Anything over 30 can stress the plants and it is best to try and avoid it if you can. Doors and roof vents open are a must and shading on the outside of the glass will help.
Fleece, old net curtains, green mesh, old bamboo curtains, Venetian blinds, etc, etc will all work.
Best on the south and south west facing areas of the roof and sides of the house to reflect the heat in the hottest part of the day.
Jerry