How big does a puncture need to be to require the tire to be replaced? Small piece of glass? nail? Can see through it 
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if you see the tire carcass deform at the spot of the puncture when inflated to the max recommended pressure, the tire should be replaced
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Anything that you can visibly see through the tire, or if there are starting to be a fair amount of cracks signifies it’s time to replace the tyre.
Anything outside the tread area should be considered unrepairable. If the hole is big enough that you can see through it, no way should it be repaired. A small puncture (not cut, tear, or rip) can usually be plugged. The best thing to do is dismount the tire and plug it, then cut the plug flush with the inner surface and patch over it. The plug will help keep the hole from opening up, and the patch will prevent air leaks (air leaking into the structure of the tire can be a real problem). If you have driven more than a few hundred yards on a flat tire, or a few miles on a tire with very low pressure, it is probably damaged beyond repair, even though it may look OK.
From the Rubber Manufacturers’ Association website http://www.rma.org/tire_safety/tire_maintenance_and_safety/tire_safety_brochure/tire_care_and_safety.cfm
Loss of Tire Pressure
When you discover a tire losing air, it must be removed from the wheel by a qualified service person for complete internal inspection to be sure it is not damaged. Tires run even short distances while severely underinflated may be damaged beyond repair.
Punctures up to 1/4 inch, when confined to the tread, may be repaired by trained personnel. These tires must be removed from the wheel, inspected and repaired using industry-approved methods which call for an inside repair unit (patch and a plug).
Plugs vs. Patches
A PLUG BY ITSELF IS AN UNACCEPTABLE REPAIR. The repair material used-for example, a “combination patch and plug” repair -must seal the inner liner and fill the injury to be considered a permanent repair. Never use a tube in a tubeless tire as a substitute for a proper repair.
Individual tire manufacturers may differ on whether the speed category applies to speed-rated tires that have been repaired. Consult the tire manufacturer for recommendations.
Serviceable Tire Injuries
Injuries larger than 1/4 inch must be referred to a full-service repair facility. No repairs to the sidewall of a tire should be made without consulting the tire manufacturer. Improper repairs can cause sudden tire failure.
Air loss due to punctures can ruin tires that might have been saved had they been removed in time for proper repair. Gradual air loss raises a tire’s operating temperature. This can cause some of the components to separate, or damage the tire body in ways that create rapid or sudden air loss.
Such internal damage may not always be readily apparent and rapid loss of air may still occur despite later installation of a proper repair.
good information, but I’m not so sure it applies to bicycle tires.
I realized that about 1 second after I hit “Post Reply.”
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Nicks and gouges are “normal” on road tires. Any hole that allows you to see daylight through the tire should be tossed. You CAN boot the tire in a pinch (I’ve done this plenty of times) with Tyvek or other suitable material, but I wouldn’t trust it 100%.
Any hole that goes through the casing will allow the tube to herniate through tire and will result in an unprovoked blowout. Consider yourself lucky if said tire is identified while airing up before heading out on a ride.
Any hole that goes through the casing will allow the tube to herniate through tire and will result in an unprovoked blowout.
So any puncture by a nail, etc. would need to be replaced? What type of road damage flat would not need to be replaced?
Sorry if this is obvious to everyone else?
I will boot a tire with more money than it is worth sometimes before replacing it. I do no throw out a tire until the tire itself is wore thing to be on the verge of causing punctures by the smallest road debris. I have not used one of those fancy boots at the cycling shops. I just grab a dollar bill and fold it in quarters and use that. Sometimes I only have larger bills, it makes for great fun when I replace the tire. One time I had $30 (a $20 and a $10) in a tire.
All of you folks that throw away tires when they have a small gash, I will send you my mailing address. If I can’t shove two fingers in it, it is still usable.
Race tires are a different story.
Any hole that goes through the casing will allow the tube to herniate through tire and will result in an unprovoked blowout.
So any puncture by a nail, etc. would need to be replaced? What type of road damage flat would not need to be replaced?
Sorry if this is obvious to everyone else?
Good question. I’ll elaborate. I wouldn’t replace a tire with a normal puncture but will if the puncture left some cuts in the casing that are large enough to let daylight through (I’ll literally hold the tire up and will see if I can see through the hole). The threads, usually filaments laid out in a textile fashion, are what gives the tire the ability to maintain its shape at pressure and a cut will leave the tire suspect since their isn’t anything left to retain the tube inside the tire. I’ve got a number of tires that I’ve booted and ride often but am keenly aware of the cut(s) and check it whenever I air up.
toss it when you find yourself wondering about the integrity of the repair.