I would do it this way... This looks like a lot now, but I'll best explain every step with a screenshot. For this method it doesn't matter where your house is placed or how it's rotated.
Let's say you already have the house with a sloped roof and those "border faces" cut which I've selected in the first screenshot to show them (your screenshot looks like you have a model like that, too):
Now press Ctrl+R for the Loop Cut tool, move your mouse to the side until the cut is shown crosswise on the building. Left-click to confirm the cut direction.
After the first click, you can edge slide the cut towards the end where you want "false" side slopes. Get them close to the end, left-click once more to confirm the cut.
Now get rid of the large face at the side end of the house and also the small rim faces at the top and bottom. Select them (I showed it in Wireframe Mode to make them all visible). Hit X > Delete > Faces.
If you now select the outmost edge on the left or right of the new hole in the house, you can fill this hole by pressing F three times, this will create three faces from left to right (or the other way round if you selected the right edge). Now your house has to "rims" at the end which only need to be sloped in the right angle as the roof.
Select the top corner edges on the roof. To move vertices along an edge without changing the angle or direction of the edge (i.e. preserving the original slope angle), you can use Edge Slide. Enable it by pressing GG (that's a double G). Note that now you can only move towards the house or down the edges. To extend the edges beyond the existing mesh, you have to disable clamping. To do that, you hit C once (or keep pressing Alt). Very important: the direction of the edge that is going to be extended along is determined in the moment you press C or Alt, so make sure your mouse is somewhere above the middle of the top roof edge. The extension will be indicated by yellow lines going beyond the mesh. If it's the wrong direction, press C again or release Alt to enable clamping, then place the mouse differently and hit C another time to disable clamping again. Sounds more difficult than it is.
Now you can move the mouse until the "false" slope sides extend as much as needed. Then left-click or Return to confirm the new position and you're finished with the sides. To create a roof on top, go to the next steps.
For the roof we first have to create a face between the extended sides. Select the three inner edges and press F to make a new face.
Before you can extrude the roof, make sure that the new face is oriented correctly. Enable Face Orientation in the overlays. If the face is red instead of blue, select it (if not already), then use Alt+N > Flip.
When they are all blue, select the top faces and simply hit E to extrude them upwards. But be aware that this will (usually) extrude along the (averaged) face normal, which means it doesn't go straight up the Z axis, but angled in the direction of the slope (the average normal of all selected faces). To change that while extruding hit Z once and you can move freely, hit Z again and it will move upwards along the global Z axis. Pressing Z a third time will get you back to moving along the face normal.
To make the roof overlapping the building, you can select all faces along the border of the extruded roof, then press Alt+E > Extrude Along Face Normals.
The difference to the normal Extrude is, it extrudes along the different face normals respectively, so that the roof borders extend to the outside. You might experience strange angles on the corners like in the following screenshot:
If you like the corners like that, slightly rounded instead of sharp 90° angles, you're fine ;) To change them into sharp corners, hit S to turn on Even Thickness. And that's it. You can of course repeat the extruding actions to create another "layer" of roof on top of the one you have now.
