Bilco Doors and Basement Stair Bulkheads: Sizing and Installation

A basement bulkhead is the sloped, door-covered stair connecting the outside of a house to the basement. The best-known version is the Bilco door, a steel hatch over a concrete stairwell. Sized right, it gives you a safe second exit, an easy route for moving a washer or boxes, and a stair that stays dry. Sized wrong, it gives you a door that will not close, steps that are too steep, water at the bottom, and headroom so low you duck on every trip.

This guide covers how to measure an opening, pick a door size, check stair dimensions and headroom, and avoid the most common leaks and code failures. Check your local code before you build, because basement egress rules vary by town, and an inspector's reading of the code is the one that counts.

What a Basement Bulkhead Actually Is

A bulkhead is a below-grade stairwell, usually poured concrete or block, with sloped doors over the top that keep out rain, snow, and debris. Some are built into the foundation wall; others are a separate exterior stairwell ending at a basement door. Either way, three things matter: stair width, rise and run, and clear headroom above the nosings.

Bilco is a brand name that has become the generic word for this door. Other makers sell similar doors, and the sizing principles here apply to all of them.

Standard Door Sizes

Sloped cellar doors are sold by the size of the opening they cover, not the overall frame size. Common sizes cover stairs about 30, 36, and up to 48 inches wide. Catalog sizes vary by maker, so work from the maker's rough opening dimensions, not a guess.

Two measurements drive the choice: the clear width between the side walls at the narrowest point, and the slope length from the top edge down to the sill at grade. If either is off by an inch, the door may bind, sit proud, or leave a gap. Never measure the old door: old doors warp and rust swells frames. Measure the concrete opening at three points each way, and use the smallest number.

Measuring Your Opening

Start with a clean stairwell, swept clear of leaves and debris. With a tape, level, and helper, measure the width at the top, middle, and bottom of the slope, and circle the smallest number. Measure the slope length along the slope, not straight down. If top and bottom widths differ by more than half an inch, the walls may need repair before a new door will sit flat.

Next, measure the stair: count the steps, measure total rise from basement floor to grade, and measure a typical unit rise and run. Run those numbers through a stair calculator before ordering. A stair that is already too steep will not be fixed by a new door. Finally, check the sill: it should sit a little above the ground so water runs away, not in. Raised mulch or soil can leave it in a low spot, which alone explains a wet stair.

Sizing the Stair: Rise, Run, Width, Headroom

Bulkhead stairs are often steeper and narrower than interior stairs, but that does not make any steep stair acceptable. Aim for a unit rise of 7 to 7.75 inches and a unit run of at least 10 inches, per the International Residential Code, section R311.7. A 36-inch clear width is the residential minimum under IRC R311.7.1 and a sensible bulkhead minimum; choose 42 to 48 inches if you will move furniture or appliances. Measure clear width between handrails, not the concrete shell, since rails can shrink a 36-inch stairwell to 28 inches.

Headroom is the most forgotten measurement. IRC R311.7.2 requires at least 80 inches from the nosing line to the door above. The pinch point is usually where the sloped frame crosses the stair, so measure at every step. Below 80 inches anywhere, a longer slope, taller door, or deeper stairwell is far cheaper before concrete is poured. See the guide to standard stair dimensions for the full set.

Choosing the Right Door

Match your smallest width and slope length to the maker's rough opening chart, and pick the door that matches, not one that is close. A slightly large door can take a filler frame; a small one leaves a gap no caulk will fix. Between sizes, ask about a custom door or rebuilding the curb, the raised concrete edge the door sits on, which is often cheaper.

Steel doors are the standard: strong under foot traffic and snow load, and easy to paint. Look for gas struts that hold the door open safely; a falling steel door is a real hazard. Aluminum is lighter and rust-free but dents easily. Avoid wooden hatches: they warp, rot, and rarely seal. Check for heavy rust-resistant hinges, a latch locking from both sides if this is an emergency exit, and a hold-open device that cannot slip.

Installation Steps

First, repair the stairwell: patch cracked concrete, repoint loose block, and level the curb. A door bolted to a crooked curb will never seal. Second, set the frame, level it both ways, shim only where allowed, and bolt it down with the maker's masonry anchors. Third, seal the frame-to-curb joint with exterior sealant in a smooth, gap-free bead. Sealant is a backup: the frame must sit flat and the curb must drain away.

Fourth, hang the doors, fit the struts, and test the swing: full opening without hitting the wall, staying open alone, and closing to an even seal. Fifth, grade soil away from the stairwell, and make sure any bottom drain is clear and drains to daylight or a sump, never to a sewer unless your town expressly allows it.

Code, Safety, and When to Call a Professional

An escape-route bulkhead must meet local egress rules, which can be stricter than general stair rules. Some towns require interior-stair width, headroom, and landing rules; others allow steeper non-exit basement stairs. Ask your building department before ordering if unsure, since a failed egress stair is an expensive redo. Hire a professional for a new foundation opening, a new stairwell, or structural or foundation water problems. A simple door swap on a sound curb is a realistic weekend job; a new bulkhead is not. Use a latch that works from both sides, secure yet openable from inside without a key.

Frequently Asked Questions

What size Bilco door do I need?

Measure the smallest clear width and slope length of your existing opening, then match those numbers to the maker's rough opening chart. Do not measure the old door. If your opening falls between sizes, ask about a custom door or rebuilding the curb.

Can I replace a Bilco door myself?

Yes, if the curb and stairwell are sound and you are only swapping the door and frame. You will need a helper, because steel doors are heavy, plus masonry anchors, a level, and exterior sealant. If the curb is cracked or you need a new foundation opening, hire a contractor.

How much headroom does a bulkhead stair need?

Aim for at least 80 inches of clear headroom above every step, measured from the nosing line to the door frame above, following IRC R311.7.2. Measure at each step, because the lowest point is usually where the sloped frame crosses the stair.

Why does water collect at the bottom of my bulkhead stair?

The usual causes are ground sloping toward the stairwell, a blocked or missing bottom drain, or a frame that does not seal to the curb. Check the grade first, clear the drain, and inspect the seal, in that order. For more on measuring before you build, see the guide to measuring stairs before you calculate.

Do bulkhead stairs need a handrail?

A handrail is a good idea on any stair you use regularly, and most codes require one on stairs with four or more risers. In a narrow bulkhead, one handrail is often enough, but it must be grippable, continuous, and mounted at 34 to 38 inches above the nosings under IRC R311.7.8.

A bulkhead door is a small project with an outsized effect: it decides whether the basement is easy to use, dry, and safe. Measure the opening, not the old door, size the stair for real headroom and width, pick a matching door, and install it on a level, well-drained curb. Done that way, it should serve for decades, opening easily, closing tight, and keeping rain outside.