Ten blades, and December got the shade
The terrace is 4.2 m along the glass and 2.6 m out from a west-facing slider, and the brief arrived in one sentence: cut the July afternoons, leave the February sun on the glass. The scheme that came back had a timber louver canopy over the whole terrace, blades on edge, spaced off a rule of thumb borrowed from an overhang study — pick the summer sun angle, size the geometry to block it, done. Built, it shaded the terrace beautifully on clear afternoons in December and did almost nothing on the evening of the longest day.
That result is not a construction fault and it is not a bad rule. It is a rule imported from the wrong elevation. The seasonal trick everybody knows — deep enough to cut high summer sun, shallow enough to pass low winter sun — runs entirely on the swing in noon altitude between the solstices. Declination moves through ±23.44°, so at any latitude outside the tropics the sun stands about 47° higher at noon in June than it does in December. A south face gets to spend that 47°. A west face never sees it, because the hours that overheat a west terrace are the last three before sunset, and the sun is low then in June, in September and in December alike.
What separates a July afternoon from a February one on that face is not height but bearing. Working in the northern hemisphere, the June sun sets well north of west — around 304° at this latitude — and spends the last hours arriving almost square onto the glass, while the December sun is away at 236° and only grazes it. A horizontal screen cannot read bearing directly — but the geometry it does read, the profile angle, is bearing and altitude combined, and once the two are combined the December sun turns out to sit higher against that screen than the July sun does. Everything below follows from that inversion: get the angle right, turn it into a depth-to-gap ratio, turn the ratio into a pitch and a count, then check the timber will hold the gap and the frame will hold the timber.
What a louver canopy over a terrace is built from
- Louver blades — stood on edge and running parallel to the glass; their depth against the clear gap between them is the only thing that sets the cutoff Exterior Timber Louver Solar Shading Angle Calculator
- Blade bearers — the beams each blade lands on, counted by the same run-over-pitch arithmetic as the blades once the canopy is too wide for a single span Pergola/Trellis Rafter Spacing Calculator
- Head and outer beams — carry the whole blade field plus wind uplift back to the posts and the wall, and their depth is a bending check rather than a preference Timber Beam Bending Stress Calculator
- Posts and wall ledger — outer edge stands on posts set out from the terrace, inner edge hangs on a ledger bolted and flashed into the building Pergola Post Spacing Calculator
- Terrace deck — the surface the shade is actually for, and the datum every post height and every soffit clearance is measured from Deck Board Calculator
Altitude is not the angle the screen feels
The quantity a horizontal shading device responds to is the profile angle — the sun's direction projected onto the vertical plane that runs perpendicular to the blades. The ASHRAE Handbook—Fundamentals sets it out in its Fenestration chapter, and the relation is short: tan of the profile angle equals tan of the solar altitude divided by the cosine of the surface-solar azimuth, that last term being the horizontal angle between where the sun is and where the face is pointing. Sun straight ahead of the face and the cosine is one, so profile angle and altitude coincide. Sun well round to the side and the cosine collapses, so the profile angle climbs far above the altitude. Blades running parallel to the glass, as they do on this terrace, sit in that same plane — one number serves both the screen and the wall behind it.
That single divisor is what makes a south elevation easy and a west elevation deceptive. Near solar noon on a south face the sun is close to dead ahead all year, the cosine stays near one, and a designer who uses altitude everywhere gets away with it. Swing the face round to due west and the two seasons land on opposite sides of the divisor. June's late sun is nearly normal to the glass, so its profile angle is barely above its altitude. December's sun is 50 or 60 degrees round to the south, the cosine drops to two-thirds or a half, and its profile angle inflates well past where it actually stands in the sky.
Run it out for a west face at 45° N and the numbers stop being an argument. At five in the afternoon on the June solstice the sun is 27° up and bearing 277°, so its profile angle on that face is also about 27°. At two in the afternoon on the December solstice it is only 16° up — more than ten degrees lower in the sky — but bearing 208°, and its profile angle against the same face is 31°. A screen tuned to intercept everything flatter than 27° cuts the December ray and passes the June one. It is doing precisely what it was built to do, in exactly the wrong month.
