I do not know the answer because this is a Lath house built with a cover without a shade rating. The usual material used is shade cloth rated at so much shade. But even with a shade rating it is a matter of trial and error to get the shading right for the orchids you grow and your particular growing situation.
Part of my shade house is pictured in the link and it is shown with the top cover
of two layers of 50% shadecloth that protected most of the plants from burning on a day with the top temp of 45C and 8% humidity.
One of those layers is removed in Autumn.
Sorry, i do not know anything about lumens and such, so this is not much help and the angle of the sun has to be taken into account especially in autumn just before you bring your plants indoors. Of course, I'm assuming that the solid bits do not let any usable light through.Your setup might be fine in Z4 MN.

This light thing has come up in another recent series of posts where the original poster assumed that direct sunlight in the tropics would burn orchids. So, the OP said that he/she was growing orchids in total shade Not so, orchids must have sun, filtered by something appropriate to what and where grown.

Here is a link that might be useful: Shadehouse picture again.

Whitecat, getting the right light is the absolutely hardest thing and biggest worry, so join the club.

I think I understand what you are describing. My orchids are on a deck which gets full sun all day *except* that on the west there is one of those lattices. These are catts and oncids.

The shade from the lattice starts about 1:00 and moves maybe a foot an hour.

I don't use shade cloth on any orchid next to the lattice, only on those a foot or more away. The orchids next to the lattice do fine. The sun thru the holes doesn't burn anything that's acclimated, and the shade arrives just in time. So I think your lattice house will give the right amount of light assuming your orchids have been gradually adjusting to more and more light as the spring wore on.

A couple of feet away there's no shade til about 2:30-3:00, and I use shade cloth there.

When there's a couple of weeks of stifling heat, I might spray them once or twice to help them cool off. About how I might do chickens or a dog :)

Next to the lattice is way too hot for phals-- they can get burned even by the sun coming thru the lattice in the afternoon.

Hope this helps. And remember I burn at least a few things every year.

Whitecat, I just noticed that my lattice isn't a single piece, but is two pieces, offset from each other by at least an inch. This makes the holes smaller.

I think with a single lattice I would add a piece of 50% shadecloth to the top and upper part of the s/w and w portion of the lattice so that 50% of the midday to 3 sun is blocked, and after 3 not blocked.

Hope I am clear

Depends what you're growing.

Not to take anything away from all the good observations but there is this. In the summers of 2001 - 2003 I was growing quite a number of things on an unshaded deck in Z5 Central Michigan. It wasn't ideal from a humidity/air movement (too little humidity, too much wind) standpoint and the drying winds and abundant sunshine resulted in sometimes twice daily watering sessions. But burned plants were not in the majority and of those that burned nothing was fatal. Those are also the only years that my only standard Catt hybrid bloomed (oh the fragrance...) I cannot tan so I am out of my depth here but is it possible after an entire Northern winter to get a decent Memorial Day bronzing without 'some' discomfort? Maybe that's not the best example but my point is one cannot eliminate all risk of spring scorch from orchids that have spent the winter in woefully underlit conditions. If you manage it then you are doing the plants a disservice. They were never meant to deal with months on end of sub 2000fc light and then within a few days go back to 3000 and 5000 for hours with no shade. Of course they scorch but they quickly adapt. 1" lattice with 1" spacing probably equals 50% shade. Sounds about right. Not sure where the deck itself fits into the overall light picture. What isn't being discussed is that MN is not AZ or HI or for that matter Burma. In a 90 day summer there will be 90 days of 10,000fc days in any of those locales. I don't think the same can be said for MN. Of course it isn't hopeless in north North America but some of the rules need to be bent to obtain the results that can be obtained in more consistently sunny areas. My mindset is never about how much light to get rid of but always how little shade can I get away with. In 1995 I was growing on a Brooklyn rooftop right out there under full screaming July sun with nary a scorched leaf to show for it. It ain't natural but when considering shading I thought it would be helpful to remember just how little you really need.

H

Howard, I'm going to have to part company with you this time. IMO, you are generalizing the light thing too far here by implying that catts won't scorch without cover if they are acclimated.

As the Bishop said to the Actress, it depends.

Today it's about 95 here, but the winds have been maybe 50 mph, and have been high all day. Even the phal leaves stay cool with that much wind. It's the heat generated in the leaves that causes the burning. And wind has a cooling effect, reducing the likelihood of a scorch.

I can *see* the wind very clearly in the tops of some tall trees in a woods behind my house. Those tree tops are almost always in motion in the morning and late afternoon.

However, I've noticed repeatedly that on most hot summer days, any wind dies down a lot around midday and it's perfectly still on my deck during the hottest part of the day.

That's when the burning happens, when there's blazing sun and no wind. But that's most of the hottest days at noon. From what I've seen, usually the hotter it is, the less likely there is to be any wind.

I've had plenty of scorch on well-aclimated plants that had a leaf tip or two sticking out from the shadecloth on one of those windless days in July or August. And I mean mush-level of scorch. These are plants that went thru all of June in full sun. But those hot windless days...

About your rooftop, I read somewhere that the wind doubles every 10 (or 20, I forget) feet off the ground. So my second story deck gets more wind than the ground level, and the tree tops still more than my deck. And your rooftop-- I assume it gets more than the tree-tops. So that accounts for your burn-free rooftop summer.

As far as lattice goes, it does cut the average light to 50%, but what actually happens is that there's a darkness/ intense light pattern, with very hot light getting thru the holes. I have seen the checkerboard of red and green on a tall plant like a big hoya that was against the lattice.

It might be possible to burn an orchid that way. I was trying to not give total assurance to whitecat that the lattice was enough, tho it does give a lot of protection.

About acclimating-- of course we do that. We learned all about that the hard way :P

I agree with all the above posts. Everyones growing conditions are different. I tend to push light levels until I see burn. I don't recommend doing that, but as Howard observed, its less than one would expect. For the last few days I have moved most of my oncidium hybrids outside in full SE sun. No shade until mid afternoon. I did this because the winds were strong and I knew they wouldn't burn. They haven't. I also have 3 hard cane dens which I leave in full sun all day - S.E.W. I get the most flowers by doing that. Some leaves burn initially, but only on the older growths. New growths do not burn. They grow and mature in that hot sun without damage. I have other dens which can't take it.

I find some plants become stressed when overheated growing on a deck. The leaves might look okay, but their roots bake when grown on cement or wood in full sun. I think that causes more damage then sunburn. My deck can be so hot it would burn the soles of your feet. I've had problems with some Catt hybrids and heat. Most do fine, but I am making an effort this year pay as much attention to the heat factor as I do the light. Mehitabel, your observation about wind is exactly what I notice. No wind, becomes too hot.

J

Thanks, everyone - For some reason, I thought this was kind of an exact science. Oh, well - orchid ambiguity abounds. If nothing else, this year's option is way more attractive than last summer's chicken wire.

Arthur, nice-looking shade house picture. The solid parts of the lattice on the patio are quite opaque and don't seem to let in any light.

Mehitabel, the deck above the lathe house blocks most overhead sun. Fascinating info about wind mph increasing with altitude. I'll make sure nothing's touching the lattice.

CJ, plants range from Phals to Potinara, Catasetae, and Den. moniliformes.

Howard, I'm with you - what's the least amount of protection that can be provided without sunburning or frying?

Jane, yeah, our wooden deck above the patio has dangerously hot temps in summer. No orchids there. The lattice fencing encloses plastic shelves w/ holes in them, so quite a bit of air circulation when the wind's blowing.

Thanks again, Whitecat8