Rooflights and upstands concentrate several difficult EPDM details into a small area: vertical bonding, four changes of direction, inside or outside corners and a termination beneath the rooflight frame. A neat field sheet will still leak if any one of those junctions is bridged, stretched or sealed with the wrong product.
The exact flashing sequence must follow the membrane and rooflight manufacturers’ details. This guide explains the planning decisions, product roles and common questions that should be resolved before cutting the sheet.
What is a rooflight upstand?
The upstand or kerb raises the rooflight above the finished waterproofing surface and gives the membrane a stable vertical surface for termination. It also separates the glazing and frame from rainwater moving across the roof.
UK flat-roof guidance commonly expects a 150mm upstand above the finished roof level, subject to the particular design and any approved threshold detail. Measure from the finished waterproofing or overburden level—not simply from the structural deck before insulation is installed.
Plan the rooflight before laying EPDM
- Confirm the rooflight’s required kerb dimensions and fixing zone.
- Check the finished upstand height after insulation and falls are included.
- Keep the rooflight clear of outlets, walls and other obstructions so details can be formed properly.
- Direct falls around the kerb without creating a backfall or trapped pond.
- Make the kerb square, rigid, dry and smooth.
- Decide the membrane cuts and corner-flashing sequence before adhesive is applied.
- Confirm how the membrane terminates beneath or against the rooflight frame.
If the surrounding roof retains water, resolve the drainage design first using Standing Water on an EPDM Roof: When Is Ponding a Problem?.
Which products are used around a rooflight?
Contact adhesive for vertical surfaces
The field membrane is normally bonded up the kerb using a compatible two-sided contact adhesive. Rubber Roofing Contact Adhesive is intended for appropriate perimeter and upstand details within its compatible EPDM system.
Apply it to the specified surfaces, allow the correct flash-off and work the membrane tightly into the base angle without bridging. Deck adhesive should not be substituted on vertical or non-porous surfaces unless the system expressly permits it.
Primer for pressure-sensitive flashing
Primer prepares the EPDM to receive compatible tape and flashing. For ClassicBond details, use ClassicBond EPDM Primer with the stated application tool and coverage. Primer is not a gap filler or general-purpose glue.
Uncured flashing for corners
Corners change direction in more than one plane and require a formable material. 9 Inch Elastoform Uncured EPDM Tape can be shaped around compatible inside and outside corner details where the system specifies it.
Do not stretch the centre thin or force one piece to cover more geometry than the detail allows. Patch sizes, overlaps and cuts must follow the manufacturer’s drawing.
A purpose-made seam roller
A Professional Everhard Silicone Seam Roller applies controlled pressure into the base angle and across flashing edges. Hand smoothing alone may leave voids at changes of plane.
Field membrane or separate curb flashing?
Some domestic one-piece systems bring the main membrane up the rooflight kerb, with controlled corner cuts completed by compatible flashing. Other systems use separate curb-flashing pieces tied into the field sheet.
Do not combine methods from different installation videos. Use the detail supplied for the exact membrane, kerb and warranty. The membrane should lie naturally without tension, fishmouths or a fold channelling water into the corner.
How to form an EPDM rooflight detail
1. Prepare and inspect the kerb
Confirm it is firmly fixed, square, dry and free from sharp edges, proud screws and contamination. Check the frame or cap can still be installed without puncturing the finished flashing.
2. Position and relax the membrane
Dry-lay the complete field sheet and confirm adequate material around every side. Allow it to relax according to the membrane instructions before marking any cut.
3. Mark cuts conservatively
A cut that runs beyond the planned flashing zone creates an avoidable repair. Use the system template or measured method, keep cuts controlled and verify each corner before proceeding.
4. Bond the field and kerb in sequence
Complete the main deck bond using the correct deck adhesive, then use compatible contact adhesive at the kerb as directed. Work progressively into the base angle so there is no bridged void.
5. Prepare the corner zones
Keep deck and contact adhesive out of areas intended for pressure-sensitive products. Clean if required, mark the flashing limits and apply compatible primer beyond the finished edge.
6. Apply corner flashing without overstretching
Shape uncured flashing gradually through the change of angle. Maintain the specified overlap onto sound primed membrane, round corners where required and avoid sharp points.
7. Roll every plane and perimeter
Roll the roof surface, vertical face, base angle and all flashing edges. Pay special attention where pieces overlap or meet another seam.
8. Complete the top termination
Finish beneath the rooflight frame, cap or specified termination so water cannot track behind the membrane. Sealant should be used only where the rooflight or membrane detail requires it.
Inside and outside corners are not interchangeable
| Detail | Main challenge | What to check |
|---|---|---|
| Outside corner | Flashing must stretch around projecting geometry | Formability, minimum overlaps and rounded edges |
| Inside corner | Excess membrane can form folds or channels | No fishmouths, bridging or open termination |
| Base angle | Membrane may tent between deck and kerb | Firm contact through the complete change of plane |
| Top of upstand | Water can track behind the flashing | Correct frame, cap or termination detail |
Common rooflight-detailing mistakes
- Installing the kerb too low above the finished roof.
- Positioning a rooflight where water is trapped behind it.
- Cutting the membrane before it is aligned and relaxed.
- Using water-based deck adhesive on the vertical kerb.
- Contaminating primer and tape zones with bonding adhesive.
- Using cured cover tape where formable uncured flashing is required.
- Stretching flashing until it becomes thin or stressed.
- Leaving a bridged void at the base angle.
- Relying on sealant to close a poorly formed corner.
- Fixing the rooflight frame through an unapproved part of the membrane detail.
How to inspect a finished rooflight detail
- Confirm all four sides are fully bonded and free from tension.
- Check every corner for fishmouths, voids and lifted edges.
- Look for primer extending correctly beyond tape without missed zones.
- Verify the top termination is continuous beneath the frame or cap.
- Check falls carry water around the kerb towards the outlet.
- Inspect after rainfall for ponding, staining or water tracking.
- Photograph the completed detail for the installation record.
Frequently asked questions
Can EPDM be wrapped around a rooflight in one piece?
Some systems use the field membrane up the kerb with controlled cuts and separate corner flashing. Follow the exact system detail rather than assuming one uncut sheet can conform to every corner.
Can ordinary roof sealant finish the corners?
No. Sealant cannot replace the specified primer, flashing and rolling sequence. Use it only at locations required by the approved detail.
Why is my EPDM lifting at the rooflight?
Possible causes include poor flash-off, contamination, bridging at the base angle, insufficient rolling, an unstable kerb or water ponding against the detail. Diagnose the cause before patching.
Can a leaking rooflight corner be patched?
A local compatible repair may be possible when the membrane and kerb remain sound. Weathered EPDM must be cleaned and primed correctly, and the patch must extend onto sound material. Wider movement or water behind the upstand requires further investigation.
Should the rooflight be installed before the EPDM?
The sequence depends on the rooflight and membrane details. The kerb is normally established before waterproofing, while the glazing unit, cap or frame may be fitted at a specified stage. Follow both manufacturers’ instructions.
Order the complete rooflight detail
Before starting, confirm the membrane, contact adhesive, EPDM primer, uncured corner flashing, seam roller and specified termination materials as one compatible system.
For adhesive roles, read EPDM Adhesives Explained. If the roof deck is still being planned, see What Roof Deck Should You Use Under EPDM?.
This guide provides general information. Follow the current rooflight and membrane manufacturers’ drawings, structural design and applicable Building Regulations for the exact installation.
