Hi Anthony,
I once had a Lattin multiband antenna in a roof space. The problem was all the TV antennas were up there too.
I suggest you make up an off centre fed dipole. Feed it with 75 ohm low loss tv coax.
Leave both ends about 2 ft long on the short end and 4 ft long on the long end. Run the dipole diagonally across the slope of the root from one corner to the opposite one. This will accommodate about 30 ft of wire. The physical centre of the wire shold be at the ridge. Bend the ends to run parallel with the 20 ft widths. Keep the wires about 4 inches clear of the roof and
4 inches away from the building. This will help with minimising detuning in wet weather.
Run the last bits of wire back along the longer sides. Keep 4 inches clear of the building.
Run the coax at right angles to the main part of the antenna as much as you can on its way to the shack.
Check for resonance in 40 and 20 m. If you are lucky the SWR will be less than 3:1. No problem if it isn’t. So long as the minimum is somewhere near the frequency on 40 m you want to use you will be able to use an ATU to make the rig happy. For 20 m the ATU should match up OK too.
If the 40 m and 20 m resonances are too low reduce the radiator length. If too high, add wire. Adjusting 40 m towards the lower end of the band will put the 20 m resonances near the top end.
I would concentrate on getting 40 m right and accept whatever you get on 20 m.
I would not expect much on other bands. This is an attic antenna not a multi band Yagi at 100 ft.
If you want the maximum number of bands then cut the dipole for 6 MHz and connect the centre to a 4:1 balun and install as before. Connect the bslun to a remote tuner with a short length of coax. Feed the tuner with 50 ohm coax. You should be able to get a match on most bands from 5 to 50 MHz
73
Ron
VK3AFW