Food

Bring your own food to construction sites since they are usually not near restaurants or convenience stores. A small cooler with a handle is perfect for packing lunch. 
Eat breakfast, and DON’T SKIMP ON LUNCH.  This is not the time to start a diet.  You will need calories to get through the day. Pack things that will not spoil in the heat. Sandwiches can be frozen ahead of time; they will thaw during the morning.
Bring an extra snack. Hard candy, nuts, granola bars, and fruit are good choices - you might need the energy. Nuts and bananas are high in potassium - a mineral your body especially needs when working hard.

Drink

Bring plenty of water and/or non-sugary drinks. Dehydration can cause headaches, muscle cramps, and fatigue, and lead to heat exhaustion or even heat stroke.
Most companies provide water on the worksite, but don’t take a chance. Wash out some of those bottles you have floating around the place, or (better yet) invest in reusable bottles. Fill them not quite to the top, loosen their lids slightly, and put them in the freezer.  In the morning, top them off, tighten the lids, and pop them in your mini-cooler – they will thaw slowly and help keep the rest of your stuff cool.

Sports Drinks

Drinks like Gatorade supply fluids, sugars, and electrolytes (minerals) to replenish what you're sweating out on the job site. Many of them, though, can cause cramping as your body tries to process the sugar on a hot day. Try diluting them with water (half-and-half), or save them for the end of your workday.

The following home-made drink mixes provide the three major electrolytes your body needs when it is working hard: potassium, chloride, and sodium. Experiment with the proportions as you please; they aren't written in stone!

3/4 cup of sugar
3/4 teaspoon of lite salt (potassium chloride and sodium chloride mix - Morton makes a version)
1 package of unsweetened drink mix
2 quarts of water

1/2 cup of orange juice (carrot, tomato, and prune juice are also high in potassium)
1/2 cup of sugar
1/2 teaspoon of salt
2 quarts of water

If you don't have the time or inclination to make your own from scratch, buy the powdered form, and try store brands, too- your dollar will stretch a lot further. Again, dilute the mix, so your body isn't processing too much sugar at once.