Roofing felt or roofing membrane? How to evaluate the two offers

A flat roof that has started to drip gives you two offers that look very different: tear off the old and lay new roofing felt, or leave the roof in place and seal the surface with a liquid roof membrane. The price difference rarely comes from the material, it comes from everything that has to be done with the roof before. Which way is the right one is determined by one question: is there water in the structure, or is the substratum degraded? If there is, no surface will help.

The technical difference between roofing felt and liquid roofing membrane

Roofing cardboard comes in strips. They are rolled out, welded or glued, and they overlap. Coverings are made for chimneys, skylights and roof wells. The roof is tight because every joint is tight.

A liquid roof membrane is applied by roller or brush, and one layer is enough. It hardens into one continuous surface, and edges and penetrations are painted with the same material.

Therefore, the leak is rarely in the middle of the roof

A roof works all year round. It expands in the sun and contracts at night, and the movement is not evenly distributed. It accumulates where the construction is locked: at fastenings, at edges, along the edge of a roof well and in the overlays. Those places are pulled and pushed again and again, while the surface in the middle of the roof lies still.

A joint is also made by hand, often in the wind, and the adhesive tolerates less movement with each passing year. The water therefore finds its way into an overlay long before the track itself is worn out. A continuous surface does not have the weak points, but it lies on top of what it lies on. If the substrate moves, it moves with it.

Half an hour on the roof tells you more than three offers

Do it in dry weather a few days after the last rain, and take a screwdriver with you.

• Go through the entire surface, also out at the edges. If the roof gives way underfoot, it is the substrate that is wrong, not the surface.
• Prick a hole in a bladder. If water comes out, there is water in the structure.
• Grab the edge of a lane. If you can lift the roofing felt free, it is not tight enough to support a seal.
• Look at the ceiling from the inside and see where the puddles are the day after rain.

When new roofing felt is the right choice

A roof membrane seals a surface. It does not repair a construct. If your half hour on the roof shows any of this, the roof must be taken up:

• Water in the insulation. Moisture that is trapped inside does not dry on its own.
• Soft or rotten substrate. If the wood under the roofing felt is broken down, it must be replaced.
• Roofing that has come off. Large blisters and loose webs. A seal follows what it is placed on.
• Lack of fall. If the water is standing because the structure is crooked, it must be built up. A surface cannot do that.
• You have to re-insulate anyway. If the roof is to be opened within a few years, there is no need to seal now.

If one of the five points is met, a membrane is a postponement and not a solution. So take the offer on new roofing felt. It's cheaper to find out now than to seal a wet roof and pay for the whole job again later.

When liquid roof membrane is the right choice

If the substrate is dry and firm, and only the surface is old, you pay to tear off something that is not faulty. Ask for the offer divided so that you can see what demolition, container and start-up cost on your particular roof.

• The substrate is firm and dry, but the surface is dull, cracked or leaking.
• The leak is in an overlay, at a covering or around a penetration.
• The roof is small: carport, garage, conservatory, dormer, balcony.
• You will do the work yourself.

If the leak is in one specific place, ask the supplier if you can settle for a limited area before you order for the entire roof.

How much do the two solutions cost per m²?

Measure the area and convert both ways to kroner per m². 20 kg membrane costs DKK 2,880 and covers up to 13.3 m², i.e. around DKK 216 per m² in material. It says up to, then on an absorbent substrate you place a margin on top.

We cannot give you a price for roofing felt here, it depends on what is underneath. In return, you can claim the offer split up:

• Demolition. How many layers are there today and how much do they cost to remove per m²?
• Disposal. Container, transport and tax according to weight, as a separate item and not in a total sum.
• New underlayment or insulation. Ask directly: what happens to the price if you open the roof and the insulation is wet? Get the answer in writing.
• Scaffolding. Set-up, take-down and rent per week is three numbers.
• Construction. A layer or two, mechanically fixed or fully glued. There lies the difference between the two offers.

Ask for lifetime and warranty in writing for both solutions. The membrane has an expected lifespan of up to 40 years and a 25-year product warranty. Ask what it covers if you roll it yourself.

Wages, time and weather

On the roofing paper side, the salary is the heavy item. Ask how many man-hours have been calculated and whether it is a fixed price or invoice work. Also ask how many days the roof is open and what happens if it rains.

The membrane is rolled or painted on, and one layer is enough. No gas burner or special tools are required. But the processing takes up a lot. The roof must be clean, dry and free of moss and loose particles.

The -40 to +90 degrees in the data sheet is the operating temperature, i.e. what the membrane can withstand when it is hardened, not what it must be applied in. Ask the supplier about the minimum temperature for application and about how many hours of dry weather there must be before you plan a weekend in November. Same question if there is water in pools several days after rain.

If you are in doubt after half an hour on the roof, measure the moisture in the structure first. And remember that the person selling a seal has an interest in the matter, just like the person selling a new roof. Ask them both to explain what works against their own solution.

See floating roof membrane from Luna Roof, and calculate how many buckets your roof will need.