Any type of roof addition will need to be tied in for it to work properly. Tying in a roof simply means linking the pieces together and connecting them. By tying the roofs together, you are making the joint much more waterproof, which will minimize the chances of allowing water inside and leaking.
Step 1 - Planning
As every single roof is different it's impossible to decide exactly how the roof will be tied together. You need to look at your individual roof and then decide how they will work with one another. This will all depend on whether or not you are building a complete two story addition, or just building a single story addition. If you're building a single story addition on a two story property, then the roof will need to be tied together against the wall. In this case, you will use flashing to seal the property from the elements.
Step 2 - Lifting the Old Tiles
When you are tying in a new gambrel roof you will need to work out which parts of your existing roof it will affect. Remove the tiles of the affected area on the existing roof so that you will be able to nail the new roof down into the joists on the original roof. Try to remove as few tiles as possible so that you will be able to limit the changes to your existing roof. Try to number all of the tiles so that you know exactly where they should be located and how they fit back together.
Step 3 - Nailing the New Roof
Now the new roof needs to be nailed into the old so that the two are connected together. Use long roofing nails to do this and apply a coat of roofing compound in the gaps and on all of the nail joints. Once the new roof is fitted you can then start to add the rafters and build up the new roof in the normal fashion. Plywood will be mounted over the rafters and then your roof covering of choice can be used.
Step 4 - Replacing the Roof Tiles
Now simply place a thick layer of roofing cement onto the space where you took the old tiles away. Then press the tiles back into the roofing compound so that they are firmly stuck. More roofing cement needs to be applied in the joints and on top of the tiles to ensure that the gap is completely watertight.
If any leaks or weak areas of the roof develop over time then these will need to be filled and dealt with as swiftly as possible to minimise the chances of costly damage.