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

A flat roof that has started to leak presents you with two seemingly different options: tear off the old and install new roofing felt, or leave the roof in place and seal the surface with a liquid roofing membrane. The price difference rarely comes from the material, it comes from everything that needs to be done to the roof beforehand. Which way is right is determined by one question: is there water in the structure, or has the substrate degraded? If there is, no surface will help.

The technical difference between roofing felt and liquid roofing membrane

Roofing sheeting comes in sheets. They are rolled out, welded or glued, and they overlap each other. Flashings are made at chimneys, skylights and roof wells. The roof is tight because each individual joint is tight.

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

Therefore, the leak is rarely located 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 distributed evenly. It collects where the structure is locked in place: at fasteners, at risers, along the edge of a roof well and in the overlays. Those places are pulled and pushed over and over, while the surface in the middle of the roof remains still.

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

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

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

• Go over the entire surface, including the edges. If the roof gives way under your feet, it is the substrate that is at fault, not the surface.
• Prick a hole in a bladder. If water comes out, there is water in the structure.
• Grab the edge of a sheet. If you can lift the roofing felt free, it is not tight enough to carry a seal.
• Look at the ceiling from the inside and see where the puddles are the day after it rains.

When new roofing felt is the right choice

A roof membrane seals a surface. It does not repair a structure. If your half hour on the roof shows any of these, the roof needs to be taken down:

• Water in the insulation. Moisture that is trapped inside does not dry out on its own.
• Soft or rotten substrate. If the wood under the roofing felt is decayed, it must be replaced.
• Roofing sheet that has come loose. Large blisters and loose webs. A seal follows what it rests on.
• Missing drop. If the water is standing because the structure is crooked, it needs to be rebuilt. A surface cannot handle that.
• You still need to re-insulate. If the roof is going to be opened within a few years, there is no need to seal now.

If any of the five points are met, a membrane is a postponement and not a solution. So get a quote for new roofing felt. It's cheaper to find out now than to seal a wet roof and pay for the whole job again afterwards.

When liquid roofing membrane is the right choice

If the substrate is dry and solid, and only the surface is old, you will pay to tear off something that is not faulty. Ask for a split quote so you can see what demolition, container and start-up costs on your particular roof.

• The substrate is solid and dry, but the surface is dull, cracked or leaking.
• The leak is located in an overlay, at a flashing or around a penetration.
• The roof is small: carport, garage, conservatory, attic, 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 ordering 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 2,880 DKK and covers up to 13.3 m², which is about 216 DKK per m² in material. It says up to, then on an absorbent surface you put a margin on top.

We cannot give you a price for roofing felt here, it depends on what is underneath. However, you can request a split quote:

• Demolition. How many layers are there today, and how much does it cost to remove them per m²?
• Disposal. Container, shipping and tax by weight, as a separate item and not in a total amount.
• 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 rental per week are three figures.
• Structure. One layer or two, mechanically attached or fully bonded. That's where the difference between the two offers lies.

Ask for a lifetime and warranty in writing on 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 on yourself.

Wages, time and weather

On the roofing felt side, the salary is the heavy item. Ask how many man-hours are included, and whether it is a fixed price or a billable work. Also ask how many days the roof will be open, and what happens if it rains.

The membrane is rolled or painted on, and one layer is enough. No gas torch or special tools are required. But the preparation takes a lot of time. 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 cured, not what it should be applied in. Ask the supplier about the minimum temperature for application and how many hours of dry weather must be left before you plan a weekend in November. The same question if there is water in puddles for several days after rain.

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