You can do a hot roof and not require any ventilation, though I would not recommend it anywhere snow accumulates on shingle roofs. Even if i don't like it, it is allowable.
Whether you need additional ventilation depends on how much you are getting now and what is allowed by the governing building code for your area. According to the International Building Code for One and Two Family Dwellings, R806 Roof Ventilation, you need the equivalent of 1/300 of the area ventilated in free vent area. If the soffit vents do not provide this, you need to install additional ventilation or opt for the hot roof concept.
Also, the attic area needs to accessible by a min 22x30 inch opening. You must also install a class 1 or II vapor retarder on the warm side of the insulation or the ventilation requirements double.
The building code does not address moisture control within the garage space, it's only concerned with moisture in concealed spaces or general ventilation of habitable spaces. You may wish to address moisture in the garage itself with exhaust fans or operable openings. This is unrelated to attic ventillation requirements.
2 LAYERS ASPHALT TO CP144
with SOLAR REFLECTIVE PAINT
on [10?]mm PEARLDECK ([.025/.023])
ON @@@@@ SC3 Joists
AT #00 CENTRES
@@ RRING [PIECES?] [10?/TO?] @@@ 1-30
CP144 is a standards document describing roofing requirements. (It's been superseded.)
Based on some cursory research, it doesn't looks like Pearldeck comes in 10mm thicknesses, so I'm not sure what that number might be.
SC3 is a European strength rating for joists that has been replaced by a different rating system. (According to the link, SC3 is roughly equivalent to C16)
I'd bet that the last line is "100 CENTRES" with 100 being cm, which seems to be a common spacing for joists.
Pearldeck is hard to find, but based on these two sources:
http://www.woodgreentimber.com/Roofing-Materials-Wood-Green-Potters-Bar.php
http://www.euroken.net/supplies/products.html
It looks like it's an insulation board for roofing.
As to whether or not it's a good roof, cold, hot, or anything else, I have absolutely no idea, and it will largely depend on your local climate. Maybe an expert can add something to this answer, or maybe my translation will let you ask someone you know.
Lastly, not only am I not a professional, but I'm also American (I'm not familiar with common European building measurements/practices), so any or all of this might be completely wrong. This is all based on deduction and Google ;-) So feel free to edit whatever is obviously wrong.
I hope that helps!
Best Answer
Thermal mass does not block heat; it stores and releases heat, and it slows down the flow of heat. It will absorb heat when it is colder than the surrounding area, and it will release heat when it is warmer than the surrounding area. These characteristics are useful in the following circumstances:
A thermal mass wall in a climate/season where the outdoor temperatures cycle above and below the indoor comfort point. The heavy walls will slow down the flow of heat so much that by the time it reaches the interior, the outside temperature will have dropped, reversing the flow of heat out of the wall. In order for this to work, the wall must be very dense and very thick; 12" thick rammed earth or adobe, for example.
A thermal mass element within a building's insulated envelope. Such a thing can be used to store free heat generated from sunlight, cooling, electricity usage, etc and store them until the sun goes down and everybody is asleep, whereupon it will release it, moderating the interior temperature and reducing its daily fluctuations, especially at night.
Insulating the underside of the floor is a great idea, as it will make your thermal mass floor fall into category 2.
Preventing the sun from hitting your walls or windows is likewise always a good idea; heat gain is driven almost entirely by direct solar gain.
Keeping the garage door closed may or may not help; try it and see.