A leak has appeared, or a wind has taken a course of shingles, and the household must now decide a question it would rather not face: whether to mend the roof or to lay a new one. It is a costly question either way, and the temptation is always to choose the smaller bill. That is sometimes the right answer and often the expensive one, because a patch upon a roof near the end of its years is money spent on a thing that will shortly need replacing regardless.

The decision is more tractable than it appears. It rests on four plain matters, and a household that has considered each in turn will usually find the answer has decided itself.

First, the Age of the Covering

A roof has a span of years. What that span is depends on the material laid and on the care it has received, and the papers from the original installation will generally say what was warranted. The useful question is not the number itself but where this roof stands within it.

A roof in the first half of its span is a candidate for repair. It has years remaining, the repair will be enjoyed for those years, and the covering around the mend is still sound enough to hold the patch. A roof in the last quarter of its span is a poor candidate. The material has grown brittle, a new shingle will not weather to match, and the household will very likely find itself paying again within a season or two, this time for the whole.

Second, the Extent of What Is Wrong

A defect confined to one place is a different matter from a defect appearing everywhere at once. A limb has come down and broken a dozen shingles: that is a local injury to a sound roof, and it is mended. But if the shingles are curling at the edges across the whole southern face, if granules are running from the downspouts in quantity, if the field is spotted with bare patches wherever one looks, then the roof is not injured. It is worn out, and the leak that prompted the inquiry is merely the first of many.

A short list serves as a rough test. Any one of these argues for replacement rather than repair:

• Curling, cupping, or cracking visible across whole planes rather than in one corner.
• Bare patches where the mineral surface has gone, seen in several places at once.
• Any sag or waviness in the line of the roof, which speaks to the deck beneath and not the covering above.
• Daylight visible through the boards when standing in the attic at midday.
• A history of repairs: three or four mends in as many years is the roof telling you plainly what it needs.

Third, What Lies Beneath

The covering is only the outermost layer. Under it lie the underlayment and the deck, and under those the framing. Where water has been entering for some while, the deck may be soft, delaminated, or rotted, and no shingle laid over soft wood will hold a nail properly or serve out its years.

This is the matter a homeowner is least able to judge from the ground, and it is the matter that most often changes the answer. A roof that appears to need a modest repair, once opened, is sometimes found to have been feeding water into the sheathing for several winters. Any honest assessment therefore requires someone to get onto the roof and into the attic and look at both faces of the same problem. That is properly the work of a roofing company that will put its findings in writing, listing what was seen, where, and what each course of action would cost, so the household is choosing between two described things rather than between a guess and a fear.

Fourth, the Household's Own Horizon

Last, and least discussed, is how long the family means to remain. A household intending to stay twenty years is buying service and should buy it once and properly. A household intending to sell within the year is in a different position, though not the position it imagines: a roof visibly at the end of its life will be found by any competent inspector, and the sum will be taken out of the price with interest. Deferring the work rarely saves what it appears to save.

The Rule of Thumb, and Its Limits

Reduced to a sentence: mend a young roof with a local injury, replace an old roof with a general one, and never lay new material over a deck that has gone soft. The rule holds in most cases. Where it does not, the reason will be visible to anyone who has been up on the roof with a straightedge and down in the attic with a light, which is precisely why the household that skips both inspections is the household most likely to pay twice.

There is no economy in guessing at a roof. There is a great deal of it in knowing what one has before deciding what to do with it.