New felt roof on top of the old?

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We have some leaks on the flat roof for the loft conversion which was "redone" by previous owner 20 years ago, according to questionnaire before exchange ownership - but unclear what the "redone" refer to.

Got a roofer had a look. He said there are some parts of the felt here and there damaged/missing - we saw a video did show there were gaps on the felt.

We thought the entire felt needs to be replaced, as it's been 20 years even if last owner has replaced the entire felt. But the roofer guy seemed to decide in the end instead of put an entire scaffolding over across the roof, he could use a long ladder from the 1st floor roof to get to the top.

Today, we've received the quote and we are a bit unsure if he meant to "repair" and "fill" the gap on the existing felt, or he'd apply a new layer of flat on the entire existing roof. We are thinking, if the old felt has already reached 20 years lifespan at least, wouldn't it make more sense to replace it? Is covering it up a common practice. The quote isn't clear to us as to what exactly to be done. Any comments will be appreciated!

"All works to be carried out to existing flat roof falls and levels unless otherwise stated.

Sweep down and make good as required the existing covering of the flat dormer roof.

Fully torch bond a single layer of 4kg/sq.m High Performance polyester reinforced mineral surfaced bituminous cap sheet to the roof surface and form upstands to the sloping roof.

Form drip edges to the remaining roof perimeters using separate lengths of cap sheet cut and bonded onto the roof surface."

Bump

Sounds like he believes the existing felt is mostly sound, he can patch the issues, and then add a hot tar sealed (torch is used to melt the underside of the new felt as it’s applied) heavy duty top coat.
How big is the area? What guarantee does he offer?
Why don’t you believe what he says? Even though he showed you a video?
The alternative of scaffolding and a full strip down would be considerably more expensive.

@OneForTheToad Thanks. It's not that we don't believe him, just thought usually felt material only last 20 years and so it's time to replace rather than spending a grand a a half on patching up. That's the main point really.

We will ask the question to the roofer, but want to get a second opinion if we could.

3 years guarantee is offered.

I would guess he’ll get up there, sweep and wash the existing felt off. Cut out any imperfections or bubbles and patch it up so the complete surface is sound, then add the new top coat.
Ask him how long he thinks it will last as opposed to a complete new replacement.
Three year guarantee isn’t bad though. He’s obviously confident it will last.

Our neighbours have a flat roof that they seem to have to get continually patched. Every year there seems to be someone up there tinkering away. I'm sure they would have been better off getting the whole thing done properly once. At it again recently, I think another leak.

Sounds like your roofer is OK, but I would like to understand a bit more about exactly what he's doing in plain English. And if he's not doing the whole roof, the relative cost if he did, and how likely other areas are to fail if he's only doing patches. Can you get up there so he can show you what's there and what he intends to do? If the guarantee is only on his work, other parts failing may not be covered so check exactly what the guarantee covers.

@OneForTheToad Thanks. What you described is really helpful. I think we have more details to ask for clarification now. Much appreciated!

@Silverbirchtwo I did get an impression this guy is far better than the previous people we used. He did some repair work in the front sloped roof before Christmas for three areas of leaking. Two out of three were fixed, but the third remaining leak is the cause of this current conversation. He believes it's a problem channel from the flat roof side as the lead is under the felt and taking in water from the higher side. DH said he could see water running underneath the damaged felt on the video he took.

The state of the felt looks alright to me in most of the area, about 3x4 m2. I attach a couple of photos.

When we got a (Flat roof replaced in Banbury ) they took all the old felt off and put new wood and new felt but ours was a complete mess maybe yours was still in good condition