ᐅ Sealing a Flat Roof with Bitumen on a Residential Building – DIY Guide

Greetings everyone,

As we are planning to install photovoltaic panels on our flat roofs, I would like to prepare them now for the next 20 years to avoid the risk of having to remove over 130 modules later on.
We are dealing with two separate flat roofs.
Both are covered with bitumen membranes, have a 2-degree slope, and no gutters.
They are 20 years old and visually in good condition.

1. Single-story residential building
- 270 sqm (2,906 sq ft) flat roof
- Roof structure: wooden beams 60/200 mm (2.4/7.9 inches) with insulation in between + 22 mm (0.9 inch) plywood panels on top
- Beam spacing 1 m (3.3 ft)
- 1.2 m (3.9 ft) roof overhang all around
- 3 to 4-layer bitumen roofing membrane (mostly with root barrier, as the roof was planned for greening)
- 1 roof dome
- 2 sanitary vents
- 1 connection, 6 m (19.7 ft) to another building

2. Shed
- 150 sqm (1,615 sq ft) flat roof
- Roof structure: Doka H-beams 60/200 mm (2.4/7.9 inches), no insulation + 22 mm (0.9 inch) Doka formwork panels on top
- Beam spacing 1.3 m (4.3 ft)
- 30 cm (12 inches) roof overhang
- 2-layer bitumen roofing membrane
- No connections

How would you suggest re-sealing these flat roofs?

I would like to do all the work myself to save costs.
My idea is to apply a new bitumen membrane layer perpendicular (90 degrees) to the current orientation. I would prefer not to penetrate the intact roofing membrane but to either use a KSK system or torch-on application.
A roofer who inspected the roofs in person said it would need to be fixed mechanically with EJOT screws into the wood.

What are your tips or recommendations? Specific suggestions for a top layer material are also welcome.

Thanks in advance.
Attached are some photos