Who created this, how it was made and why it exists
- Who
- Published under the organizational identity Pergola Wiki Editorial Team; neither AI nor the organization is presented as a named professional individual.
- How
- AI assistance was used for drafting, translation, source-metadata organization, static-site coding and automated QA in a publisher-directed workflow. Automated checks do not establish technical correctness.
- Why
- To provide free bilingual educational synthesis; one disclosed Oscar Pergola industry link does not determine conclusions, source tiers or rankings.
Read a detailed text description
Arrows follow rainfall from the roof or closed louvers to falls and joints, then into gutters, outlets and downpipes before reaching a safe discharge point. A separate overflow path represents the backup route needed when the primary outlet is blocked or rainfall exceeds the assumed capacity.
Key takeaways
- Every load needs a complete path.View claim record ·
PWC-POSTS-BEAMS-RAFTERS-1 - Notches and holes can reduce capacity.View claim record ·
PWC-POSTS-BEAMS-RAFTERS-2 - Frame geometry controls stability as well as appearance.View claim record ·
PWC-POSTS-BEAMS-RAFTERS-3
New or rewritten sections in this topic use section-mapped citations; the claim register remains editorial synthesis, not external fact-checking or independent review.
On this page
Roles
Rafters or louvers collect local actions, beams gather them, posts carry combined vertical and lateral effects, and bases deliver reactions to foundations. Connections must transfer the forces assumed in the design model.
Coordination
Lighting channels, gutters, concealed wiring and decorative cuts should be coordinated before fabrication. Field drilling at a highly stressed zone can invalidate the intended section behavior.
Structural aluminum design reference cited by the International Building Code.
Read the frame as a hierarchy
Roof elements deliver reactions to rafters or louvers, then to beams, posts, bases and soil—or into an attached building. A discontinuity anywhere can govern the system. Decorative sleeves and internal gutters often conceal the actual joint, so inspection and documentation should identify the load-bearing component rather than the visible cover.
Alignment is performance evidence
Out-of-plumb posts, beam twist, unequal diagonals and inconsistent roof falls can indicate erection error, foundation movement or connection slip. Establish tolerances before installation and record a baseline at handover. Later comparison is more reliable than judging movement from memory or a single photograph.
ANSI-approved wood design standard referenced by 2024 US model codes.
A catalogue size is not a clear span
The size a buyer types — 3 × 4 m, 4 × 4 m, 6 × 4 m — usually names the marketed footprint or the roof plan, not the structural quantity a designer has to check. Two systems sold as the same size can put the posts in different places, leave a different unobstructed distance between those posts, project a different amount of roof past the end supports, and disagree about whether the quoted figure includes the overhang, the blade tips, the fabric valance or the gutter. A louvered bay, a fabric canopy and a fixed roof can also span different distances inside the same outline, because the covering is carried by its own rails or purlins inside the perimeter. The practical step is to stop comparing advertised sizes and compare the clear span instead: the unobstructed distance between the members that actually carry the roof, the post centres and base positions, the projection beyond each end support, and the datum used for height — the underside of the beam, the top of a closed blade, or the highest point when the louver field is open. Those facts belong in the brief, because the clear span and the actions are what a member is checked against, and a drawing that only repeats the catalogue size is not a design record. The perimeter size still matters for a different reason: it sets the plan, the falls, the circulation, the guarding interfaces and the approval footprint, so the same structure can be right for the site and wrong for the span.
ANSI-approved wood design standard referenced by 2024 US model codes.
Official standard hub for US structural actions including wind, snow, rain, seismic and load combinations.
Span sets the members, and the covering sets the actions
A member's job is fixed by the span it crosses and the load delivered to it, so a section that is comfortable over a short clear span can become the governing element over a longer one. In the hierarchy this page describes, covering elements deliver reactions to rafters or louvers, rafters to beams, beams to posts and posts to bases; lengthening any link increases the demand on that link and on everything below it. For a uniformly loaded member under standard support conditions — the relationships set out as beam formulas in standard wood design references — the bending moment varies with the square of the span while deflection varies with the fourth power, so doubling a clear span does not double the demand. An overhang or cantilever adds bending at the support and changes what the post has to hold, including the tension and uplift that govern the anchors. Two further things travel with the span. First the covering: an open rafter frame, a fabric shade, a closed louver field, a glazed or solid roof and a side screen each deliver a different set of actions over the same plan area, which is why ASCE 7 and Eurocode 1 treat wind, snow, rain and imposed actions as separate cases and why the covering state has to be stated as part of the design basis rather than discovered later. Second the material: aluminium's elastic modulus is roughly a third of steel's — about 70 GPa against about 200 GPa — so at similar geometry a member deflects more under the same load and deflection often governs before strength does; the Aluminum Design Manual is the aluminium reference the model codes point to, the AWC NDS supplies the corresponding framework for wood, and Eurocode 5 treats deflection as a serviceability check alongside resistance. None of this produces a number for a particular pergola: it explains why the clear span, the covering state and the material designation have to be fixed before anyone quotes a member size.
ANSI-approved wood design standard referenced by 2024 US model codes.
Structural aluminum design reference cited by the International Building Code.
Expert-prepared implementation material for Eurocode 5 timber design, hosted by the European Commission JRC.
European framework for actions including self-weight, imposed, snow and wind actions.
Official standard hub for US structural actions including wind, snow, rain, seismic and load combinations.
