Bitumen felt roofs
Bitumen felt roofs, meaning roofs covered in felt or roofing paper over a boarded deck, are mounted with hanger bolts or bracket plates sealed by a flashing collar bonded or torched into the membrane itself. Selection turns on one thing: the age and condition of the felt. Sound, recently laid membrane takes a sealed penetration well, a brittle one should be re-covered first. Rails, clamps and roof accessories come from the pitched roof range. Guide
Related felt roof categories
- Shingle roofs
- Flashings and sealing washers
- Rail profiles and clamps
- Pitched roof systems
- Flat roof systems
Bitumen felt roofs: the membrane decides, not the timber
A felt roof is a waterproof membrane laid in overlapping sheets on a solid deck, bonded or torched at the laps, and it works because that sheet is continuous. Every fixing driven through it is a deliberate breach of a system designed to have none. The timber underneath behaves as it does on a shingle roof, the fixing goes into the rafter, but the sealing detail differs: a flashing collar or a sleeve of matching membrane is bonded, torched or hot-air welded over the base plate so the patch becomes part of the covering rather than a seal sitting on it. That is why membrane compatibility matters: where a modified sheet or a plastic protection layer is present, the patch and the adhesive must be the ones the membrane manufacturer approves.
FROM THE FIELD
"Before I quote a felt roof I press a thumb into the membrane in a couple of places and look at the laps. If the surface crazes or the laps have lifted, the roof has less life left in it than the array. Re-covering costs a fraction of stripping a system off in eight years, and fresh membrane gives you clean, weldable material at every penetration."
Ladislav Proc · Mounting systems specialist, ONSA Plus
How to choose a mounting system for a bitumen felt roof
Assess the covering before the array. The condition of the felt decides whether you mount now or re-cover first, and the membrane type decides which collar and adhesive may touch it. Only then does the usual sequence apply: find the rafters, set the fixing grid, size the rails.
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Judging the age and condition of the felt
Bitumen membrane fails gradually and visibly. The array will be on the roof for twenty-five to thirty years, so the honest question is whether the covering can match that.
- Surface condition: granule loss, crazing, blistering or a shiny, dried-out look means the bitumen has lost its volatiles and is going brittle. Brittle felt cracks around any new fastener.
- Laps and upstands: lifted or open laps, and upstands pulled away at abutments, mean water is already tracking. Fix that before adding fixings.
- Weldability: sound membrane takes a torched or welded patch cleanly. A weathered surface must be cut back, primed or overlaid before a collar will bond.
- The re-cover decision: if the felt has under ten years left, re-cover before mounting. Refitting a full array to re-roof later costs far more than the new membrane does today.
Sealing the penetration into the membrane
The aim is a detail that behaves like the rest of the roof, not a plug in a hole. A hanger bolt or base plate is fitted into the rafter, then a collar or piece of matching membrane is dressed over it and bonded down so water runs across a continuous surface. On a pitched felt roof the sheet above must always lap over the collar, never the other way round. Where a protection or separation layer is present, check that collar, primer and adhesive are approved for that layer as well as for the felt, because a bitumen product against a plastic sheet can soften or embrittle it.
Tip from practice
Get the sealing detail signed off by the roofer in writing before you start. On most buildings the membrane carries a workmanship warranty, and a penetration made by a third party voids it unless the covering contractor agreed the detail. That email is cheaper than a disputed leak two winters later.
Above the membrane: rails, spacing and standing water
Once the fixings are in and sealed, the layout follows the rafter grid, commonly 0.6 to 1.0 m in European timber roofs, and that spacing sets the rail span. Keep base plates out of valleys and off the low side of any dip, because water lingers longest there and a collar sitting in standing water ages fastest. Route DC cabling in clips or a tray rather than letting it lie on and abrade the membrane.
Frequently asked questions about mounting on bitumen felt
What is a bitumen felt roof in mounting terms?
It is a roof covered with bitumen felt, also called roofing paper, laid in overlapping sheets on a solid boarded deck. For mounting it behaves like any solid-deck roof: fixings pass through the covering into the rafters and are sealed with a bonded flashing collar.
How is the penetration sealed on a felt roof?
With a flashing collar or a patch of matching membrane bonded, torched or hot-air welded over the base plate so it becomes part of the covering. The sheet above must always lap over the collar, so water runs across a continuous surface.
Should I re-cover the roof before installing panels?
If the felt has less than about ten years left, yes. An array lasts twenty-five to thirty years, and stripping it off later to re-roof costs far more than a new membrane now. Fresh membrane also gives clean, weldable material at every penetration.
How do I tell whether the felt is still sound?
Look for granule loss, crazing, blistering and a dried-out sheen, all signs the bitumen has gone brittle. Check laps and upstands for lifting, and walk the deck for soft spots indicating trapped moisture. Brittle felt cracks around any new fastener.
Does the collar have to match the membrane?
Yes. Use the collar, primer and adhesive approved for that membrane and for any protection or separation layer above it. Bitumen products against some plastic sheets cause softening or embrittlement, and an unapproved patch will not bond reliably anyway.
Will fixings invalidate the roof covering warranty?
They can. Most membrane workmanship warranties exclude penetrations made by others. Agree the detail with the roofing contractor in writing first, and where possible have them execute or inspect the collars. Sealing components are in roof accessories.
The age and condition of the felt covering decide whether it can be penetrated at all. Have it assessed before mounting.
About ONSA Plus
Why installers across Europe order roof fixings from us
ONSA Plus is a European B2B distributor of photovoltaics, battery storage, heat pumps and EV charging stations. In our own EU warehouses we keep over 1,500 products from 25+ brands, including Huawei, Solis, SolaX and Dyness.
We are not just a trading company. We design and service energy systems ourselves, so we answer questions on felt roof fixings and sealing from engineering practice, not from a catalogue. You handle a warranty claim directly with us and you buy per job.
