Joints
Board joints are staggered relative to insulation joints and membrane seams. No joint presses on another joint.
VlamShield DEK · Thin mineral cover board below the membrane
The firm layer
below the membrane.
DEK is a thin, rigid mineral cover board laid between insulation and membrane on new roofs or during re-roofing. It gives the waterproofing a flat, compression-resistant mineral substrate exactly where PV supports, ballast and foot traffic put their loads.
01 · Function
Function and installed position
PV supports, ballast and maintenance traffic apply local loads to the membrane. Substrate deformation depends on the compressive strength of the insulation, the contact area and the duration of loading. An appropriate cover board helps distribute that local pressure over a larger area.
DEK places a thin, rigid board in between. Loads spread over a larger area, the membrane lies flat and the insulation stays out of direct contact with supports and tools. The board replaces neither insulation nor structure nor waterproofing; it makes the layers below and above work better together.
Schematic representation. Not a construction drawing; the actual build-up and detailing follow from the project.
What loads act here?
Point loads from PV supports
Feet of mounting systems and ballast pavers press on a few square centimetres. The cover board spreads that pressure across the insulation.
Installation and maintenance
During PV installation and inspections the roof is walked on and material is set down. A firm substrate limits indentation and damage to the membrane.
Fire load from above
When a fault occurs on or between PV modules, the membrane sits in the fire load. A mineral board directly beneath the membrane puts a non-organic layer above the insulation. What that means for the complete roof assembly follows from the evidence for that assembly, not from the board alone.
02 · Material
Material build-up
The reference for DEK is a cover board of the DensDeck Prime type: a modified gypsum core faced with glass mat on both sides, factory-made in thin, rigid boards. No solid concrete, no soft fleece, no wood fibre.
- Modified gypsum core with glass mat on top and bottom faces
- Thin, flat board with straight edges for butt joints
- Stored dry and installed dry according to system instructions
- Supplied with the data sheet and classification report of the specific board
03 · Installation
Installation on the roof
- 01
Check the substrate
Insulation flat, dry and completely laid. Height differences at joints and junctions are resolved beforehand.
- 02
Lay and stagger the boards
Boards are butt-jointed with joints offset from the insulation joints so that no continuous joint runs through.
- 03
Fix according to the system
Mechanically fixed, adhered or loose-laid under ballast: the method follows the system instructions of membrane and insulation and the wind load on the roof.
- 04
Cut and fit
Infill boards are sawn or scored and snapped. At outlets, penetrations and upstands the board fits closely so the membrane is supported everywhere.
- 05
Apply the membrane immediately
The cover board is not exposed to rain. The membrane follows in the same working pass, or the board is covered.
Joints, outlets and upstands
Outlets and penetrations
Around outlets the board is cut to fit; the membrane remains the waterproofing. The board must not raise or block drainage.
Upstands
At edges and upstands the board stops against the rising structure. For protecting the rising face itself, WEER is the matching line.
04 · Data
Technical data
Reference values from manufacturer documentation
Values for the reference board DensDeck Prime according to the Sika data sheet (June 2026). They apply to that make and that document. The board we select for your project is offered with its own data sheet and classification report.
| Material | Modified gypsum core, glass mat both faces |
|---|---|
| Thickness and surface mass | 6.4 mm ≈ 5.9 kg/m² · 12.7 mm ≈ 9.8 kg/m² |
| Reaction to fire (EN 13501-1) | A1, as stated for the reference board |
| Position | Above the insulation, below the membrane |
| Storage | Dry, flat, covered |
Defined per project
These points are set out in the project proposal, with the supporting documents.
| Thickness and format | Per project, matched to membrane, insulation and loading |
|---|---|
| Fixing | Mechanical, adhered or loose under ballast, per system instructions |
| Added roof load | Surface mass × area, for assessment by the structural engineer |
| Evidence | Classification of the board and assessment of the complete roof assembly, named separately |
Source: Sika data sheet DensDeck Prime (June 2026) · Manufacturer information Georgia-Pacific
05 · Selection
When yes, when no
Suitable
- New build or complete replacement of the membrane
- PV on ballast or supports above soft insulation
- Roofs where the membrane needs a firm, flat substrate
- Projects where dry, fast board installation fits the working sequence
Not suitable
- An existing roof whose membrane stays in place: then VLOEI, or WEER at details
- As a finish exposed to the weather
- As a replacement for insulation, structure or waterproofing
- When the roof cannot carry the added mass of the selected board (reference roughly 6 to 10 kg/m²)
06 · Questions
Questions from practice
Does DEK replace the membrane?
No. DEK sits below the membrane and is not waterproof. Waterproofing remains the job of the membrane.
Can DEK be laid on an existing membrane?
Not as a cover board. If the existing membrane is removed or a new build-up is created, DEK sits on the insulation below the new membrane. If the membrane stays, we look at VLOEI.
How much weight is added?
That follows from the thickness and surface mass of the selected board. According to its data sheet, the reference board is around 5.9 kg/m² (6.4 mm) to 9.8 kg/m² (12.7 mm). Multiply by the roof area and have the structural engineer assess capacity.
Does that make the roof fire-safe?
The board has its own reaction-to-fire class; the roof assembly as a whole is assessed separately. In the proposal we name which evidence belongs to the board and which to the complete assembly. A material classification is not an approval of the whole roof.
Project enquiry for VlamShield DEK
What we need for a quotation
With this information we prepare a concrete proposal: configuration, quantity, documentation and programme. If something is missing, we ask.
Project enquiry- New build or membrane replacement, with programme
- Planned membrane and insulation (type, thickness)
- PV arrangement: ballast, supports or integrated
- Roof area and available capacity according to the structural engineer
- Section or drawing of roof edge and penetrations
