The range, not the average
Take one reading in the middle of a garage door and you learn almost nothing. Take nine across the opening and the shape of the slab appears. The default readings run from a quarter inch at both jambs to thirteen sixteenths in the middle — an ordinary dished slab on a sixteen foot opening, and a range of nine sixteenths.
Nine sixteenths of range is what the seal has to absorb, and a seal quoted at half an inch of range is a sixteenth short of it. Set that seal to touch at the quarter inch ends and it hangs short in the middle; set it to reach the middle and it is crushed at the ends every time the door closes, which is how bottom seals get torn out of their retainer. That is a single number that decides the product, and it does not appear anywhere in an average.
| What you measure | What it tells you |
|---|---|
| Average gap | Roughly how much air is getting under, nothing about the seal |
| Widest gap | How far the seal has to reach at its worst point |
| Range, widest minus narrowest | Whether one seal can do the whole opening at all |
What the gap adds up to
Integrating the default readings across the door — trapezoids between the measured points — gives 97.5 square inches of open area, which is a square 9.9 inches on a side. Assuming a flat gap at the average, which the page reports as 15/32 in and 0.479 decimal, would have given 92 square inches, so on a smooth dish the average is within about six percent; on a slab with one sharp low corner between two readings it would be badly wrong, which is the reason to integrate rather than multiply.
Either way it is a large opening, and it is worth keeping in proportion. It is the single biggest hole in most houses that have an attached garage, which is why a garage tracks the outdoor temperature however well the walls are built. It is also, on most houses, outside the conditioned space, so closing it is a comfort, dust, water and vermin decision rather than a heating one. The wall and the door between the garage and the house are where the heating argument lives.
Three answers to a dished slab
A taller bulb seal buys more range, and is the first thing to try. A bonded floor threshold raises the slab to meet the door at the point where it dips, which also solves water running in under the door on a drive that falls the wrong way — at the cost of a permanent bump across the opening. Grinding or patching the slab is the real fix and almost nobody does it. What does not work is setting the seal deep enough to reach the low point, because the ends then take the whole closing force through a crushed seal and it fails within a season.
Related pages
For the geometry above the door rather than under it, the garage door headroom calculator covers track type and the space a door needs. For where this gap sits against the whole house, the leak area calculator puts a measured fan reading and a list of gaps side by side. For the door between the garage and the house, weatherstrip and sweeps is the take-off, and foam coverage covers the static gaps around the framing.
The part that is not about rubber
A sectional garage door is held up by springs storing a very large amount of energy, and they are under that load whether the door is up or down. Springs, cables, bottom brackets and the lift mechanism are not homeowner work and nothing on this page asks you to go near them. The other hazard is the one people forget because the door is open when they think about it: a vehicle running in a closed garage produces carbon monoxide quickly, a tighter garage concentrates it faster, and an attached garage shares a wall and a door with people. How that separation has to be built is a requirement that varies by jurisdiction and belongs to the building department, not to a seal.
Questions people ask
How do I measure the gap under a garage door?
Close the door and take readings at intervals across the whole width, including hard against both jambs, with a ruler or a set of shims. Five is a minimum and nine is better on a sixteen foot door. Write down the distance from the left jamb with each reading so the shape of the slab is visible afterwards, because a smooth dish and a single low corner call for different answers and an average hides both.
What size bottom seal do I need?
Long enough to run the door width plus whatever overhang the retainer takes at each end, and with a compression range at least as large as the variation you measured. A 16 ft door with an inch of overhang each side needs 194 inches, so an 18 ft piece covers it in one length. The height or bulb size is the part that has to match the range, and that is a product figure — a tall U-shaped bulb covers far more variation than a flat brush.
Why does my new seal touch at the ends but not in the middle?
Because the slab dips, which is normal. Concrete is poured with a fall towards the door and it settles unevenly over years, so the middle of a wide opening is usually the low point. The seal cannot stretch to cover more range than it has. The real options are a taller seal with more range, a bonded floor threshold that raises the low area to meet the door, or repairing the slab. Adjusting the door down until the seal reaches the middle just crushes it at the ends.
How much air is actually getting under the door?
On the default readings, integrating across the opening gives 97.5 square inches, or a square 9.9 inches on a side. That is geometry rather than measured leakage, since a real gap is partly closed by whatever seal is there and does not flow like a clean hole. It is also worth remembering that on most houses the garage is outside the heated space, so closing it is about comfort, water, dust and vermin rather than about the heating bill.
Is it safe to seal a garage up tightly?
Sealing the perimeter is fine; treating the garage as part of the house is not. A running engine produces carbon monoxide quickly, and the tighter the space the faster it concentrates and the more of it pushes through into the house. Never run a vehicle, generator or fuel-burning heater in a closed garage. How the separation between an attached garage and the living space has to be built and sealed is a specific requirement that varies by jurisdiction, and it is a question for the building department rather than something to solve with weatherstrip.