What live load does a patio cover have to be designed for?

Not less than 10 psf vertical live load under IRC Appendix AH, except that snow loads are used where they exceed that minimum. It is a quarter of the 40 psf a deck carries, which is why a patio cover frame is lighter.

How tall can a patio cover be?

Appendix AH limits patio covers to one-storey structures not exceeding 12 ft in height. Above that the appendix no longer applies and the main body of the code does.

Can I park a car under a patio cover?

Not under Appendix AH. It states that patio covers shall be used only for recreational, outdoor living purposes, and not as carports, garages, storage rooms or habitable rooms. A carport is designed for different loads and uses.

How many posts does a 12x16 pergola need?

Six, three each side at 8 ft spacing, for a freestanding structure. An attached one needs a ledger on the house side and half as many posts, with the ledger rules applying in full.

How far apart should pergola rafters be?

Whatever suits the span and the look, commonly 16 to 24 in on centre. There is no prescriptive table for it, because the load is so light. What matters more is the beam span between posts and the size of the rafter across the width.

Does Appendix AH apply to my project?

Only if your jurisdiction has adopted it. Appendices are not automatically part of an adopted code, and adoption varies. Ask your building department before relying on the 10 psf load or the 12 ft height limit.

Can I add a roof to an existing pergola?

Structurally that turns it into a patio cover, and in a snowy climate that trebles or quadruples the load on a frame designed for an open top. Check the beam and rafter capacity before adding anything solid.

Home › Decking › Pergola and Patio Cover Calculator

Pergola and Patio Cover Calculator

IRC Appendix AH: 10 psf live load, 12 ft high, outdoor living only

A patio cover has its own appendix in the residential code and almost nobody reads it. Appendix AH sets a vertical live load of not less than 10 psf, a quarter of what a deck carries, caps the structure at 12 ft and one storey, and says plainly that it may not be used as a carport, a garage or a habitable room. That last sentence is the one that catches people out.

Pergola and patio cover

Width (the rafter span)

Length

Height to the beam

Post spacing

Rafter spacing

Rafter overhang (each end)

Ground snow load

psf

Roof

Quantities from your dimensions, with the Appendix AH load and height limits applied.

Specification summary

Generated from on . Reopen that address to reproduce these figures exactly.

Entered

Result

Posts, beam and rafters, under a 12 ft ceiling

Worked example

A 12 ft by 16 ft pergola, posts at 8 ft, rafters at 16 in on centre with a 12 in overhang each end, beam at 9 ft, no snow.

Posts. 16 ft at 8 ft spacing is 2 bays, so 3 posts each side: 6 posts.
Rafters. 13 rafters at 16 in on centre, each spanning the 12 ft width.
Design load. 10 psf live, the Appendix AH minimum, plus about 7 psf dead.
Enter a 30 psf snow load and the live load jumps to 30. Appendix AH says snow governs where it exceeds the minimum, and that trebles the load on a frame people size by eye.
Raise the beam to 13 ft and the verdict fails: Appendix AH caps a patio cover at 12 ft.

The snow step is the one worth playing with. A patio cover frame designed at 10 psf and a deck at 40 psf look very different, and a snowy climate pushes the cover most of the way to the deck.

The formula

Counts, and one load rule taken straight from the appendix:

posts = 2(⌈ L / s ⌉ + 1) rafters = ⌊ 12L / r ⌋ + 1 live = max(10, snow)

L, W: length and width, ft. The rafters span the width
s: post spacing along each side, ft
r: rafter spacing, in on centre
live: AH101: not less than 10 psf, except that snow loads govern where they exceed it
12 ft: the height limit Appendix AH puts on a patio cover

The load rule is a maximum, not a sum: you do not add snow to the 10 psf, you replace it. Ten is a floor for the case where there is no snow at all.

An appendix most people have never opened

Patio covers have their own appendix in the IRC, AH in the 2021 edition and H before it, running from AH101 to AH106. It is short, and it decides three things that change how the structure is built.

