How to Tie New Shingles Into an Existing Roof for Additions and Partial Replacements
New shingles have to lap under the old ones, match the exposure, and meet at a line that the eye does not catch.
- Advanced
- 3 to 8 hours per tie line
- Updated
The short answer
To tie new shingles into an existing roof, lift the old course above the tie line with a flat bar, slide the new shingles under it, and nail below the old course so nails are covered. Match the old exposure by measuring it first, weave the joint over two or three courses, and choose a natural break such as a ridge, valley or wall where possible. Seal lifted tabs afterward.
Tying new shingles into an old roof is one of the most common jobs on a house. A new addition, a repaired section, a replaced skylight or a partial reroof after storm damage all leave a line where new meets old. The goal is a line where water cannot get in, the courses match up, and the eye does not catch the joint.
Structurally, tie-ins are a matter of weaving. New shingles sit under the course above, and the old course above laps over them, so water goes from old to new across the joint in the right direction. The trouble is that old shingles are brittle, the old nails pull out, and the old exposure may not match the new product. A sloppy tie-in leaks at the joint and also looks like a patch.
This guide covers tie-ins on the field of the roof. For replacing a single damaged shingle, see how to replace a damaged shingle. For a full replacement, see how to tear off a shingle roof. Colour and ageing differences are covered below.
Tools
- Flat bar
- Roofing hammer
- Roofing coil nailer and compressor
- Hook blade utility knife
- Chalk line
- Tape measure
- Caulking gun for roof cement
- Fall arrest harness and anchor
Materials
- New shingles, as close a match as available
- Underlayment and ice and water shield
- Roofing nails, 1-1/4 inch galvanized, or longer for an overlay
- Asphalt roof cement
- Flashing for any new walls or valleys
Safety first
- Old shingles are brittle, and the old roof may have soft decking or weak spots near the edge of the section you are opening. Test the deck before you put your weight on it.
- Stay tied off. An open tie line is a fall and a leak hazard, and you have a steep edge to work along.
- Weather is the threat. Do not open more of the old roof than you can close before dark, and tarp the tie line when you leave.
Step by Step
Step 1: Choose the tie-in line
A joint at a ridge, valley, hip or wall is the least visible, because the eye expects a change there. A joint in the middle of a slope shows colour and texture differences at every course. If you must tie in mid-slope, pick a straight course line and plan to weave over 2 or 3 courses.
Mark the line with a chalk line along a course of the old roof. Remember that the old courses may not be perfectly straight, so use the old course as your guide, and adjust the new courses to meet it.
Step 2: Measure the old exposure
Old 3-tab shingles commonly have 5 inches of exposure. Many laminated shingles have about 5-5/8 inches, and metric or older products can differ by a fraction. Measure over 5 or 6 courses and divide, since the old roof may have drifted. If the old exposure is different from the new product, blend the difference over several courses, changing it by no more than about 1/8 inch per course.
Check the manufacturer's allowed exposure range for the new product. Stretching exposure beyond it voids warranties and reduces coverage.
Step 3: Open the tie line without damaging the old roof
Slide the flat bar under the old tabs, break the sealant and locate the nail heads. Lift each nail with the bar and remove it, working gently since old shingles are brittle and crack when bent. Remove the old shingles below the tie line, back to the deck.
Keep the exposed opening to a size you can close in a day. See how to replace a damaged shingle for the lifting technique.
Watch out: Cold or aged shingles crack when lifted. Work in warm weather when you can, or accept that a few old shingles will need replacing.
Step 4: Prepare the deck and underlayment
The underlayment at the tie line follows the lapping rule: the upper layer goes over the lower one. At the top of the opening, slide new underlayment under the old. At the sides and bottom, lap it over the old. See the shingle lapping rule.
If the deck has been cut or replaced, see how to replace roof sheathing and how to install synthetic underlayment.
Step 5: Lay new shingles up to the tie line
Start at a solid edge, with starter if the section begins at an eave. Lay courses up to the tie line as on any roof, following chalk lines snapped to line up with the old courses. See how to lay shingle courses and offsets.
Keep the offsets between the new and old joints at the manufacturer's minimum, commonly 4 to 6 inches.
