How to Install Box (Static) Roof Vents: Placement, Hole and Flashing
Passive exhaust in a row near the ridge. Count by net free area, cut clean, and flash like a pipe penetration.
- Advanced
- 1 to 2 hours per vent
- Updated
The short answer
To install a box vent, locate it on the roof face near the ridge, between rafters, cut a hole to the vent's template, slide the flange under the shingles above, nail the flange on the top and sides, and lap the shingles below over it. Space vents evenly along the ridge side of the roof, and choose the count from the net free area on the label.
A box vent, also called a static vent or turtle vent, is a low-profile square or rectangular cap over a hole in the roof. It has no moving parts. Warm air rises out of the attic through it, drawing replacement air in at the eaves. A row of them along the upper part of the roof forms the exhaust side of the ventilation system.
They are simple and cheap, but their performance depends on how many you install, where, and whether they are paired with enough intake. Each one has a modest net free area, so a typical attic needs several. They are also each a hole in the roof with flashing, so every one is a potential leak if flashed badly.
If you are choosing between exhaust types, read how attic ventilation works and how to install a ridge vent. Box vents should never be mixed with a ridge vent on the same roof, see why you should not mix roof exhaust vents.
Tools
- Drill with a long bit
- Reciprocating saw or jigsaw
- Tape measure
- Pencil
- Roofing hammer
- Hook blade utility knife
- Flat bar
- Caulking gun
Materials
- Box (static) roof vents, with published net free area
- Roofing nails, 1-1/4 inch
- Roofing cement or sealant, only where the manufacturer specifies
- Matching shingles
- Self-adhered membrane patch, if the roof is being reworked
Safety first
- Work tied off. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall could cause serious injury. You will work on the upper slope near the ridge.
- Check the attic before cutting. Make sure no rafters, wires or ducts are under the planned hole.
- Metal vent flanges have sharp edges. Wear gloves when handling them.
Step by Step
Step 1: Calculate how many vents you need
The BC Building Code asks for vent area of at least 1/300 of the insulated ceiling area, or 1/150 for low slope or joist roofs, and intake and exhaust are each about half of that. The method is in how to calculate attic ventilation.
Example: 360 square inches of exhaust divided by a label rating of 60 square inches per vent gives 6 vents. Labels vary, so use the actual figure. Do not guess the rating from the size of the vent.
Step 2: Confirm intake is open before adding exhaust
Look in the attic for blocked eaves or missing soffit vents. Fix intake first. See how to add soffit intake vents and how to install insulation baffles.
Check for existing exhaust vents and do not add a different type beside them.
Step 3: Choose locations near the ridge, evenly spaced
The higher the vent, the better it works, because hot air collects at the top. Keep it low enough that the flange can tuck under the ridge cap or upper shingles and high enough that it is above the normal flow of water. Avoid valleys, skylights and chimneys.
Spread the vents evenly along the length of the ridge so the whole attic is ventilated, rather than clustering them in one spot. Where the ridge faces a direction, place more on the leeward side, since negative pressure on the lee side helps the vent draw.
Step 4: Mark the hole from inside
Drill from the attic so that you can see the rafters and avoid them. Check on the roof that you are where you want to be. Place the vent over the pilot hole and trace the opening.
The hole is sized to the vent's neck, commonly a square or rectangle of a few inches per side, but use the template or instructions that came with the vent.
Step 5: Cut the hole and lift the shingles above
Score and cut the shingles, then cut the deck with a jigsaw or reciprocating saw. Remove nails and loosen the shingles above the hole with a flat bar, without tearing them.
Vacuum sawdust out of the hole. Patch the underlayment so it is continuous under and around the vent.
Step 6: Slide the flange under the upper shingles and lap over the lower
This is the same lapping logic as any penetration: the upper layer over the lower layer. Upper flange under the shingles above, bottom flange over the shingles below. Water running down the roof passes over the vent, not into it. See the shingle lapping rule.
Check the vent sits flat on the roof plane and the opening lines up with the hole.
Step 7: Nail the flange on the top and sides only
Use roofing nails and follow the manufacturer's pattern. Nail only through the top and side flange areas, which are covered by shingles above or beside. Never nail at the bottom of the flange, where water runs.
Do not add sealant under the flange as a substitute for the lap. Apply sealant only where the instructions say, for instance at exposed nail heads. See roofing sealants.
Step 8: Shingle up to and over the flange
Lay shingles at the sides and above so that they cover the flange and nail heads, stopping clear of the vent opening. Shingles below stay over the lower edge of the flange, never under it.
Check that no opening is covered and that the vent's louvres are clear. Repeat for each vent, then look from the attic to confirm each hole lines up with the vent.
How Many Is Enough
A single box vent has a modest net free area. A typical attic needs a number of them, evenly spread. Too few, and exhaust is undersized compared to intake. More is not automatically better, since it is the net free area and the balance with intake that count. Always count by net free area.
Box Vents on Their Own, Not With a Ridge Vent
Box vents work as a group of exhaust vents. If you add a ridge vent later, remove or seal the box vents, because a mix of exhaust types can pull air in through one and out of the other without moving air through the whole attic. See why you should not mix roof exhaust vents.
Common Mistakes
- Placing vents too low on the roof. They do not catch the warm air that collects at the ridge.
- Cutting into a rafter. Mark from inside the attic first.
- Flange over the lower shingles reversed. Water runs under the flange.
- Nailing the lower flange. Nail holes where water runs become leaks.
- Too few vents. Net free area falls short of the requirement.
- Mixing with a ridge vent. The airflow short circuits.
When to Call a Professional
If the roof is steep, high or has limited access, or you are not sure how many vents are required, have a roofer lay out the system. Each box vent is a roof penetration and a potential leak.
If the attic shows mold or wet insulation, the ventilation layout should be reviewed as a whole, not one vent at a time.
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 Install Box (Static) Roof Vents: Questions
How many box vents do I need?
Divide the exhaust net free area you need by the rating on the vent label, and round up. For example, 360 square inches of exhaust with vents rated at 60 square inches each needs 6 vents. The total comes from 1/300 of the insulated ceiling area under the BC Building Code, split about equally between intake and exhaust.
Where should box vents be placed on the roof?
High on the roof, commonly within a couple of feet of the ridge, evenly spaced along its length and on both slopes where possible. Keep them out of valleys and away from chimneys and skylights. The higher the vent, the better it catches the rising warm air.
Can you use box vents and a ridge vent together?
It is not recommended. Air can enter the lower box vent and leave through the ridge vent without crossing the attic, leaving the lower roof stagnant. Choose one type of exhaust for the whole roof. If you add a ridge vent, remove or seal existing box vents.
Do box vents leak?
They can if they are flashed wrongly. The flange must lap under the shingles above and over the shingles below, with nails only along the top and sides. Old vents with cracked plastic, rusted flanges or dried sealant also leak. Replacing the vent when the roof is replaced avoids most problems.
Related Guides
All Ventilation and Attics guides|The full How-To Library
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.
Want someone to look at your roof?
Kory & Johnny have roofed homes across the Fraser Valley since 2016. Free inspections, honest estimates, no pressure.
604-728-0146Text UsEmailServing Hope, Agassiz, Chilliwack, Rosedale, Abbotsford & the entire Fraser Valley