Fall is the rainy season and the time of year you need to make sure your roof is in good shape. I don’t want to think about it, but winter is on its way with snow and ice, freeze-and-thaw cycles and strong winds. Obviously, your roof protects your home year round, but in the fall and winter any weak spots in your roof will be tested. And, when the temperature drops, it’s the worst time of year for someone to be working on your roof – assuming you can find anyone who’ll do it – so make sure your roof is in good shape now.

Your roof gets worn over time, and through exposure to sun, rain, snow, wind and debris. And, it might look OK from the ground, but without a professional taking a good close look at it, you won’t necessarily know what shape it’s in. You may assume it’s fine. But wishful thinking won’t keep you dry.

Most homeowners notice they’ve got a problem with their roof when they have a leak. Whether it’s just water stains on the ceiling or a full drip, the source of the leak can be hard to find. Water can travel horizontally along the underside of the roof sheathing or along roofing boards, and drip through to your home far from where it’s getting through your shingles.

You need to understand that your roof is a system – all the parts work together. From the exterior surface – the parts that most people think of first – to the underlayment, to the flashing and the sheathing, all parts of the system must be in good shape for the roof to stand up to what the weather throws at it.

Most roofs have some slope. They are pitched so the rain hits the surface, and flows down the slope to the gutters, where it’s taken away. Depending on the degree of pitch, some roofing materials are more appropriate than others – you wouldn’t use standard shingles on a low-pitched roof, for example.

Certain roof types have more problems than others. I find that a low-pitched roof is more likely to have problems with ice dams, since there is less room beneath the roof sheathing for insulation. In general, the steeper the pitch of your roof, the longer it will last, because water and snow load will be shed more quickly. On a low-pitched roof, the snow will probably sit longer, melting and refreezing, and have more time to get through to your house.

You may find problems if your roof has lots of detail, such as dormers, gables and angles. In spots where roof planes join, there is a potential for leaking at the flashing, peaks and valleys.

Most roofs are covered in asphalt shingles – not that they are my favourite roofing choice. The shingles are overlapped with each shingle partly covering the one under it, so the water flows down from one shingle to the next. They are also staggered so the seams between the shingles don’t line up and let water get through the crack and find its way to the roof sheathing.

A leak occurs when water somehow penetrates the roof’s first layer of protection: the shingles. Maybe some shingles are damaged or missing. Or, maybe an intense rainstorm with high winds will drive the water up behind the shingles.

That’s why your roof system has a backup under the shingles: the underlayment. Traditionally, black felt roofer’s paper is rolled on to the surface of the sheathing, and depending on your local building code and climate, a three-foot length of ice-and-water shield is laid along the bottom perimeter of the roof. I prefer to use ice-and-water shield all over, and think it’s well worth the extra money.

Think about how your roof is installed: The shingles are nailed on to the roof sheathing. Thousands of nails, all over your roof’s surface, penetrate the shingle and the sheathing. Every one of those nails is a spot where water can trickle down and drip into your attic space. That’s one reason ice and water shield is a good idea to use all over your roof – it’s a rubberized adhesive membrane, so it’s waterproof, and it seals around the nails much better than roof paper.

You want to make sure water stays off your roof sheathing. Roof sheathing is wood and wood is porous. It’ll soak up the water until it’s saturated, then the water will need to go somewhere. It will drip, or run along the rafter until it finds a better place to drip. When the rain stops, the drip stops, but the problem remains. Over time, that section of roof that has been repeatedly wet then dry again will rot. It might mould. It could lead to major roof damage.

Your roof system also includes flashing and counter flashing- the thin sheets of metal, like copper or galvanized aluminum – that you’ll find around the chimney, skylights and vents, anywhere the roof shingles meet another surface. This flashing has to be properly caulked to prevent water from getting in behind it and under the shingles.

You’ll also find flashing in the roof valleys -that’s the spot where two sections of your roof slope downward and meet. Valleys are also a great spot for water to collect, and for snow to sit, so you need to make sure the valley is properly flashed.

Check your roof system before the stormy season arrives and a leak finds you.

Catch Mike in his brand new series, Holmes Inspection, airing Thursdays at 8 p.m. ET/PT on HGTV. For more information visit hgtv.ca.

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