Two qualifications keep this honest. The first is that a glancing ray delivers far less than a square one: the two o'clock December sun in that table meets the face at a cosine of incidence of about 0.46, so the winter half of the mistake costs far less energy than the July half earns. The second is that the table below is one latitude on two dates, given in solar time, and solar time is not clock time — it runs off the local meridian and the equation of time, and daylight saving moves it another hour. Read your own site off a sun-path chart or the solar position data in the Handbook's Climatic Design Information chapter, at the hours the client actually uses the terrace, before any of this arithmetic starts.
| Date and solar time | Solar altitude | Solar azimuth | Profile angle on a west face |
|---|---|---|---|
| 21 June, 17:00 | 27° | 277° | 27° |
| 21 June, 18:00 | 16° | 287° | 17° |
| 21 June, 19:00 | 7° | 297° | 7° |
| 21 December, 13:00 | 20° | 195° | 55° |
| 21 December, 14:00 | 16° | 208° | 31° |
| 21 December, 15:00 | 10° | 221° | 15° |
Blade depth over clear gap, and nothing else
Strip a louver canopy down and it is one interception test. A ray entering a gap between two blades has to fall the full depth of a blade before it clears the underside; while it falls that depth it travels sideways by the depth divided by the tangent of its profile angle. If that sideways travel is longer than the clear gap, the ray runs into the next blade and never reaches the terrace. Flat rays travel a long way sideways per unit of fall and are caught easily; steep rays drop almost vertically through the opening and are the hard ones to stop.
So the widest gap that still intercepts everything arriving flatter than a chosen cutoff is the blade depth divided by the tangent of that cutoff, and the ratio is the entire design. A 145 × 45 board stood on edge — ordinary stock in most markets, close in size to a nominal 2 × 6 — at a 30° cutoff wants a 251 mm clear gap. Push the cutoff to 40° and the gap closes to 173 mm; back it off to 20° and it opens to 398 mm. Nothing about the timber species, the finish or the fixings enters into it. Two lengths and an angle.
One conversion is worth writing on the drawing in words. What comes out of that ratio is a clear opening, and what a setting-out dimension and a fabrication drawing use is an on-centre pitch, and the difference between them is one blade thickness. Turn 251 mm of clear gap into a 251 mm pitch and every gap on the terrace shrinks to 206 mm, the cutoff climbs past 35°, and the canopy blocks noticeably more sky than the brief asked for — a mistake that shows up as a gloomy terrace rather than as anything anybody calls a defect. Add the thickness once, early, and carry the on-centre figure from there.
Enter the depth of the board you intend to stand on edge and the cutoff profile angle you settled on in the previous section, and the spacing that comes back is the widest clear gap that still intercepts everything arriving flatter than that cutoff — add the blade thickness to it before it becomes a pitch.
The depth of each blade measured in the direction the sun passes through the canopy — for blades on edge in an overhead array, that is the blade's height.
The sun angle above which the louvers should fully block direct sun.
Required louver spacing
0.5 ft
This gives full shading only for sun angles above the cutoff angle you enter — determine your target cutoff angle from your latitude and the season/time you want to block direct sun (e.g. using a solar path chart for your location).
They open the calculator with your figures already in it
Exterior Timber Louver Solar Shading Angle Calculator: 0.5 ft — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
What this calculation does not cover
- Works from the sun's ALTITUDE, and blades respond to the profile angle — that altitude projected onto a plane cut square across the blade run. The two match only when the sun is square to the blades; swing round toward the ends of the run and the profile angle drops below the altitude, so sun the cutoff says is blocked comes straight down the channel between blades. Noon is the easy case; mid-afternoon is the one that leaks.
- Treats the blades as lines with no thickness. What this returns is the clear opening between blade faces, so a 45 mm (1.8 in) blade on a 115 mm (4.5 in) result sits at 160 mm (6.5 in) pitch — read it the other way and the array is either a third more open than designed or a third more timber than it was costed at, with the daylight and the view through it changing to match.
- Sizes the array optically and stops there. Blades that shade well are deep, on edge and close together, which is a lot of timber spanning between the bearers, and each blade still has to carry its own weight, wind uplift, and in the wrong climate wet snow packed into the gaps. The spacing fixes the shading; the blade's span, its section and the fixing at each end are what stop the array sagging into a wave visible from the ground.
Turning a gap into a cutting list
Blades in a louver canopy are the rafters: they span the bearers, they carry themselves and whatever wind does to them, and they are counted the way any repeated member is counted — the run divided by the pitch, rounded up so no gap ever exceeds the design, plus one to close the far end. The rounding always tightens the spacing rather than opening it, which is the right direction for a shading device and the wrong direction for anybody who ordered exactly the divided figure.