The load. Not less than 10 psf vertical live load, except that snow loads are used where they exceed it. That is a quarter of the 40 psf a deck carries, and it is why a patio cover frame is genuinely lighter than a deck of the same span rather than just being built lighter by habit.

The size. One storey, not exceeding 12 ft in height. Past that it is not a patio cover under the appendix and the main body of the code applies.

The use. Recreational, outdoor living purposes only, and specifically not a carport, garage, storage room or habitable room. Parking under a patio cover, or enclosing one later, changes what it is and what it should have been designed for.

The catch is that it is an appendix. Appendices apply only where a jurisdiction adopts them, and adoption is not automatic. Check before relying on the 10 psf, because without the appendix you are back to the main code and a heavier load case.

A pergola and a patio cover are the same frame

Posts, a beam each side, rafters across, and usually purlins on top. What differs is what goes over the rafters, and that changes the load rather than the structure.

An open pergola carries essentially nothing but its own weight and wind. A solid patio cover carries whatever lands on the roof, which is where the snow provision bites. A pergola that gets a polycarbonate roof retrofitted onto it two summers later is a patio cover that was designed as a pergola, and in a snowy climate that is a real problem.

Wind matters more for the open one than people expect. AH101 sends you to R301.2.1 for wind loads, and an open structure with a large flat top still generates uplift.

Attached or freestanding

An attached cover hangs off the house on a ledger and every word on the deck ledger page applies: flashing, fastener spacing, and the rule that a ledger may not be fixed to masonry veneer.

A freestanding one needs a post and beam each side, which is what this calculator assumes, and it needs to resist lateral load on its own. Diagonal knee braces at the post-to-beam joints are what usually do that, and they are the part most often left off for the sake of appearance.

Either way the posts land on footings, and the footing calculator is the relevant page. Frost depth governs and it is local.

Frequently asked questions

What live load does a patio cover have to be designed for?: Not less than 10 psf vertical live load under IRC Appendix AH, except that snow loads are used where they exceed that minimum. It is a quarter of the 40 psf a deck carries, which is why a patio cover frame is lighter.
How tall can a patio cover be?: Appendix AH limits patio covers to one-storey structures not exceeding 12 ft in height. Above that the appendix no longer applies and the main body of the code does.
Can I park a car under a patio cover?: Not under Appendix AH. It states that patio covers shall be used only for recreational, outdoor living purposes, and not as carports, garages, storage rooms or habitable rooms. A carport is designed for different loads and uses.
How many posts does a 12x16 pergola need?: Six, three each side at 8 ft spacing, for a freestanding structure. An attached one needs a ledger on the house side and half as many posts, with the ledger rules applying in full.
How far apart should pergola rafters be?: Whatever suits the span and the look, commonly 16 to 24 in on centre. There is no prescriptive table for it, because the load is so light. What matters more is the beam span between posts and the size of the rafter across the width.
Does Appendix AH apply to my project?: Only if your jurisdiction has adopted it. Appendices are not automatically part of an adopted code, and adoption varies. Ask your building department before relying on the 10 psf load or the 12 ft height limit.
Can I add a roof to an existing pergola?: Structurally that turns it into a patio cover, and in a snowy climate that trebles or quadruples the load on a frame designed for an open top. Check the beam and rafter capacity before adding anything solid.

Check these numbers yourself

IRC Appendix AH, Patio CoversUpCodes, 2021 IRC as adopted in Connecticut. Sections AH101 through AH106: design loads, height limit and permitted use. Confirm whether your jurisdiction has adopted the appendix.
IRC R301.2.1 wind design criteriaUpCodes. Appendix AH sends patio cover wind design here.
AWC DCA 6: Prescriptive Residential Wood Deck Construction GuideAmerican Wood Council, free PDF. Post, beam and footing detailing for the same kind of structure.

How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.

Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.

Related calculators

Appendix AH applies only where adopted. Appendices are not automatically part of an adopted code, so confirm with your building department before relying on the 10 psf live load or the 12 ft height. Snow governs where it exceeds 10 psf. A patio cover may not be a carport, garage, storage room or habitable room. This page counts members; it does not size them.