Special Roof Techniques
Although pre-fabricated trusses are used to frame the vast majority of new roofs, there's almost always a need for hand-framing to some degree to finish the roof framing. And there are some roofs that just can't be trussed.
We've collected helpful information on a few of those hand-framing situations here on this page.
For definitions throughout this project, see Framing Glossary.
Tying a New Roof Into an Existing Roof
How a new roof ties into an old roof will obviously depend on the existing structure and the planned addition. In fact, "tying in" is often not even covered on an addition plan, being left to the framers' discretion. But the process usually follows these steps:
Establishing ridge point on old roof: Run a string along the peaks of the new trusses, establishing a ridge line, and extend the string to the old roof along that line to determine where the ridge would hit.
Establishing valley lines: Snap chalklines from the ridge point to the points where the new walls intersect the old roof.
Cutting and securing valley boards: Measure the valley lines, cut two boards to that length with 45 degree angles on each end, nail those in along the valley lines.
Cutting and securing ridge board: Measure between the ridge point and the peak of the last new truss, cut a board to that length with an angle on one end to match the slope of the old roof, nail that board to the last truss and to the old roof at the ridge point.
Cutting and securing valley jacks: Mark 24" on center along new ridge board, use framing square and straight-edge to mark points on valley boards 90 degrees from each mark on ridge board, cut boards to fit with compound miter cuts for valley jacks at the roof pitch needed (angles given on framing squares) and nail each in place.
Hand-Framing a Small Hip Roof
Framing a full hip roof is beyond the scope of this page, but we can give you the basic steps for framing a small hip roof like the one we did in one of our projects.
The first step was to secure a 4-foot ledger board to the house wall centered between the addition side walls to anchor a series of common rafters, which will run down over the outside wall at a 2/12 roof pitch.
We also nailed up angled boards running from each end of the ledger board down along the house wall to the tops of the addition side walls (as nailers for the roof sheathing).
We decided to have the common rafters cross right over the studs in the outside wall, so we just measured out the same 16" on center layout from the side of the addition to determine the matching positions for the ends of the rafters on the ridge/ledger board. Then we measured down from the top of the ledger board at each mark to the outside of the wall plate at each corresponding mark and transferred those measurements to the 2x8 boards we were using for rafters.
NOTE: To transfer that measurement EXACTLY you must account for the height of the rafter where it crosses the wall on the board you're measuring. We first cut out a template of the plumb cut and seat cut we wanted on each rafter where it rested on the wall plate (known as the birds mouth cut), lined that up along the bottom of the rafter so that the top corner of the template was exactly the same length from the end of the rafter (measured from the top) as the length we measured on the wall. Then we traced the pattern on the rafter and cut each birds mouth.
The angles for the plumb cut (which you also need at the end of the rafter) and the seat cut are determined by your roof pitch, and they're listed in roof framing tables. Any overhang you want should be measured out past the plumb side of the birds mouth.
After cutting and securing the common rafters, we next measured and cut the hip ridges. For those, we measured from the ends of the ledger board to the outside corners, added extra for the overhang (which we would cut to proper length later) and cut 2 boards to those lengths with the appropriate birdsmouth cuts (plumb cuts set for a 2/12 pitch but the seat cuts for a 2/17 pitch, following the "rule of 17" for hips and valleys to account for the shallower pitches of those members).
We secured the hip ridges at the ends of the ridge/ledger board and ran those over the 2 outside corners of the addition.
We needed hip rafters running from the hip ridges out over the wall plate in the same layout as the common rafters. To mark the rafter positions on the ridges, we lined up a framing square with one leg resting on the wall plate and used a straight edge along the other leg to mark perpendicular points on the ridges corresponding to the rafter positions on the wall plates. Then we measured between the marks, transferred the lengths to 2x8's as described above and cut the rafters with the same birds mouth cuts as the common rafters.
NOTE: The end of a hip rafter requires a compound miter cut to rest solidly against the hip ridge, which runs at a 45 degree angle to the wall plates. You mark for the plumb cut angle for your roof pitch as on all the other rafters, but set your saw blade for 45 degrees and make your cut.
After installing the hip rafters, we then plumb cut the rafter tails to the overhang length we wanted and sheathed the roof.
Framing a Gable Dormer
Gable dormers are smaller pitched roof and walls units framed on top of the main roof. Shed dormers are built about the same but have a low-pitched roof. Gables add character and space to the roof level.
A gable dormer is framed by marking existing rafters and cutting out the area the dormer will occupy. The remaining outside rafters are doubled with same size lumber.
A doubled rafter header caps the cut rafters at the bottom. The front dormer wall sets on top of this header. The two side walls set on the doubled roof rafters. Top plates cap all three dormer walls.
A down-sized gable (end) truss sets flush on the top of the front wall. A dormer ridge board runs level from the gable peak back to the old roof surface -- or make a top rafter header if you have exposed rafters there.
Once that dormer ridge board is nailed in, each valley rafter can be cut. The valley rafters form the slope (valley) where the dormer roof meets the main roof.
Valley rafters go on each side of the dormer; between the point where the ridge board meets the old roof or top rafter header and the point where the side wall top plates stop.
After nailing the valley rafters to the roof/top header and through the side wall top plates, dormer rafters are bevel cut on the bottom, mitre cut on top, spaced on 16" centers and nailed to the ridge board and side walls.
If you have enough trusses exposed, additional jack rafters may be nailed 16" on center (o.c) to the back side of the valley rafters up to the roof's ridge board.
Some dormers may have added rakes and eaves to protect and decorate the dormers. A rake is built by nailing short perpendicular lookouts (16" o.c.) to the gable truss rafters. The lookouts are capped with fly rafters that run at the same angle as the truss rafters. A soffit board can be nailed on the bottom of the lookouts to finish off the rake.
Eaves are created by extending the dormer rafters past the side walls. Lookouts are nailed between the rafters and the side walls, and a soffit board nailed to the lookout bottoms completes the overhang.