A flat roof framing plan is, structurally, very close to a floor framing plan: joists span between bearing walls or beams, openings are framed with headers, and the plan is drawn looking straight down. The key differences are the loads a roof carries and the fall built in for drainage. That similarity is good news — it means you can lay out a flat roof in a floor framing tool. Here is how.

For the wider roof picture (pitched roofs, rafters, trusses), see the roof framing plan guide.

Joists spanning across the short dimension onto two red bearing wallsspanJ1 @ 450bearingbearingA flat roof framed with joists spanning onto bearing walls, like a floor

Why a flat roof is framed like a floor

A flat — more accurately low-slope — roof uses joists spanning between supports, exactly like a floor. The same decisions apply: joist direction, on-centre spacing, bearing walls, beams over openings. That is why a preliminary floor framing tool is a natural fit for laying one out, where a pitched roof would need rafter or truss design.

What's different from a floor

Two things set a flat roof apart:

  • Loads. A roof carries different loads from a floor (for example weather and maintenance access rather than occupancy). This affects member sizing — strictly engineering territory.
  • Falls for drainage. A flat roof is never truly flat: it has a slight fall so water runs to outlets and doesn't pond. The framing plan is drawn as a flat layout, with the fall direction and drainage points annotated. The fall may be built into the structure or formed above it.

Laying out a flat roof framing plan

  1. Footprint — set the roof outline.
  2. Joist direction — run joists across the shorter span, bearing on the longer walls or beams. (See floor joist spacing.)
  3. On-centre spacing — pick a sensible spacing and check the preliminary span feedback.
  4. Bearing — confirm the walls and beams the joists land on. (See load-bearing walls.)
  5. Openings — frame roof lights, hatches, and any penetrations with headers.
  6. Annotate — note the fall direction, drainage outlets, member callouts, and dimensions.
  7. Export — PDF for review, or DXF for CAD.

Because this mirrors the floor workflow, the step-by-step in how to draft a preliminary floor framing plan applies directly — just read "floor" as "flat roof".

Using Framing Plan for a flat roof

Since a flat roof frames like a floor, you can lay it out directly in Framing Plan: set the footprint, joist direction and spacing, bearing walls, and openings, then annotate the fall and export. Add the drainage and fall notes as annotations so the intent is clear on the drawing.

Keep it preliminary

Roof loads, member sizes, falls, and drainage are engineering matters. A flat roof framing plan you draft is a preliminary layout aid for coordination — have a qualified engineer confirm the structure, loads, and drainage design for your project and local code.

Try it

Lay out a flat roof in your browser the same way you'd frame a floor. Open the framing studio to start, or read the full roof framing plan guide.