Both are neutral cure silicone. Both come in the same 20 fl oz sausage. On the shelf they look identical. They do completely different jobs, and the difference is load.
Quick answers
Can weatherproofing silicone be used for structural glazing?
No. Weatherproofing silicone keeps water and air out of a moving joint and carries none of the building. Structural silicone holds the glass on the building, and its bite and glueline are calculated by the project engineer.
Which grade does a four-sided or hidden-frame system need?
Structural, engineered. A cap bead over a captured system, a perimeter joint or an expansion joint is weatherproofing work.
What if the specification names ASTM C920 or ASTM C1184?
The product on the job has to carry that standard by name. Where ASTM C1184 structural compliance is required, order DOWSIL 795; Silande MF899 declares GB 16776 and ASTM C920, not C1184.
Weatherproofing silicone
Its job is to keep water and air out of a joint that moves. It is not carrying any of the building. Perimeter joints around windows and doors, panel-to-panel joints, expansion joints, cap beads on a curtain wall — that is weatherproofing work.
What matters here is movement capability. Ours is rated plus or minus 50 percent, meaning a 1/2" joint can open to 3/4" and close to 1/4" without the bead failing. Silande MF898 is the weatherproofing grade, in five colours.
Structural silicone
Its job is to hold the glass on the building. In a hidden-frame or four-sided structural glazing system there is no cap, no pressure plate, no gasket doing the holding. The silicone bead is the attachment. It carries dead load, wind load and thermal movement, and it does so for the life of the facade.
That is a different product with different tensile requirements, and it is designed and sized by the project engineer — bite and glueline are calculated, not chosen on site. Silande MF899 is the structural grade.
How to tell which one the job needs
| If the joint... | You need |
|---|---|
| keeps weather out and carries nothing | Weatherproofing |
| is what holds the glass to the frame | Structural |
| is a cap bead over a captured system | Weatherproofing |
| is four-sided or hidden frame | Structural, engineered |
The part that is about liability, not performance
Read the specification before you order. If it calls out ASTM C920 for a weather seal, or ASTM C1184 for a structural seal, or requires the manufacturer's structural warranty, then the product on the job has to carry that certification by name.
Our Seimeda sealant is laboratory tested to GB 16776-2005 and GB/T 13477, and we will send you the full four-page report and a sample so your own QC can prove adhesion on your actual substrate.
Silande prints its production standards on its own packaging: MF898 to GB/T 22083 and ASTM C920, class 100/50, and MF899 to GB 16776 and ASTM C920. That is the manufacturer's declaration, not an independent test report and not a US code approval, and neither Silande grade declares ASTM C1184. We send the manufacturer's data sheet with any sample.
Where the spec demands ASTM C1184 structural compliance by name, order DOWSIL 795 — Dow lists it as compliant with ASTM C1184 and ASTM C920 Class 50. We stock it by the case of 16, in white, black, grey, bronze and limestone.
One thing both grades share
Test adhesion on the real materials before you commit a facade to anything. Coatings change, anodising batches change, the mill finish on this job is not the mill finish on the last one. Ask us for a sample; we would rather send one than argue about a failure.