Ok, after you ungrouped the object, the walls, you should think about using different textures for the roof, the ground and the walls. we want a nice grass texture for the ground. so, select a nice grass texture in the texture browser, select the brush that should have the grass texture and click on the "Apply Texture" button.
Viola... now you have a nice grass ground.
we do the same thing with the roof, but we use the "Sky" texture for it. it will be rendered as the sky ingame. remember that the sky texture is not within the esf textures. but since you need the zhlt textures, you have the sky texture already in it. i recommend using the half-life.wad also, because it contains the sky texture, too.
after our edits it looks like this