Okay, so it's not strictly medieval, although it's certainly olde worlde enough to play the part :)
It's made from mountboard, balsa and thick (270gsm) card for the roof.
Thanks for your comments :) The grey slate panders to those playing LOTR (I build to sell for the most part) and grey slate has become the Bree look since the Perry's gaming table was featured in WD.
As for the slate being too expensive for a stable, I guess that depends on where the stable is in relation to the quarry, and the slate could be reclaimed too, but whatever :)
The building is clean because it's to sell, and most of my clients prefer to muddy their pieces up themselves. As for the grey beams, wood greys with age, it's rarely brown, but aesthetically I accept your point.
A few criticisms: It's too clean, and I think the beams should be brown, not grey. They look like metal right now. If you want to make one that looks more medieval, try giving it a thached roof - use an old towel soaked in glue or something. A slate roof would be too expensive for a stable back then... And make it nice and grungy...