Step 6: Weave the last new courses under the old
The last new course tucks up under the old course above. Nail the new shingles in their normal nailing zone, where the old shingle will cover the nails. A nail that would sit in the exposed part of the old course should not be used.
Weaving over 2 or 3 courses, cutting each new course slightly shorter to stair-step up, makes a more watertight and less visible joint than a single straight line.
Step 7: Re-nail and reseal the old courses
Put the old course back down over the new one, nail it through both layers with nails that are long enough, and press it flat. Old sealant will not re-bond, so put quarter-sized dabs of roof cement under each lifted tab. See how to reseal lifted shingles.
Cover any exposed nail heads with cement. Replace any old shingles that cracked when lifted.
Step 8: Flash and seal where the new meets walls and penetrations
A tie-in at a wall or a chimney needs the flashing replaced or extended so it laps correctly with both old and new shingles. See how to install step flashing and how to install counter flashing.
After the job, walk the joint and check that tabs are flat, nails are covered and the joint has no gaps. If the roof was left overnight, tarp it.
Where the Joint Shows
New shingles are darker, cleaner and sharper than old ones, which have faded, lost granules and may be covered in moss. Even the same colour from the same line will look different next to a 15 year old roof. The difference softens within a year or two as the new shingles weather, but it will always be visible where the two meet in the open field.
The best fixes are to tie in at a natural break, blend the two roofs across several courses, and pick the closest available colour. On a roof that is near the end of its life, a partial replacement may not be the right answer, and a full roof will look better. See roof replacement.
Old Roof Condition Matters
A tie-in works best where the old roof is sound. Brittle shingles crack when lifted, old nails pull out, and the deck may be soft. If the old roof is near the end of its service life, opening it up can cause more damage than the repair solves. A roofer will check a few shingles before committing to the plan.
Common Mistakes
- Tying in on a straight line across the open field. It shows, and the colour difference is obvious.
- Ignoring the old exposure. New courses that do not match the old ones produce a kink in the pattern at the joint.
- Nailing through the exposed part of the old course. The heads show and leak.
- Laying new shingles over the old ones at the top. Water then runs under the old course, which is the wrong direction.
- Skipping the hand seal. Old sealant will not re-bond, and lifted tabs blow off.
- Opening too much roof. Weather can arrive before the tie line is closed.
When to Call a Professional
Tie-ins are a repair skill that depends on knowing how old shingles behave. If the roof is steep, more than 15 to 20 years old, has a hip or valley at the tie, or is a wall junction with flashing, have a roofer do it. A roofer can also tell whether a patch makes sense or the roof is near the end of its life.
If you have a leak at an old tie-in, have it inspected before repeating the repair. The fault is often the flashing or the nail placement and not the shingle.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Roof Replacement service or the towns we cover.
How to Tie New Shingles Into an Existing Roof: Questions
How do you tie new shingles into an old roof?
Lift the old shingles above the tie line with a flat bar, slide the new shingles under them so the old course laps over the new, and nail in the nailing zone so the old course covers the nails. Match the exposure, weave the joint over several courses, and hand seal the lifted tabs, since old sealant will not re-bond.
Will new shingles match old shingles?
Not exactly. New shingles are darker and sharper than weathered ones, even in the same colour, and the difference fades slowly. Tying in at a ridge, valley or wall hides the joint, and blending across several courses softens it. A full reroof gives a uniform look if the match matters.
Can you put new shingles next to old ones?
Yes, when the roof is sound and the tie-in is done correctly, with the new under the old above the joint. Exposure should match, nails should be covered, and the tabs should be sealed. If the old roof is near the end of its life, a partial replacement may not be worthwhile.
How do you match shingle exposure on an old roof?
Measure the exposure over 5 or 6 courses of the old roof and divide, since old roofs drift. If it differs from the new product, change the new exposure by no more than about 1/8 inch per course over several courses until they meet. Stay within the range the manufacturer allows.
Is it worth repairing a section of an old roof?
It depends on the age and condition of the rest of the roof. A sound roof with a damaged section is a good candidate for a partial repair. A roof that is 20 or more years old, brittle and granule-worn may fail elsewhere soon, so a full replacement may cost less over time than repeated repairs.
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This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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