The dimension that goes into that division is the one this job gets wrong most often. Blades that cut west sun have to run parallel to the glass, so they are spread across the 2.6 m the canopy projects, not along the 4.2 m it is wide — the width is the number everybody quotes and it is the blade length, not the run. At a 296 mm pitch the projection gives nine bays and ten blades, each of them 4.2 m long. That length is the second thing the orientation decides: a 145 mm board on edge does not cross four metres without a visible sag, so a middle bearer goes in and the blades become two spans, which is a beam the elevation drawing did not have on it.
The table below is worth putting in front of whoever is choosing the cutoff, because it prices the decision. Every five degrees of extra cutoff is more blades, more fixings, more linear metres of finish, more self-weight on the bearers and more of the sky gone from the view. Going from 20° to 50° on one modest terrace takes the order from seven blades to seventeen — the same canopy, the same timber section, two and a half times the material — and buys shade only in a band of profile angles that, on a west face, is mostly winter.
Set out from the bearers, not from the wall. The two edge blades sit hard against the ledger and the outer beam, and their gaps are the ones that end up odd if the projection does not divide cleanly; splitting the remainder equally between the two end bays keeps the discrepancy below what the eye reads as a mistake, and it keeps both end gaps tighter than the design rather than looser. Where a canopy is deep enough to need an intermediate beam running parallel to the glass, treat each bay as its own run: a shared pitch carried across a break puts a blade centre-line into the beam, and that is a joint nobody wants to detail.
- Measure the run the blades are spread across — the projection from the ledger to the outer beam, not the width along the glass.
- Add one blade thickness to the clear gap from the shading ratio to get the on-centre pitch.
- Divide the run by that pitch, round up, and add one for the closing blade.
- Recompute the actual pitch from the rounded count, then recompute the gap and the cutoff it now gives.
- Split any remainder between the two end bays and mark both edge dimensions on the setting-out drawing.
- Check the blade length against the bearer layout, and add an intermediate bearer before the span rather than after the sag.
| Cutoff profile angle | Clear gap | On-centre pitch | Blades across 2.6 m |
|---|---|---|---|
| 20° | 398 mm | 443 mm | 7 |
| 25° | 311 mm | 356 mm | 9 |
| 30° | 251 mm | 296 mm | 10 |
| 35° | 207 mm | 252 mm | 12 |
| 40° | 173 mm | 218 mm | 13 |
| 50° | 122 mm | 167 mm | 17 |
The blades are the rafters here, so the count is the ordinary one: run divided by pitch, rounded up, plus the closing member — enter the projection the blades spread across rather than the width they span, together with the on-centre pitch from the step above, and the answer is the quantity that goes on the order.
The overall width of the pergola or trellis in the direction the rafters span across.
The maximum on-center spacing allowed between rafters per the design or span table.
Rafters needed
8 rafters
Maximum rafter spacing depends on the rafter material, size, and any planned cover/planting load — confirm the spacing against the specific rafter species/size's span table or a designer's calculation for your load case.
- Bays across the pergola
- 7
- Rafter centres, as laid out
- 1.86 ft
They open the calculator with your figures already in it
Pergola/Trellis Rafter Spacing Calculator: 8 rafters — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Only the two lengths you type enter the arithmetic — the width is divided by the maximum spacing and one rafter is added — so rafter thickness never appears and the count sets out centre lines rather than faces; if the two outer rafters must finish flush inside the beam ends instead of being centred on them, take one rafter thickness off the width before entering it.
- The answer is the count on its own, and the tighter on-centre spacing that the rounding-up produces is not reported back, so mark the beams out by dividing your width by one less than the rafter count instead of stepping the maximum spacing along from one end and squeezing whatever is left into the last bay.
- One width and one spacing describe a single straight run divided evenly end to end, so a doubled rafter at each end, a widened bay over a gate or path, and an L-shaped or multi-bay frame all have to be run through separately with the shared rafter at each junction counted once.
- Nothing but the rafters is totalled here: the beams they sit on, the posts, the hangers and screws, and any lattice, battens or purlins laid across them on a trellis top are outside the figure.
- Rafter length is never asked for and no waste percentage is applied, so the number is how many sticks stand in the finished frame rather than a timber order — multiply it by your own cut length and add your own allowance for off-cuts and warped or split lengths.
