Three repairs, one porch ceiling, and the same brown mark

When a repair does not hold, most people assume the roofer did a poor job. That can be true. It can also be true that the roof is the wrong slope for what sits on it, in which case no patch will keep the water out for long. The question to ask is simple: how steep is this roof?

The roofer can give you the number, and the rest of this guide tells you what to do with it.

Roofers describe slope as rise over run. A 2:12 roof climbs 2 inches for every 12 inches it runs sideways, and a 4:12 roof climbs 4 inches.

Do not climb up to measure it. Ask the roofer to measure and to write the number into the proposal. A number is a fact you can check, and "it looks fine" is not.

New York's residential code allows asphalt shingles only on slopes of 2:12 or steeper. Between 2:12 and 4:12 it also asks for double underlayment beneath the shingles. Anything flatter than 2:12 is outside what shingles are allowed to cover.

Put that beside your porch. If it measures under 2:12 and wears shingles, the repairs are fighting the slope. Tell the roofer the number and the history, and ask whether the roof has the right product for its pitch.

Houses that are decades old may have had a porch, back kitchen or addition added over the years, which is why the slope question is worth asking. It is a reason to check, not a measurement of any roof, and yours may be steep enough.

No dataset counts low-slope porch or addition roofs in this county, so be wary of anyone who gives you a percentage. Ask the roofer to measure yours instead.

Roofs flatter than 2:12 take a product made for low slopes. Single-ply membranes such as EPDM, TPO and PVC are one family, and the code sets a design slope of at least a quarter unit in 12 for them. Mineral-surfaced roll roofing is not allowed below 1 unit in 12.

The name of the product matters less than the match between product and measured slope. Ask which product is proposed and what slope it is made for, and expect both answers in writing.

Even a correct roof leaks at its edges if the wall joint is wrong. Where a low roof meets a taller wall, the base flashing should run in one piece or in steps, and a kick-out at the bottom should turn water into the gutter. If the stain sits near the house wall, ask about the joint as well as the slope.

A roof that does not slope to its edges also needs a drain at each low point. Look from the ground after rain; water still sitting there a day later is something the roofer should hear about.

The porch ceiling tells its own story. If the stain sits near the edge, the water is probably leaving the roof there. If it sits against the house wall, the wall joint is the first suspect. If it is in the middle and spreads, ask about the roof boards and where the water collects.

Take a dated photo of each stain and draw its edge. When the roofer arrives, hand over the pictures and the history of repairs. That saves the visit from repeating work that has already been tried.

Sealant is quick, which is why it is the first thing most people try. On a roof that is too flat, it closes only the spot where water got in this time. The next rain finds another seam, a nail or a lap, and the ceiling stains again, a little to the left of the old mark.

If you have already used sealant more than once, you have learned something useful: the leak moves. Tell the roofer that, because a leak that moves is a strong sign of a roof asked to do more than its slope allows.

An enclosed porch can behave like a small attic. If it is heated, warm room air can meet a cold roof deck above it, and the same dew point rule applies as in the main attic. A stain after a clear, cold night may then be condensation, not a leak at all.

Check your weather notes before you blame the shingles. If the stain follows cold nights and not rain, say so when you book, and the roofer will look at the porch ceiling and the deck above it first.

The measured pitch decides this, and the roofer should say which side of 2:12 and 4:12 your roof falls on. At 2:12 or more, a repair at the real cause may hold, and a double layer of underlayment may be needed if the pitch is below 4:12. Under 2:12, the product has to change.

Whichever way it goes, get the pitch measured before the discussion moves on. The porch measurement answers only the porch.

• What is the pitch of this roof, and how did you measure it?
• Is that steep enough for the covering that is on it now?
• If not, which product do you propose, and what slope is it made for?
• How will the water leave the finished roof?

Ask these before the roofer talks about price. If the answers come easily, you have someone who has looked at the roof. If they dodge the first one, treat the rest of the conversation with care.

• The measured pitch of the porch or addition roof, as a number.
• The product proposed, and the slope it is made for.
• How the wall joint and the low edge are finished.
• Where the water leaves the finished roof.

If a proposal repeats the work already done, ask what is different this time and why. The low-slope roofing page lays out the visit step by step, and the flashing page covers the wall joint.