Draw a basic room

Keep your existing model. Pick the Box tool below and draw a new box on empty grid space beside it — two corners, then the height.

Right-click the box → Assign Building Role → Room. Walls, a slab and materials come along.

Same trick as before: click the top face to grab it, then press the ⊕ button at its center.
Draw a line across the face (click two points).

Made it with Rectangle and pushed it up instead? That's an extrusion — it keeps its profile, so turn on Allow Free Edit in the right panel first.

Then click that new edge and drag the blue (Z) arrow up — the roof pitches as you pull.