What to require for a specific span
For a specific span, ask for the submittal rather than the label. A set that makes a span claim checkable contains six things. First, the clear span between the members that carry the covering, with post centres, base positions and the projection beyond each end support dimensioned. Second, a member schedule that names the governing member for each span, its material and its grade or temper, and — for wood — the strength grade and the treatment or use category rather than a colour name. Third, the assumed actions and load cases the check rests on, including the wind, snow and rain basis, the dead load of the covering as supplied, and any maintenance live load the supplier has assumed. Fourth, the deflection limit that was applied, because a statement that a frame “carries” a span says nothing about how much it will sag or how a roof will still shed water once it does. Fifth, connection details showing how forces move between rafter, beam and post, together with base reactions for the anchor and footing design. Sixth, the covering and enclosure state the design assumes, so that adding a solid roof, glazing, a side panel or a screen later is handled as a change to the design basis rather than an accessory. Where none of these exists for the configuration actually proposed, the honest reading is that the span is unverified for that configuration — not that it is unsafe, and not that it is adequate. This page publishes no span, capacity, price or approval outcome for any product or project.
ANSI-approved wood design standard referenced by 2024 US model codes.
Structural aluminum design reference cited by the International Building Code.
Pergola span and size FAQ
Does a 4 × 4 m pergola mean a 4 m clear span? Not necessarily. The figure usually describes the marketed footprint or the roof plan, and it may or may not include the overhang, the blade tips, a valance or a gutter, while a louvered bay or a fabric canopy can be carried by rails inside that outline. The distance that matters structurally is the unobstructed distance between the members that carry the covering, together with the post centres, the base positions and the projection beyond each end support. Ask for those dimensions on a drawing, with the height datum stated, before comparing two systems. Can the same frame be extended over a longer span? Not by assumption. A member's demand grows much faster than the span itself — for a uniformly loaded member under standard support conditions the bending moment varies with the square of the span and deflection with the fourth power — and a longer span also changes the reactions, the uplift the anchors must resist and often which member governs. Treating a longer span as the same system at a larger size turns a checked design into an unchecked one; it is a re-design, and it should go back to the supplier or the designer with the new span and the unchanged or restated actions. Does the roof or covering type change what span is possible? Yes. An open rafter frame, a fabric shade, a closed louver field, a glazed or solid roof and added side screens or panels each deliver a different set of actions over the same plan area, which is why the covering state belongs in the design basis. A frame that was checked as an open structure is not automatically checked once it is covered, glazed or enclosed, and the same applies in reverse when a cover is removed. Record the covering, the enclosure state and any added screens or electrical equipment, and re-check the members, connections, anchors and drainage whenever they change. What should a supplier's span or member schedule contain? At a minimum: the clear span and the supporting positions; the governing member for that span with its material and grade or temper, plus the treatment or use category for wood; the assumed actions and load cases, including the covering dead load; the deflection limit applied; connection details and the base reactions needed for anchor and footing design; and the covering and enclosure state the design assumes. Also ask who is responsible for the structural design and for any approval, and which items are excluded. A schedule that lists members without the span, the actions or the deflection criterion does not let anyone verify the claim.
ANSI-approved wood design standard referenced by 2024 US model codes.
European framework for actions including self-weight, imposed, snow and wind actions.
Official standard hub for US structural actions including wind, snow, rain, seismic and load combinations.
Nominal-size-to-span check
Use this table to record what a marketed pergola size actually commits. Each row is a fact to obtain in writing from the supplier or designer for the configuration proposed; it is not a capacity, span or approval conclusion for any product.
| Checkpoint | Define | Retain |
|---|---|---|
| Advertised size and datum | What the quoted dimension measures — roof plan, frame outline or blade field — whether the overhang, valance or gutter is inside it, and the height datum used | Dimensioned supplier drawing with the datum written on it |
| Clear span and supports | Unobstructed distance between the members that carry the covering; post centres, base positions and base fixity | Dimensioned plan and elevations; assumed support conditions |
| Overhang and cantilever | Projection beyond each end support and whether a cantilever action is assumed at the connection | Drawing plus the assumed support and cantilever conditions |
| Covering and enclosure state | Open rafters, fabric shade, closed louver field, glazed or solid roof, screens or side panels, and any added equipment | Written design basis listing the covering dead load and the enclosure state |
| Member schedule | Governing member for each span, material and grade or temper, treatment or use category for wood, connection schedule and the deflection limit applied | Member and connection schedule with the deflection criterion stated |
| Reactions, responsibility and exclusions | Base reactions for anchor and footing design; who is responsible for the structural design and for approvals; what the quote excludes | Design-responsibility statement, calculation basis and exclusions list |
Evidence check before acting
This article provides general educational synthesis; these three checks turn reading into project-specific verification work.
| Confirm | Stronger evidence to retain | What this article cannot establish |
|---|---|---|
| Geometry, connection, load-transfer and drainage details at every interface | Approved details, manufacturer instructions and installation records | A rendering or component list does not establish an assembled system's load or water path. |
Registered cited sources and scope
Structural aluminum design reference cited by the International Building Code.
ANSI-approved wood design standard referenced by 2024 US model codes.
Official standard hub for US structural actions including wind, snow, rain, seismic and load combinations.
European framework for actions including self-weight, imposed, snow and wind actions.
Expert-prepared implementation material for Eurocode 5 timber design, hosted by the European Commission JRC.
Cite this page
Pergola Wiki Editorial Team. “Posts, beams and rafters.” Pergola Wiki. Content version 1.7.50; editorially reviewed 10 Oct 2026. https://pergola.wiki/articles/posts-beams-rafters/