The gap you drew is not the gap in August
Louver blades on a terrace canopy live in one of the harshest exposures timber gets: full weather on the top face, no coating on the underside once the first refinish is skipped, water sitting in the fixing holes and driving rain reaching every surface. AWPA U1 puts that at Use Category 3B, exterior above ground with poor water runoff or no coating; Eurocode 5 calls the same condition service class 3 in EN 1995-1-1, and the NDS applies its wet service factor for exactly this. All three are saying the same thing about the material, and none of them are optional because the piece is decorative.
The dimensional consequence is small for the shading and large for the fixings, and it is worth separating the two. Take a flat-sawn softwood blade delivered around 19 per cent and drying to about 12 in a long hot spell: at the tangential coefficient the USDA Forest Products Laboratory Wood Handbook tabulates for common construction softwoods, roughly 0.0026 per point of moisture change, a 145 mm depth gives up a little over 2.6 mm and the 45 mm thickness gives up under a millimetre. Against a pitch fixed by the frame, that moves the cutoff by about half a degree — nothing. Against a blade screwed tight through both edges into an unyielding frame, that same 2.6 mm has nowhere to go, and it goes into a split that runs the length of the board. Slot the outer hole, fix from one edge, and size the slot off the movement rather than off habit.
Set the level count to one, enter the blade depth as the cross-grain dimension, and run the moisture content the blades arrive at against the exterior figure they will settle to — the movement that comes back is the length the fixing slot has to allow, not a number to design the shading around.
The summed depth of horizontally laid timber the load path passes through at one level.
The meter reading taken on the timber at the moment it was fixed in place.
Where the timber will settle once the building is finished and running.
The fraction of its depth the timber moves for each single point of moisture change.
How many platform-framed levels the movement accumulates through.
Cross-grain movement at one floor level
0.2749 in
Straight arithmetic on the coefficient and the moisture change entered. The movement is across the grain; along the grain the same change moves the timber by a small fraction of this and is normally ignored.
- Accumulated movement over all floor levels
- 0.82 in
- Depth remaining once the movement has happened
- 11.48 in
- Movement for a single point of moisture change
- 0.03 in
- Change in moisture content
- 9 %
- Proportion of the depth lost
- 2.34 %
They open the calculator with your figures already in it
Timber Cross-Grain Shrinkage Calculator: 0.2749 in — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 0.2749 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- Movement across the grain only. Timber moves along the grain by a small fraction of this, and a long member's length is effectively stable.
- Says nothing about whether the movement will be RESTRAINED. A member free to shrink simply shrinks; the split the guide describes happens when two rigid fixing lines hold a deep section apart while it tries to move, and that is a detailing question this arithmetic cannot answer.
- Uses one coefficient for the whole stack. Where the plates, the rim and the joists are different species or different sawing patterns, run them separately and add the results.
- Ignores creep, mechano-sorptive movement under sustained load, and any moisture cycling. A detail that dries, wets and dries again does not simply return to where it started.
What holds it up, and what wind does to a row of blades
An open blade field is not a light roof and it is not a solid one either, and the wind case is the part of a louver canopy most likely to be waved through. Wind pressures on an attached canopy or an open-frame structure come from the governing loading standard — ASCE/SEI 7 in the United States, EN 1991-1-4 in Europe, AS/NZS 1170.2 in Australia and New Zealand — and none of them let a designer reason from the solidity ratio by inspection. A terrace canopy is usually at the corner of a building, which is the zone where local pressures are highest, and it is usually cantilevered or simply supported with a free edge, which is where uplift concentrates.
The blade fixings are the failure that actually happens. Each blade is a small aerofoil held by two screws in withdrawal, and screws driven into end grain hold a fraction of what the same screw holds in side grain — the NDS publishes its withdrawal design values for penetration into side grain and offers no end-grain equivalent, which is the standard telling you something rather than merely being quiet. Fix through the side of the frame into the blade face, or use a proprietary bracket, and treat the two-screws-into-the-end-of-the-board detail as a sketch rather than a specification.
Above the blades, the frame and beams are ordinary timber design with one adjustment most people forget: exposure. The section that works dry does not automatically work at service class 3 or with the NDS wet service factor applied, and a beam sized off a dry-service span table and then left out in the weather has quietly lost part of its capacity before it carries anything. Deflection is usually what governs a canopy beam rather than bending, because a visible sag over a terrace is a complaint long before it is a structural concern.
