store.astm.org/d2240
ASTM C509
Elastomeric Cellular Preformed Gasket and Sealing Material — the closed-cell counterpart to C864. store.astm.org/c0509
ASTM C1115
Dense Elastomeric Silicone Rubber Gaskets and Accessories, where the silicone chemistry is specified by name. store.astm.org/c1115
ASTM C1087
Compatibility of Liquid-Applied Sealants with Accessories Used in Structural Glazing Systems — the framework behind sealant-manufacturer compatibility letters. store.astm.org/c1087
ASTM E1300
Determining Load Resistance of Glass in Buildings. Sets the glass build, and therefore the lite weight the blocks carry. store.astm.org/e1300
ASTM C920
Elastomeric Joint Sealants — the wet sealants the blocks live against. H-O does not supply wet sealants. store.astm.org/c0920
Get a setting block quote
Send the block callout, the lite size and glass build, or the drawing — partial information is welcome. We typically respond within one business day with a material recommendation traced to a published data sheet and a lead time.
See also: related H-O application pages
Where this standard is actually applied, and the adjacent fenestration reference pages.
Material data & standards. Durometer bands, block dimensions and placement practice here come from the cited standards and general glazing practice; they are a reference, not a project specification. H-O converts materials tested to these methods on the source manufacturer's technical data sheet and does not independently certify materials against the standards unless explicitly stated on the quote. The GANA Glazing Manual, the system manual and the engineer of record govern.
Conversion scope. H-O converts roll, sheet and bun stock to drawing in Winsted, Connecticut: die-cut blocks, spacers, shims and gaskets, kiss-cut-on-liner parts, slit rolls and laminations, with lot-code TDS records. H-O does not extrude or mold profiles in-house and does not supply wet sealants.