EPDM is waterproof, but it does not create drainage. Installing an ordinary exposed EPDM roof deliberately dead level is poor practice: tolerances, joist deflection and low outlets can quickly turn “no fall” into a backfall with persistent ponding.
For most UK flat roofs, the target is positive drainage and a minimum completed fall of 1:80—equivalent to 12.5mm per metre. The design fall must allow for the actual structure, tolerances and deflection so that this completed fall is achieved.
What do no fall, zero fall and backfall mean?
- No fall: the deck has been built nominally level, without an intentional route to drainage.
- Zero-fall roof: a specialist technical category that may only be acceptable for specifically certified systems and designs.
- Backfall: the surface falls away from the outlet or gutter, trapping water. This requires correction.
A normal domestic EPDM roof should not be described as zero fall simply because the rubber can resist water.
What fall should an EPDM flat roof have?
Current BS 6229:2025 guidance retains a minimum completed fall of 1:80 on most flat roofs. Older blanket advice often used a 1:40 design fall—25mm per metre—to allow for construction tolerances and achieve 1:80 finished. The current standard expects the project design fall to be derived from structural analysis or detailed survey information rather than applying one assumption to every roof.
Manufacturer requirements, warranty conditions and project specifications can be more demanding. Check the current documents for the chosen system.
Why does ponding matter if EPDM is waterproof?
- water adds permanent load and can increase deck deflection, which then holds more water;
- debris and biological growth accumulate around low points;
- blocked outlets become harder to detect;
- ice and slippery growth create access hazards;
- a small puncture, open seam or failed outlet has greater consequences under standing water;
- persistent water can conceal wet insulation or a sagging deck.
Water remaining more than about 48 hours after rain is commonly treated as ponding in manufacturer guidance. It does not prove membrane failure, but it deserves investigation. Use Standing Water on an EPDM Roof: When Is Ponding a Problem? for diagnosis.
How can a fall be created?
Sloping structure or firrings
Joists may be installed to fall, or correctly designed firring pieces can create the drainage plane before the deck is fixed. This keeps the roof build-up simple but must account for support and edge heights.
Tapered insulation
A designed tapered scheme can create falls while improving thermal performance. It must coordinate high points, crickets, sumps, outlets and upstand heights—not just place wedges at random.
New deck or overlay build-up
Localised correction or a redesigned deck may be appropriate when the existing surface is irregular. Read EPDM Roof Deck: OSB3 vs Plywood. Adding a flat board over sagging joists does not correct the structure.
Relocated or additional drainage
An outlet can sometimes be moved to the true low point or additional drainage introduced. Capacity, overflow provision and structural openings need proper design.
Choosing the correct outlet
The outlet type follows the drainage route. The EPDM Horizontal Drain Kit serves an appropriate through-edge detail, while the Internal Drain 68mm Kit suits a compatible internal discharge arrangement. A Horizontal Outlet 100mm offers another edge-outlet format.
These products do not determine how many outlets a roof needs. Outlet number and capacity must be calculated for roof area, rainfall and blockage risk. Enclosed parapet roofs also need independently functioning overflow provision.
How to assess an existing roof with no fall
- Inspect after rain and mark the extent and duration of puddles.
- Use a level or laser survey to map high points, low points and backfalls.
- Clear and test gutters, outlets and overflows without flooding the roof.
- Inspect internally for deflection, staining and condensation.
- Check the deck and insulation for moisture and decay.
- Confirm that added falls will leave sufficient height at rooflights, walls and doors.
- Decide whether correction requires firrings, tapered insulation, outlet changes or structural repair.
Do not hide a drainage defect beneath a re-cover. Read Can You Lay New EPDM Over an Existing Flat Roof? before retaining old layers.
Installing EPDM after the falls are corrected
Use a sound, smooth and dry substrate. EPDM Water Based Deck Adhesive (WBA) bonds field membrane to suitable porous timber, while Rubber Roofing Contact Adhesive is used in specified perimeter and upstand zones. Adhesive does not fill dips or create a fall.
Locate seams and details away from unavoidable low points where possible, form outlets to the new waterproofing level and protect the work from damp conditions; see Can You Install EPDM in Cold or Damp Weather?.
Decision guide
- Roof drains fully with no backfall: proceed after normal deck and system checks.
- One shallow low spot: investigate the cause and use a designed local correction.
- Widespread ponding or sagging: assess the structure and deck before waterproofing.
- Parapet roof with one outlet and no overflow: redesign the drainage strategy.
- Intentional water storage: specify an engineered blue-roof system; this is not ordinary ponding.
Frequently asked questions
Is a 1:80 fall enough?
It is the usual minimum completed benchmark, but outlet capacity, geometry, deflection and the chosen system still matter.
Can tapered insulation fix a level roof?
Often, yes, when designed as a complete scheme with drainage and upstand heights. It cannot stabilise a failed structure underneath.
Will more adhesive stop ponding?
No. Adhesive bonds the membrane to the immediate substrate; it does not reshape the drainage plane.
Can I install EPDM over a sagging deck?
No. Establish why it has moved and repair or replace the structure and deck first.
Is a blue roof the same as a ponding roof?
No. A blue roof is an engineered attenuation system with controlled outlets, overflows and a certified build-up. Unplanned standing water is a defect to investigate.
Design the water route before the membrane
A flat roof should be flat in appearance, not dead level in performance. Establish positive falls, place correctly sized drainage at true low points and verify the finished surface before covering it with EPDM.
