Pergolas and Outdoor Timber Structures
In an outdoor timber structure you do not design the timber: you design the water. Every detail that holds it becomes a point of decay, and durability is almost entirely a matter of geometry — covered end grain, raised bearings, surfaces that shed — long before it is a matter of species and treatment. The second check is not weight: it is wind, because a covered pergola is a sail.
Overview
TAV. 00The process covers pergolas, canopies and light timber structures exposed outdoors, from choosing the use class through to the maintenance plan. The two checks that govern everything are less obvious than they seem: the first is the path of the water, which decides where the timber will decay in ten years, the second is wind action, which in light covered structures produces uplift instead of load and inverts the logic of the anchorages. The phases follow that order: first establish how wet the timber will get, then build a base that never keeps it in contact with water, then make joints that shed, then brace and anchor against uplift. The finish comes last and replaces none of these choices: it only extends them.
The site score
TAV. HThe detail of the craftThree conditions, three answers
Establish how wet the timber will get
The first decision is not about species but about exposure. An element under a continuous roof, one exposed to rain but shedding well, and one in permanent contact with the ground are in three completely different conditions and call for three different answers in terms of natural or conferred durability. So every element in the structure is classified by its actual condition — not by the average for the works — and each is assigned the appropriate species or treatment, accepting that different solutions coexist in the same pergola.
Classification of the structural elements by moisture exposure condition and assignment, for each class, of the corresponding timber species or durability treatment, presented as a schedule by element.site notebook — editorial synthesis
click a beat · arrow keys ← → walk the site · dashed pauses are the chemistry at work
PLATE 01
You do not design the timber, you design the water
Three details decide the service life of an outdoor structure: the base, the end grain and the top face. On each the choice is between a geometry that traps and one that sheds, and no treatment compensates for the wrong geometry.
The diagram illustrates the logic of the details. Durability classes, sections and performance requirements are to be taken from the design and the applicable standards.
PLATE 02
Covering a pergola means building a sail
While it is open, wind passes through it. The day it receives fabric or sheeting, the same structure acquires a surface the wind can lift, and its anchorages go from working in compression to working in tension.
The diagram illustrates the change of regime. Design actions, coefficients and anchorage checks are to be taken from the design and the applicable standards.
How it ages (and what betrays it)
TAV. PQuestions from the site
TAV. QBetter pressure-treated timber or modified timber?
It depends on where the element is and what is underneath it. Pressure treatment confers durability with chemicals and costs less; thermal and acetylated modification achieve comparable results by acting on the structure of the wood, without biocides, and cost more. Above a vegetable garden, a play area or a pool, the difference matters; on a concealed post, much less.
How often does the finish need redoing?
There is no single interval, and anyone promising one is simplifying. South- and west-facing faces consume the product much faster than north-facing ones, and horizontal surfaces faster than vertical ones. The practical criterion is observation: when water stops beading and starts darkening the timber, that face needs refreshing, whatever the calendar says.
Does every post really need a pad?
You need something that keeps the timber clear of the ground and resists wind uplift. On a small open pergola an anchorage to an existing slab may be enough; on a covered structure the pad is not just a bearing, it is ballast and uplift anchorage at once, and at that point its size is decided by the wind check, not by habit.
Is timber that has turned grey ruined?
No. The grey is surface lignin degraded by ultraviolet light and washed by rain: it affects a fraction of a millimetre and does not touch the structural section. It is an aesthetic matter, and in many species it is exactly the look expected over time. What to watch is not the colour but the post bases and the beam ends, where degradation is not superficial.