A builder’s net zero envelope course by Xerox Builders · October 2026
Door sills sit low, take foot traffic and face pooled water, so they leak more often than windows. This Detail covers the door sill pan, large sliding doors, low and accessible thresholds, the air seal under the sill, and protecting the sill during construction.
Why doors are harder than windows
A door is a window you walk through, and its sill is the weak point. It sits low, close to decks, patios and landings where water collects. Rain running down the glass pools at the threshold. Owners want the threshold low and easy to cross; water management wants it high. Large sliding doors add weight and width, and every door sill takes foot traffic for months before the house is finished.
The sill pan
Every exterior door gets a sill pan, either a flexible membrane or a formed pan, running under the full length of the sill. The pan does three things:
- Slopes to the outside, so water under the sill runs out, not in.
- Has dams on three sides: a back dam at the inside edge and end dams turned up each jamb. Door makers commonly ask for a back dam of at least about 13 mm (1/2 in.) and end dams about 150 mm (6 in.) up the jambs. Use the figures in your door’s installation guide.
- Laps out over the water-resistive barrier (WRB) below the opening, so water drains onto the face of the wall or into the rainscreen cavity.
Never fasten through the floor of the pan. Seal the corners completely; pinholes at the corners are a classic leak.
Setting the door
Bed the door sill in a continuous bead of sealant along the inside edge of the pan. Leave the front edge open, or with drainage gaps at the ends as the door maker shows, so any water that gets under the sill can escape. Seal behind the jamb and head flanges, fasten as specified, then lap jamb tape, a sloped metal head flashing with a drip and end dams, and finally the WRB flap over the head flashing, exactly as for a window (Detail 07).
Large sliding doors
- Flat and level: the sill must be flat along its whole length. A sag or hump causes sticking panels, poor seals and leaks.
- Fully supported: solid blocking and shims at every jamb and mullion. Anchor the middle of wide frames so the head and sill don’t bow.
- Full-width pan: one piece, or overlapped and sealed joints, with end dams at both jambs.
- Safe handling: two people or suction lifters. Set the panels, adjust the rollers with the weight relieved, and check that the weatherstrip still seals.
Thresholds and accessibility
A step down from the door to the landing is the best defence against water. Low and flush thresholds are increasingly requested for aging in place and wheelchair access, and they are a good goal. But every millimetre you take off the sill must be paid back with protection:
| Approach | Water | Access |
|---|---|---|
| Step down to the landing | Best | Harder |
| Low threshold with roof or canopy, drain channel and deck sloping away | Good, if all three are done | Good |
| Flush threshold, exposed, no drain | Poor: water runs to the sill | Good |
In buildings that must be accessible or adaptable, the BC Building Code’s accessibility rules govern doorways and thresholds, so check them with your designer. The BC Building Code 2024 adaptable-dwelling requirements, phased in from March 10, 2025, apply to a share of the units in apartment buildings (units off an accessible common corridor and entrance), not to single houses or typical townhouses, but house owners increasingly ask for low thresholds anyway. Agree the threshold design with the designer and envelope consultant before framing, because it can change floor heights and deck framing.
Air sealing under the threshold
The bottom of a door is a common blower door leak. Seal the inside edge of the sill to the pan and subfloor with a continuous bead, seal nearby subfloor joints, and connect the door frame to the wall’s air barrier on all four sides. Adjust the sweep, weatherstrip and corner pads so the door closes tight, then check under the threshold and at the bottom corners with smoke during the test (Detail 18).
With exterior insulation
In a thick wall the sill must sit on structure: a buck or solid blocking that carries the full sill, never foam alone. The pan still laps onto the WRB so water drains into the cavity, and the head flashing spans the cavity and projects past the cladding, with a drip and end dams. Door position follows the same logic as windows in Detail 08.
Site checklist
- Sill pan under the full sill, sloped to the outside
- Back dam and end dams in place, corners sealed
- Pan lapped out over the WRB below
- No fasteners through the pan floor
- Sill bedded in sealant at the inside edge; front free to drain
- Slider sill flat, level and supported at every jamb and mullion
- Low thresholds have an overhang, drain channel and slope away
- Air seal complete on all four sides; checked with smoke
- Rigid sill guard installed
- Sill water-tested and photographed before decking or paving
Test yourself
Try to answer each question, then click it to check.
Where is the door sill sealed to the pan, and why?
With a continuous bead at the inside edge. The front edge is left open, or sealed with drainage gaps as the door maker shows, so water that gets under the sill can drain out.
Name the three dams on a door sill pan.
A back dam at the inside edge and two end dams turned up the jambs.
What should you add when you lower a threshold?
More protection: a roof or canopy overhang, a drain channel, and a deck or paving that slopes away from the door.
Next: Detail 10, Exterior Insulation & Cladding Attachment. See also our Windows, Doors & Water Management Field Guide.
Planning a high-performance home in Vancouver, Burnaby or Surrey? Talk to our team about getting the envelope right from day one.