In a large bowl, stir together the warm water and sugar. Sprinkle the yeast over the top and let stand until foamy, about 5 minutes.
Stir in the salt, wholemeal flour and rye flour. Mix in the bread flour 1 cup at a time until the dough is not sticky. Knead on a floured surface until smooth and elastic, about 10 minutes. Place in an oiled bowl, turning to coat. Set aside to rise until doubled in size, about 1 hour.
Punch down the dough and divide into 4 or 5 portions. Form each one into a ball, and let rise again until doubled, about 20 minutes.
Place each ball onto a baking tray and flatten out into an oval 1-2cm thick. Poke holes all over using the tines of a fork. Let rise again until doubled, 20 to 30 minutes. Preheat the oven to 180 degrees C.
Bake for 20 minutes in the preheated oven, or until browned on the top and bottom. Brush the tops with butter while still warm.