Setting blocks for roof lights

Setting blocks are small, firm supports placed between a roof light unit and its upstand before the perimeter is sealed and fixed. They look insignificant on site, but they carry the full dead weight of the glass at the bearing points and keep the unit at the correct height for weathering. Poor block selection or placement is one of the more common hidden causes of toughened glass breakage and seal compression faults.

Why blocks matter

A flat roof light can weigh from tens to hundreds of kilograms depending on size and build-up. The glass edge must not bear on an uneven kerb, sharp corner or trapped membrane fold. Setting blocks:

• Transfer load to the upstand at controlled points.
• Level the unit across minor height variations in the bearing plane.
• Hold position while perimeter sealant and fixings are applied.
• Keep the glass clear of the raw substrate so sealant can form a continuous fillet.

Toughened glass has high bending strength but is sensitive to edge stress from point loads or twist. Laminated inner panes add weight. Blocks are part of the structural interface, not packing shims chosen from site offcuts.

Block material and hardness

Use blocks specified by the roof light manufacturer or firm neoprene / EPDM rubber of approximately 80–90 Shore A. Soft foam, cork or multiple layers of compressible tape are unsuitable — they creep under sustained load and create uneven support as the unit settles.

Blocks should be:

• Non-staining — no bitumen or oil that attacks sealant bonds.
• Dimensionally stable — cut cleanly to width; typically 50–80 mm long depending on edge depth.
• Of consistent height — mixed thicknesses across one edge induce twist.

Framed roof light kits sometimes include proprietary nylon setting shoes. Use the supplied components rather than substituting generic rubber unless the manufacturer approves an equivalent.

Placement rules

Exact positions vary by product, but the industry pattern is well established:

1. Two blocks per bearing edge, set inward from corners — often near quarter-points along the length.
2. Inset from corners — corners are stress concentrations; blocks too close can wedge the glass.
3. Within edge cover — blocks must sit where the unit is fully supported by the upstand, not overhanging the void.
4. Clear of drainage paths — on units with internal drainage or rear channels, blocks must not block water paths specified in the detail.
5. Symmetric layout — unequal spacing induces twist when the unit expands under sun load.

Mark block positions on the upstand before lifting the unit. Dry-run with the frame or glass carrier to confirm the unit sits level and that all blocks make contact without rocking.

Installation sequence

A typical frameless installation sequence:

1. Clean and dry the upstand bearing surface.
2. Apply primer to the kerb if required by the sealant system.
3. Position setting blocks on adhesive pad or spot-bonded as per detail — they must not shift when the unit is lowered.
4. Lower the unit onto blocks using suction lifters or the manufacturer’s lifting instructions.
5. Check uniform block contact and equal edge reveal before fixing.
6. Install perimeter fixings, compression gaskets or cappings as specified.
7. Apply weathering and internal sealant, maintaining the ventilation gap at the internal perimeter.

Do not stand on the glass to adjust blocks after placement. Adjust from the perimeter using approved lifting gear.

Interaction with sealant and ventilation

Blocks set the unit height for the perimeter sealant fillet and must not block the internal ventilation gap between glass and lining.

Common mistakes

Mistake

Consequence

Soft foam instead of firm rubber

Uneven creep, edge stress, eventual crack

Blocks at corners only

Mid-span edge sag on wide units

Block over a membrane buckle

Point load into glass

Different block heights on one edge

Twist, seal gap on opposite side

Removing blocks after sealant cures

Loss of designed support — never do this

Specification note for architects

When writing performance specifications, require setting blocks as defined by the roof light manufacturer, installed to their layout drawing. Generic clauses such as “glazier to provide suitable packing” invite non-compliant substitutions.

For made-to-order frameless roof lights, fixing layouts are confirmed at order stage from your opening size. Build the upstand to those dimensions and use the block positions supplied with the installation guide.

Every Vant Glass roof light is made to order in Britain, backed by a 20-year guarantee and free UK mainland delivery. Configure frameless or framed sizes in the online calculators or call 03330 902 592.