Timber size for a pergola: posts, beams and the 4 measures that decide

The timber size for a pergola depends on four things: the span between posts, the roof load, the height and whether it stands alone or leans against a wall. Posts typically range from 75mm by 75mm for a small freestanding structure to 150mm by 150mm for a wide span or a roof carrying climbing plants. Beams follow the span: a 2.4-metre gap might need 100mm by 200mm, while 4 metres asks for 150mm by 200mm or doubled-up sections. This guide walks through the measures, the logic behind them and how to match timber to your garden. Blakesleys covers the practical choices so you can plan with confidence.

Span and spacing: where size begins

The distance your pergola reaches between posts—the span—decides most of the timber sizing. A short span of two to three metres asks less of the beams than a wide one of four or five. Freestanding pergolas often use posts spaced around 2.4 metres apart; lean-to designs fixed to a house wall may stretch further because one side carries no post at all.

Post spacing affects beam thickness. Closer posts mean lighter beams; wider gaps need thicker sections to prevent sagging. A 100mm by 200mm beam works well for spans up to 3 metres; beyond that, move to 125mm by 200mm or double up two beams side by side. Doubling also stiffens the structure without moving to heavier single sections.

Post dimensions for different heights and loads

Posts carry the weight of beams, rafters and anything growing on top. A 75mm by 75mm post suits a lightweight pergola under 2.5 metres tall with a simple open roof. For heights above 2.5 metres or a roof carrying polycarbonate panels or dense climbers like wisteria, step up to 100mm by 100mm. Wide or tall pergolas—anything over 3 metres high or spanning more than 4 metres—need 125mm by 125mm or 150mm by 150mm posts for stability against wind and load.

Taller posts amplify sideways movement, so thicker timber compensates. A slender post might feel rigid at waist height but flex noticeably at head height. If your design includes glass or solid roofing rather than open slats, treat it as a heavier load and size up one step.

Freestanding vs lean-to

A freestanding pergola stands on four or more posts and bears its own weight entirely. Each post does equal work, so uniform sizing across all corners keeps the structure balanced. A lean-to veranda fixes one edge to a house wall with a ledger board, meaning only the outer posts take the full vertical load. Those posts can often match the freestanding size, but if the span from wall to post exceeds 3 metres, thicker beams or closer post spacing prevents droop.

Beam and rafter thickness

Beams run horizontally between posts and carry the rafters; rafters sit on top of the beams and span across them. Beam thickness matters more than rafter thickness because beams cover the longer distance and hold more weight. For a span up to 2.4 metres, a 100mm by 150mm beam handles most open-roof designs. Between 2.4 and 3.5 metres, move to 100mm by 200mm or 125mm by 200mm. Spans beyond 3.5 metres need 150mm by 200mm or two narrower beams bolted together.

Rafters can be lighter: 75mm by 100mm or 75mm by 150mm works for most layouts when spaced 400mm to 600mm apart. Closer spacing lets you use thinner rafters; wider gaps ask for thicker ones to stop them bowing. If the roof will carry anything solid—panels, shade cloth stretched tight, heavy vines—treat rafters like beams and size them one step thicker.

Doubled beams and laminated sections

Doubling two beams gives you the strength of a thicker single piece without the weight or cost of large-section timber. Two 100mm by 200mm beams bolted face to face perform like a single 200mm by 200mm beam but are easier to lift into place. Laminated beams, made from layers glued under pressure, resist warping better than solid timber and suit long spans where droop over time is a concern.

Choosing timber for a pergola: the practical steps

1. Measure the span from post to post in both directions—this gives you the longest distance any beam or rafter must cover without support.
2. Decide the roof type: open slats, partial cover with plants, or a solid roof with panels or glass. Heavier roofs need thicker timber throughout.
3. Set post size based on height and load—start with 100mm by 100mm for most garden pergolas up to 3 metres tall, then size up for taller or wider builds.
4. Match beam thickness to span: 100mm by 200mm for spans up to 3 metres, 125mm by 200mm or doubled sections for longer reaches.
5. Space rafters between 400mm and 600mm, using 75mm by 100mm for open roofs and 75mm by 150mm or thicker if covering them.
6. Check the timber grade: structural-grade softwood (C16 or C24) for all load-bearing parts, with pressure-treated finish for outdoor durability.

