Does Attic Mold Mean You Need a New Roof?

Does attic mold mean you need a new roof? In most cases, no. Mold grows on the surface of roof sheathing and can be cleaned off wood that is still solid. You only need roofing work if the sheathing has gone soft or delaminated, or if worn shingles or failed flashing are letting water in.

Surface mold and rot are different problems

Mold lives on the surface of wood and feeds on what is available there. It discolours the wood but does not take away its strength. Rot is caused by different fungi that digest the wood fibre itself, and it needs the wood to stay quite wet for a long time. An attic can have heavy black staining on sheathing that is as strong as the day it was nailed down.

The two are related only through moisture. Sheathing that gets damp every winter and dries every summer usually grows mold and stays sound. Sheathing that stays wet all year, under a leak or beside a fan duct, can go on to rot.

How to tell if the sheathing is still sound

• Press on it. Push a screwdriver tip against a stained area with firm hand pressure. Sound wood resists. Damaged wood lets the tip sink in or flakes apart.
• Look at the layers. Plywood that is failing separates into curling plies. OSB swells at the edges, turns crumbly and loses its flakes.
• Look along the roof from outside. Dips or a wavy surface between rafters suggest the deck has lost stiffness.
• Check the nails. Heavily rusted nails that have lost their grip, or dark rings where nails have pulled, mean the area has been wet for a long time.
• Ask how it felt underfoot. A roofer walking the roof will notice spongy sections immediately.

If the wood passes these checks, it can be cleaned. Do the pressing gently and with a respirator on, and stay on the framing.

When attic mold does mean a new roof or new sheathing

There are real cases where a roofer has to be involved.

• The sheathing is soft, delaminated or rotted. Those sheets get replaced, and that can only be done from above with the shingles off in that area.
• The shingles are at the end of their life and leaking in more than one place. Cleaning the attic under a failing roof is money spent in the wrong order.
• Flashing has failed at a chimney, valley, skylight or vent stack. This is a repair, not a reroof, but it has to come first.
• There are not enough roof vents, or the wrong mix. Adding or changing vents is roofing work, though it is a small job.

Even with rot, the damage is usually limited to a few sheets near the cause. Replacing a section of deck and patching the shingles over it is a normal repair. A full reroof is only needed when the roof covering itself is finished.

Why a new roof does not fix attic mold

Homeowners are sometimes told that new shingles will solve the mold. They will not, for two reasons. First, most attic mold comes from humid house air condensing on the underside of the deck. New shingles on top change nothing about that. The same moisture will condense on the same wood next winter.

Second, a reroof leaves most of the old sheathing in place, along with every rafter and truss. Any growth on those surfaces is still there under the new shingles. Even a full sheathing replacement leaves moldy framing behind, and the new plywood will grow mold too if the cause is not corrected.

If your roof is due for replacement anyway

This is the one case where timing can save money. If the shingles have a year or two left and the attic has mold, plan the two jobs together.

1. Have the attic assessed first, so you know which sheets are soft and where the moisture is coming from.
2. Give the roofer that list. Bad sheets get replaced while the deck is exposed, which is far cheaper than a separate visit.
3. Have the roofer install proper hoods for bathroom fans and correct the roof venting during the same job.
4. Schedule the mold removal on the remaining sheathing and framing after the roof work, so the new deck stays clean and one report covers the finished attic.

What to ask before you sign anything

If one contractor says you need a new roof and another says you need cleaning, ask each for the evidence. A roofer should be able to show you where the shingles or flashing have failed. A mold company should show you moisture readings and explain the cause. Take your own screwdriver test and photos of the worst area up front. Solid wood plus a clear moisture source from inside the house means the roof can stay.

Common questions