The connection back to the building is the other end of the same load path and the one that reaches the rest of the envelope. A ledger bolted to a wall, flashed and drained, is covered in principle by the deck and ledger provisions of the International Residential Code — Section R507 in recent editions, as adopted and amended locally — and the reason those provisions read the way they do is that a fastener pattern into a rim board is a very different thing from a fastener pattern into masonry or into a rainscreen with a cavity behind it. Uplift on a canopy reverses the load on that ledger, so a detail sized for gravity alone has been checked for half its cases.
Posts land at the corners and wherever a side runs past what the beam can span, so put the terrace footprint in against the maximum span the beam section actually gives at the exposure it will live in — not the dry-service figure off the table.
The length of the pergola structure.
The width of the pergola structure.
The maximum span your beam size can safely support between posts.
Posts needed
8 posts
This is a general layout planning estimate — final beam sizing and post spacing should be confirmed against your local building code and beam span tables for the actual lumber species, grade, and expected roof/shade structure load.
- Posts along each long side
- 3 posts
- Posts along each short side
- 3 posts
- Post centres down each long side
- 6.5 ft
- Post centres across each short side
- 5 ft
They open the calculator with your figures already in it
Pergola Post Spacing Calculator: 8 posts — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
What this calculation does not cover
- Every answer describes a freestanding four-sided frame: the total adds the posts on two long sides and two short sides and then subtracts the four shared corners, and there is no input for a pergola attached to a house or garden wall, where one whole run of posts is replaced by a ledger and the count drops.
- Only two maximum spans exist in the dropdown, 6 ft and 8 ft, and whichever you choose is divided into both dimensions at once, so a heavier beam that clears more than 8 ft between posts, or a layout that runs a long beam one way and lighter members the other, cannot be described here.
- Each side rounds its bays up on its own, so the two sets of centres above are usually different from each other and both sit under the maximum you picked — a 4 m side at the 8 ft setting is two bays of roughly 6.6 ft. That is the frame set out square. What it is not is a beam layout: the centres are post positions, and a beam carrying them still has to be checked against its own span.
- The metric option labels are rounded while the arithmetic is not — the option shown as 1.8 m divides by 6 ft (1.829 m) and the one shown as 2.4 m divides by 8 ft (2.438 m) — so on some footprints the bays behind the answer run a few centimetres wider than the metric figure you thought you had selected.
- The length and width you type are treated as the post run itself, so no allowance is made for the beam and rafter tails that normally cantilever past the corner posts, nothing distinguishes an overall outside dimension from centre-to-centre post positions, and only true rectangles are handled — an L-shaped or angled footprint has to be split and each part run separately.
- What comes back is a count of posts and nothing more: no post height or section size, no footing depth or embedment, no diagonal or knee bracing against racking, and no post bases, brackets or fixings.
Prove it at the glass, not at the blade
A canopy over a terrace and a shading device on a window are two different jobs, and the louver ratio only answers the first. The canopy controls what lands on the deck; what reaches the glass depends on how far the canopy projects, how high its underside sits above the window head, and how tall the glass is — because a low ray coming in from the west arrives under the outer edge and walks straight up the pane. That is the calculation to run before anyone signs off a shading claim, and it is the one that decides whether the canopy needs a dropped fascia, a fin at the outer edge, or simply a bigger projection.
It also decides how much the shade is worth. The glass has its own solar heat gain coefficient off the NFRC label or the supplier's data, and ISO 15099 sets out the detailed method for glazing combined with a shading device; the quick screening version treats the shaded portion of the pane as receiving negligible direct gain and prorates the coefficient by the unshaded fraction. That is a screening number, not a load calculation, but it is enough to show whether a 600 mm projection is buying anything real at four in the afternoon or whether the honest answer is different glass.
One trap sits in the code path rather than in the physics. Both ASHRAE Standard 90.1 and the IECC allow a permanent projection to relax the prescriptive fenestration requirement, expressed as a projection factor, and it is tempting to claim that credit for a louver canopy. Read what the adopted text actually defines a permanent projection as before assuming a device that is half open counts as one, and get the authority having jurisdiction to agree in writing if there is any doubt. A screen with 250 mm gaps in it is not a solid overhang, and the compliance path is not the place to discover that.
