Design and Build A Patio Roof To Tie In To An Existing Roof
Two videos cover the design questions and construction process of a post and beam patio roof structure
Video 1: Design a Roof extension over an existing patio
Existing patio is outside the dining room on the west side of the house. the patio is essentially unusable on a sunny afternoon in the summer.
This video looks at a couple of design options, including attaching the roof ledger below that of the existing eaves and setting the patio roof atop the existing roof.
The latter option was chosen because the low roof and roof beam would have blocked the view from the dining area. Did we say that the house is in Bozeman, MT, where the view is worth viewing?
Work from the view back, not from the house, out.
- Calculate the sightline clearance height, about 6 feet, 2 inches, in this case.
- Determine the beam size: 8 inches in this case.
- Choose a roof pitch (3/12) and roof structure (2x6)
This places the new roof on top of the existing roof. The next question is what to do with the existing eave under the new roof: kick it or keep it?
To tie the design together, Peter chose to cut the rafter tails off the extend the exterior wall up to the underside of the new roof. As it turned out, the existing roof structure is a truss system, but they were able to contact the original truss manufacturer to verify that cutting the tails off would not jeopardize the structural integrity of the trusses.
From the we-hope-this-is-obvious desk: Never alter engineered trusses without the approval of a licensed professional engineer. Ever.
The next design decision: material selection
This new patio roof structure will be a prominent focal point, so it should look good. The team chose a rough-hewn look: 8x8 rough sawn posts and beam and 2x6 rough sawn rafters.
The underside of the roof is exposed 1x6 atop the 2x6 rafters, and it was prestained.
Video 2: Construction process
The rough cut edges of the posts and beam were eased with a power planer and all of the material was prestained.
Because the posts are rough-sawn, custom post bases had to be fabricated. The post bases consist of a bottom plate and two vertical fins. The plate is bolted to the concrete with 5/8 inch expansion bolts.
The posts were cut to length, fastened to the post bases with lag bolts, and plumbed into position.
The horizontal beam was chamfered along the top at the bearing point of the rafters so that the rafters did not need a bird's mouth cut.
The beam was temporarily braced in place and the two outer rafters placed into position to show how much of the existing roofing needed to be removed.
The eave was removed, truss tails cut off, and the exact bearing point of the rafters determined.
A continuous 2x6 ledger is placed between the two outer rafters, and the subsequent rafters were notched to sit atop the 2x6 ledger.
The ledgers were then fastened to the 2x6 with an 8 inch structural screw through the top of the rafter down into the 2x6 ledger.
Blocking between the rafters (prestained) gives the wall below a clean termination point. The blocking is held low, to hide the roof/wall intersection and to provide a 1 inch air space at the top for a ventilation channel.
At the beam end, the rafters are fastened with a 10 inch structural screw through the top of the rafter into the beam.
The roof deck is 1x6 attached to the top of the rafters with 1/4 inch space between boards. Black roofing felt accentuates the shadow gaps from below.
5/8 OSB is applied on top of the felt paper and 1x6s to make sure no roofing nails penetrate the 1x6 or shadow gaps
Temporary bracing on the post/beam connections was then replaced with custom connection plates.