The shingles for a room addition are applied the same as for any new construction. The only difference is in joining the new shingles to the old.
In preparation, nail a metal drip edge along the bottom eave, cover the roof with 1 5-pound underlayment lapped 4 inches (19 inches for roof slopes less than 3 in 12), and nail drip edge over the felt at the rake edges. Some areas may also require ice and water guards along the starter row and in valleys.
Shingles are applied starting with a double course at the eave. As you work up roof slope, maintain a consistent pattern in lining up grooves between individual tabs. The easiest installation is to split the gaps (offset each course by 6 inches), but this pattern shows deviations and mis takes the most clearly. A 4-inch pattern gives a strong diagonal look, wears better than a 6-inch pattern, and is recommended for slopes less than 4 in 12. Many professionals use a 5-inch pattern, which is the most random appearing.
If the new roof is an extension from the gable end of the old roof, start each course at the intersection and maintain the same pattern as the old roof. Weave new shingles into old for a more durable seam by removing partial shingles from the end of each old course and continuing with a new full shingle.
If the new roof is perpendicular to the old, the intersection will form two valleys. There are two methods of roofing a valley: open and full-laced closed. For an open valley, install metal valley flashing over underlayment felt, tucking it up under house shingles. As you lay each course of new shingles, trim the valley opening to flare outward, eliminating edges that might catch water or debris.
To form a closed valley, flash the valley with a full sheet of roll roofing turned upside down to minimize cracking. Interweave shingles of alternating courses over valley and at least 1 foot up opposite side. Trim (dub) bottom of final shingle of each course about 2 inches to prevent water from collecting against it.
As you apply ridge shingles along the top of the addition, split the last shingle where it butts up against the house roof and tuck the two flaps up under the row of house shingles.
To install other types of roofing, see our guide How to Replace and Install Roofs and Sidings.