Gingerbread Exchange Community

That's definitely the best way to handle tricky shapes, since you can esaily access the inside seam for where you want your royal, but in the past when I haven't had any cardboard to hand (think 2am On Christmas morning!) I have used a couple of other techniques with pretty good success.

The tricky part with multi piece roofing is always the downward force of gravity pushing the bottom of the pieces apart or pushing the tops down onto one another, so if you can prevent the pieces from sliding out or falling down then 50% of the battle is won.  I used to use tin cans or other weighted objects to keep my pieces from sliding out, but now I use tins only as props in the center and scrap fondant around the base. The rest of your battle will be fiddling the pieces into position precisely.

On my work surface I place a template outlining the shape of the base of the roof, I then use some scrap fondant rolled out roughly and pushed down to stick onto the surface to create a "barricade" in the outline of the base.  Unlike tins or books around the outside, which can impede your hand movements, the fondant stays out of your way.   I also find a tin, glass, cup, bowl or anything that will help to prop up my roof pieces from the center as I am stacking them. Depending on how good the fit is, it may remain in there until the royal is dry, or may remain only so long as I need it to hold up 2 or more pieces in an approximatoin of where they will end up finally and while I incorporate the other pieces.

The main disadvantage to this technique is that the inner seams are on the underside and are just a little more tricky to access for application of royal, but still definitely achievable. 

Although the method that Adam described is definitely the easiest, I prefer to see how the outside of my pieces fit together rather than how the inside of my pieces fit, especially if I want flush outer seams, so I use the fondant barricade technique when I'm not planning to cover the roof with decorative pieces that may disguise slightly gappy roof fitting.

As with every technique, patience is the key ingredient; waiting an appropriate amount of time for the royal to cement is essential.  I run workshops every year, and every year, the houses that fall are by those people who rush their roof and don't wait long enough for the royal to set properly, either in their walls or roof.