Glazing Packers vs Setting Blocks: What Each Does and When to Buy Both

professional architectural glass and window glazing tools  · 

Two small parts, two different jobs

A setting block is a load-bearing pad. The bottom edge of the glass rests on it, so it carries the weight of the pane and keeps the glass edge off the metal, vinyl or timber sill. A packer is a position-holding shim. It sits between the glass face or edge and the frame to hold the glass central in the opening and maintain the clearances the glass maker specifies.

The two are often sold in the same bag or bin, which is why they get swapped. The mistake costs money in one direction and breaks glass in the other. A packer used where weight must be carried can compress or creep. A block used as a general spacer can pin the glass in the channel and stop it moving with the frame.

What setting blocks do and the specs that matter

Setting blocks transfer the dead load of the glass into the frame. Glass maker guidance, such as Pilkington’s general glazing bulletin, puts each pane on two blocks centred at the quarter points of the bottom edge. If that is impractical, the end of the block may move toward the corner, but not beyond 6 inches (152 mm) or one-eighth of the glass width from the quarter point.

Hardness is the first spec to check. ASTM C864 covers setting-block material, and the usual callout is 80 to 90 Shore A (ASTM D2240), as in U.S. federal glazing specifications and glass-fabricator technical notes. That is firm enough to hold the pane without squashing, but not rigid. A block that is too soft creeps under load, and one that is too hard concentrates stress on the glass edge.

Length and width are the second check. Length comes from the glass weight and the manufacturer’s table, and a block that is too short raises bearing stress even at the right hardness. Width must suit the glass or unit thickness, and the block must not stop water leaving the glazing channel. Where the glass is structurally sealed or sits in silicone, the material must also be silicone-compatible. Tristar and Oldcastle instructions allow silicone or silicone-compatible EPDM.

How much weight the blocks actually carry

Annealed or tempered glass weighs about 2.5 kg per square metre for every millimetre of thickness. A 1.2 m by 1.8 m insulating unit with two 6 mm lites has 12 mm of glass in total. That gives 2.16 m² x 30 kg/m² = roughly 65 kg, before the spacer and sealant, resting on two blocks.

This is why a flat plastic shim is the wrong tool under a heavy unit. Setting blocks are specified by material, hardness and length because they are the only parts in the sill that carry the full pane weight. Load rises quickly with larger lites, laminated makeups and triple glazing, so re-check block length for each size, not just for the first pane of a job.

What packers do and where they go

Packers hold the glass where the design puts it. They keep the pane square and plumb in the rebate, help hold face and edge clearance, and keep it from shifting sideways under handling or wind. Glass-maker tables set the minimum clearances. Mid-American Glass, for example, lists 1/8 inch (3.2 mm) face and edge clearance and a 1/2 inch (12.7 mm) glazing bite for a 1/2 inch insulating unit, and the figures grow with thickness.

Flat packers are typically supplied as cut strips in several thicknesses. One UK range lists a grey packer at 20 mm x 100 mm x 4 mm, and boxes colour-coded for 24 mm and 28 mm widths. Packer thickness is how you fine-tune the fit, because a 4 mm packer is a different size from a 2 mm one. The same maker advises placing packers at least 30 mm in from the frame corners.

Where a continuous cushion is called for around the glass face, as with gaskets or tape, Pilkington specifies a cushioning material of 65 ± 5 Shore A, such as neoprene, and says intermittent face shims should not be used. Packers are for position and are not a substitute for that continuous seal.

Terminology differs by region and system

In North America the sill part is a setting block, and side parts that keep the glass centred are spacers, shims or edge blocks. In UK and European window practice the word packer covers more. Glazing packers can be support blocks that carry load, or distance pieces (location blocks) that hold position. The Roto glazing guide separates the functions the same way, saying support blocks transfer dead load, while other packers deal with position and movement.

So a product listed only as a packer may be a load-bearing block, a spacer, or a simple shim. Read the described function and the material hardness, not just the name. If the maker’s specification does not say whether the part is rated to carry glass weight, do not put it under a pane.

When a job needs both

Almost every fixed or hung pane in a conventional captured frame needs setting blocks. Packers or spacers are added where the frame does not otherwise limit the glass position: the glass is thinner than the rebate, the frame has play, or the unit has to sit square against an out-of-plumb opening.

A new-build window in a frame with tight rebates might need two blocks and only a few shims. A replacement unit going into a worn, oversize rebate usually needs two blocks plus packers on the sides, and the pack thickness must be matched to the actual gap. Operable sashes add anti-walk or location blocks at hinge and lock points so the glass cannot slide as the sash opens and closes.

A useful buying split is to keep dedicated, correctly sized setting blocks in stock for each common unit thickness, and a mixed assortment of flat packers in graded thicknesses for site adjustment. Keep a thickness gauge in the kit so the rebate and glass are measured, not guessed.

Failure modes buyers hit

The common failures are mismatches. Soft rubber blocks bought for hardness-agnostic jobs creep, and the pane slowly sags until it touches the sill. A block too short for the glass area concentrates load and can chip the edge, and an overlong block can block weep paths so water sits against the seal.

Incompatible materials are a quieter problem. Some plastics and rubbers react with sealants or tape, so check the maker’s compatibility notes before mixing parts from different sources. And never stack random packers under a pane to fake a block. The stack can shift under load, and the pane’s weight then lands on whichever piece is smallest or hardest.

FAQ

Can I use glazing packers instead of setting blocks?

Only if the part is specified by its maker as a load-bearing support block, because ordinary flat shims are for position and clearance. Setting blocks are normally called out at 80 to 90 Shore A under ASTM C864, and a thin packer of unknown hardness may compress or creep under the pane. Check the maker’s data sheet before putting any part under the glass edge.

Where do setting blocks go on a pane?

Glass manufacturers such as Pilkington place two blocks on the bottom edge, centred at the quarter points. If that is not practical, the block can move toward the corner, but within 6 inches (152 mm) or one-eighth of the glass width of the quarter point. Always follow the glass maker’s table for your unit.

How do I know what size setting block to buy?

Size the block from the glass weight and the thickness of the unit. Calculate weight at about 2.5 kg per square metre per millimetre of glass, then compare it to the block length and width in the glass manufacturer’s guidance. If the supplier’s listing does not give hardness, material and dimensions, ask for the data sheet before buying.