Put the projection, the gap from the underside down to the window head and the glass height in against the altitude at the hour you are designing for, and the shaded fraction it returns tells you whether the canopy is reaching the glass at all or only shading the deck in front of it.
The glass's own solar heat gain coefficient with no shading applied.
The sun's angle above the horizon at the time of day/year being checked.
How far the horizontal overhang or shading device projects out from the wall face.
The vertical distance from the underside of the overhang down to the top of the window.
The full height of the window glass being shaded.
Effective SHGC
0.15 SHGC
This simplified geometric approximation is for a window facing directly toward the sun (zero azimuth difference) and treats shaded glass as receiving negligible direct gain, which overstates shading benefit since diffuse/reflected radiation still reaches shaded glass. For precise energy modeling, use ASHRAE 90.1/LBNL Projection Factor tables or full solar gain software.
- Shaded fraction of window
- 0.63
They open the calculator with your figures already in it
SHGC Effective Shading Fraction Calculator: 0.1495 SHGC — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
Add the equipment this sizes
This result is a specification — 0.15 SHGC — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
What this calculation does not cover
- One instant, not a season. This is the shading at a single solar altitude — one time of day on one day of the year — and a fixed overhang cannot be tuned to more than a few of them. Whether the overhang is any good comes from the balance across the year: summer shading set against the winter sun it also blocks, and how it behaves through the mid-afternoon hours when a cooling peak usually lands rather than at noon, when it always looks best.
- The overhang is treated as infinitely wide. The shadow here drops straight down the glass with no ends to it, but a real overhang stops somewhere, often at or just past the jambs, and sun reaches in around both sides for much of the day. The narrower the overhang relative to the window, and the further the sun swings off the facade's normal, the more that end effect eats into the shaded fraction this returns.
- A horizontal overhang is close to useless on east and west glass, and nothing here will say so. The sun is low whenever it faces those elevations, so the shadow drop the formula returns is small at exactly the hours the gain is worst. Those windows need vertical fins, exterior blinds or glass with a lower SHGC — size an east or west shade off a noon altitude figure and you build a device that shades nothing when it matters.
When fixed geometry will not do it
If the brief genuinely is cut July and keep February at the glass on a west elevation, fixed horizontal blades cannot deliver it, and saying so early is cheaper than saying it after the timber is cut. The devices that can are the ones that read bearing instead of height: vertical fins, canted toward the equator so they present an open face to the winter sun and a closed one to the summer sun, are the classical answer for an east or west elevation and have been since the first brise-soleil went up. An egg-crate of horizontal blades over vertical fins does both, at the price of a much heavier-looking screen and a great deal more timber.
The alternatives worth putting on the table are operable blades, which turn a geometry problem into a maintenance and controls problem and solve it completely; deciduous planting, which is the only shading device that knows what month it is without being told; and the option of accepting that the terrace and the glass have different needs. Shading the deck so it is usable at six in the evening in July is a comfort goal, and a fixed blade field sized off a 25° or 30° cutoff does that job well. Keeping winter sun on the glass is an energy goal, and on a west face it is mostly answered by glass selection and by the two hours around noon that no west-facing device ever intercepts anyway. Separating the two goals usually turns one impossible screen into two straightforward decisions.
Before the timber order goes out
Everything on a louver canopy is fixed by two lengths and an angle, so get those settled and dated on the drawing first — the quantities fall out of them, and re-cutting a blade field because the cutoff moved five degrees is a full re-order.
- Design hours and their profile angles — The dates, the solar times and the latitude the cutoff was chosen against, written down. Without them nobody can check the screen later or defend it in a review.
- Blade section and cutoff, as a pair — Depth over clear gap is the whole shading device; changing either one without the other changes the cutoff, so they belong on the same line of the specification.
- Blade count and on-centre pitch — Run divided by pitch, rounded up, plus the closing blade — where the run is the projection the blades spread across and the pitch is the clear gap plus one blade thickness.
- Movement allowance at every fixing — Cross-grain movement from delivery moisture to exterior equilibrium, converted into slot length. Fixed rigidly at both edges, a 145 mm blade splits rather than shrinks.
- Frame, posts and the ledger back to the building — Sized at the exposure the canopy actually lives in, with uplift as a separate case from gravity. Wet service and service class 3 both reduce what the section gives.
- What the shading claim is at the glass — Projection, head gap and glass height against a stated hour, plus written confirmation of whether an open screen counts as a permanent projection in the adopted code.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.