For all intents and purposes, zero.

According to http://www.climate4you.com/ClimateAndClouds.htm, "a 1 percent increase in global cloud cover corresponds to a .06 degree (C) decrease in global temperature".

A one percent increase in global cloud cover is equal to covering an additional 2 million square miles of the planet's surface with clouds.

If, instead of clouds, we substituted your white roofs, how much surface area would that be?

Since only 30% of the world is land, we'd need to paint about 3% of the "land" white to get that 0.06 degree decrease. In the US, about 3% of the land is urban area, so that would be possible just by painting roofs. However, in the rest of the world, the land area covered by housing is much lower on average.

If we assume 1% of the worlds land area has housing on it (probably too high), and everyone painted their roof white, this means we could see a .02 degree (C) decrease in global temperatures - or about 1/200th of the overall temperature increase scientists are concerned about.

Again, for all intents and purposes, zero.

It wouldn't have a direct effect on global warming by changing the planet's albedo (reflectivity), but could substantially affect the energy expended on temperature control.

How much would depend on where the house in question is located. When the sun shines a white roof will not heat up your house so much as a black one, reducing the cost of air condition. The difference is greatest in hot sunny climates for buildings with a large roof area relative to the volume, and smallest for buildings in Arctic regions where heating costs are the dominant ones.

How much you will save is possible to calculate based on your location and air condition bills. There are such calculators on the net. How much we will save globally depends on where people will live and how the buildings are constructed.

Many more people are living in hot climates than cold, lately the trend has been to move towards hotter climates, and as poorer countries, overwhelmingly in hot climates, become more affluent the demand for air condition is set to grow. Getting a white roof is a very cheap and quick way of reducing air condition costs, relative to for instance building a new energy economic house. Even without air condition a white-roofed house would be economically (and indirectly environmentally) beneficial. Workers are considerably more efficient in cool ambient temperatures than at high temperatures. It would be bad advice, in workplaces at least, to ask people to forego temperature control where needed.

Closer towards the poles the problem shifts from keeping cool in summer to keeping warm in winter. Here the sun heating becomes more irrelevant, in the spring a black roof can actually be beneficial as it reduces the heating costs, but by the way houses are constructed in this region the colour of the roof is becoming irrelevant, and the way the house retains heat paramount. In other words here insulation beats insolation.

The idea is stimulating though the impact on reducing global warming seems doubtful, basically due to two issues.

Firstly, even if we assume that all the rooftops around the world are universally painted white, the total surface area would hardly be 5% of the Earth's total surface.

Secondly, when the white rooftops would reflect sunlight back from the Earth, it will amount to an increased albedo, but the as the reflected sunlight passes through the atmosphere, the greenhouse gases would hold back much of the radiation, nullifying the effect.

Nonetheless, overall reduction may occur in terms of cooling achieved inside the houses on the individual level, power savings and reduced need for airconditioning.