Timber grades and treatment

Structural timber comes graded by strength. C16 is the everyday grade for most garden pergolas; C24 is stronger and better for long spans or heavy loads. Both are softwood—usually spruce or pine—and widely available. Hardwoods like oak or iroko resist rot naturally and look richer, but cost more and weigh more, making them harder to work with unless appearance or longevity justifies the expense.

Pressure treatment forces preservative deep into the wood and protects against rot, insects and moisture for decades. Untreated timber lasts a few years at best in contact with soil or damp air. All posts, beams and rafters should be pressure-treated before assembly; painting or staining afterwards adds another layer but does not replace treatment. Pay attention to cut ends: any saw cut after treatment exposes untreated wood, so brush those surfaces with end-grain preservative before fixing them in place.

Common sizing mistakes and how to avoid them

Undersizing beams for the span causes visible sag within months, especially once plants grow or rain collects. A beam that looks flat when new may bow under summer wisteria or a wet autumn. Measure the span honestly and choose the next size up if you are between two options. Oversizing posts is less common but adds cost and weight without benefit—100mm by 100mm posts suit most domestic pergolas, and going thicker only makes sense for very tall or wide structures.

Spacing posts too far apart to save timber puts all the strain on the beams and leads to flex or failure. Keep freestanding post spacing at or below 2.4 metres unless you are doubling beams or using heavy sections. For lean-to designs, the span from wall to outer post should not exceed 3.5 metres without intermediate support or thicker timber throughout.

Fixing and anchors

The way timber connects to the ground and to other pieces affects how size performs in practice. Post shoes—metal brackets that lift the timber above ground level—keep the end grain dry and prevent rot at the base, meaning a 100mm post in a shoe lasts longer than a 125mm post buried in soil. Bolted joints hold better than screws for beam-to-post connections, and galvanised or stainless fixings resist rust where treated timber is acidic.

Adjusting for roof weight and future planting

An open-slatted roof with rafters every 600mm and no covering weighs little and needs only the baseline timber sizes. Add polycarbonate sheets, glass panels or a dense canopy of climbing plants, and the load multiplies. A mature wisteria in full leaf can weigh as much as a person sitting on the pergola; wet foliage after rain adds more. If you plan to grow anything substantial, size beams and posts one step thicker than the span alone suggests.

Solid roofing—whether transparent or opaque—also catches wind. A sail effect pushes sideways on the posts, so taller structures with solid covers need thicker posts or diagonal bracing between them. Planning rules for garden structures sometimes limit height and footprint; check what applies locally before committing to large sections or wide spans. More detail on when formal permission is needed appears in planning permission for a veranda.

Sourcing and cutting timber to size

Most timber merchants stock structural softwood in common sections—75mm by 75mm, 100mm by 100mm, 100mm by 200mm—and can cut lengths to order. Buying pre-cut saves time and ensures square ends; cutting on site needs a decent saw and attention to angles. Hardwood usually comes as sawn boards rather than planed sections, so expect to plane or sand it yourself if appearance matters.

Timber dimensions are nominal: a 100mm by 100mm post often measures 95mm by 95mm after planing. The difference rarely affects structural strength, but check actual sizes if joints need to be tight. Lengths come in 300mm increments—2.4m, 2.7m, 3.0m—so plan your layout to use standard lengths and avoid waste. A 3.6-metre span needs a 4.2-metre beam, not a 3.6-metre one, because you lose length at each end where the beam sits on the post.

Maintenance and long-term performance

Timber left outdoors weathers grey, develops surface cracks and collects moss or algae in shaded corners. Pressure treatment keeps the core sound for years, but surface maintenance—cleaning off growth, reapplying stain or paint—keeps it looking deliberate rather than neglected. A spring clean with a stiff brush and a mild detergent solution lifts dirt without stripping treatment; avoid jet washers at high pressure because they gouge the surface and push water into the grain.

Check fixings and joints each year. Bolts may loosen as timber shrinks and swells with the seasons; posts in shoes stay drier than buried posts and need less attention. A well-sized pergola with treated timber and sound fixings stands for decades with minimal work; undersized or untreated sections start to fail within five years. For attached structures, the way a lean-to connects to the house wall matters as much as timber size—more on that in lean-to veranda options.

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