First thought is, do you know if the roof is weight bearing and what kind of load its designed for?

Well you can certainly try clematis in a large pot (minimum 2 feet deep, 12 inches wide), but you'd need to shade the pot in some way to keep the roots cool, perhaps by placing other pots in front with other planting in - grasses would probably do well in that situation.

or a realy big pot and put some nice natural stone and or other plants in front of the stem as like bamboo sais it needs a lot of shade.im sure you can plant a clematis 6" deeper than the pot it came in to help with this which is quite unusual to do in most cases.

Your clematis is going to want to climb a lot higher than your railings...

I read somewhere that clematis like there root run to be damp but well drained rather than just having their roots in shade. It has been common practise to plant or place stones around the base of clematis to give them shade. Do you have enough space at ground level in front of the garage to plant in to the soil. You can then run the clematis up on a narrow support until it reaches the roof. Sit back and watch it explode. You can train it along the railings. Any of the montanas will survive happily. I particularly like Marjorie because the flowers last a long time. When planting clematis, plant it at least four inches deeper than the surrounding soil level because if it happens to catch the disease 'Wilt' it will die back and you will need to remove and burn or certainly remove from your garden all of the dead foliage. to stop the wilt spreading to other nearby clematis. The plant will regenerate from the roots. You have done wonders in your garden. It is lovely. If you can plant in the front of the garage, Hydrangea petiolaris is another suggestion and would give better year round interest. It will need a little support at first but should be suitable for training along the railings. It is easy to train by tying in the stems.

scotsgaran has the best idea if you can i think .

That will be beautiful. Why not choose a later flowering clematis than the montana as it is a bit of a thug and will want to take over whereas one of the later summer flowering clematis will prolong your colour and enhance the whole picture. It will be cut down before the hydrangea flowers and will be less greedy of space. If you choose a montana you could just keep the H pet. as a climber up to the roof and then the C mont. can have the space above that. Pyracantha is another trainable shrub. It will need to be tied in but it is spikey and a deterrent to anyone thinking of scaling the wall. The rose Knifsgate is another suggestion. You are spoilt for choice. Spitzhendry has a great blog on clematis choice ,care and pruning.

The blog was
Clematis - Prune or not and posted on 5th June 2009. She shows lovely examples in her own garden.
If you use the A-Z at the foot of the page, choose C and then clematis or Climbers for other examples. It has stopped raining so I must go and plant out my peas and water the potatoes.