For a house roof the recommended minimum slope is 2 5 vertical inches for every horizontal foot but a nearly flat roof is fine for under deck installations.
Pitch for under deck roof.
Then stretch a string between them.
Minimum slope is 1 4 inch vertical in 12 units 2 slope and ideally 1 2 vertical in 12 units 4 slope.
Install the purlin along the house and the outer purlin.
Step 2 install a wood support structure for your panels screwing a row of 2 by 4s running parallel to your wall every 2 feet along the underside of your deck.
If you consider it as similar to a roof than according to code.
The slope or pitch of the under panels can be at any degree that will shed the water.
Modified bitumen roofing requires a minimum roof pitch of 1 4 12 or greater.
If you choose to add a roof to an existing deck you may need to add additional supports so it can hold the weight of the roof.
Adding a roof to your deck is best done as the deck is being built.
If the span under your deck is 12 ft for example the purlin at the outer end of the deck should be 3 in.
The minimum pitch for a roof is 1 4 12 which translates to 1 4 inch rise to 12 inches of run.
The footings for porch decks must be larger than deck footings.
As a result most decks have inadequate footings to support a new porch roof.
So let s say a roof rises 6 inches for every 12 inches in depth towards the peak.
Typical decks are designed to support 55 psf lbs per square foot.
Thermoset and thermoplastic single ply roofing requires a minimum roof pitch of 1 4 12 or greater.
Many porches use 22 diameter corner and intermediate footings.
First mark all the purlin locations on the deck joists.
The most common styles are gable hip and shed.
That is to say the roof pitch expressed as a ratio tells you how many inches the roof rises for every 12 inches of depth.
The roof pitch would be described as a 6 12 pitch.
However you can only use this pitch with built up roofing or specialized synthetic roofing.
Consider including an ice and water barrier too.
Lower than the purlin along the house 12 x 1 4 3.
This is for metal roofs that have a sealant between their lapped seams.
Sprayed polyurethane foam roofing requires a minimum roof pitch of 1 4 12 or greater.
Porch decks require 25 additional psf for roof loads for a total of 80 psf.
Among the newest is the raintight system introduced at deck expo 2014.
The primary reason roof pitch exists in the first place is to redirect water.