Before you tile your floor you need to answer this: Is your floor stiff enough for tile?
Flex is what breaks grout, cracks tile, and turns your beautiful new floor into an expensive lesson.
This calculator estimates joist deflection (how “bouncy” your floor is) and returns an L/### rating.
It’s based on the same concept as the old John Bridge “Deflecto”*** calculator many DIYers used for years.
Enter your joist size, spacing, span, and wood type/condition. You’ll get an estimated deflection rating plus a simple pass/fail for tile and stone.
Important: This checks joist deflection only. It does not guarantee your subfloor is adequate. Be sure to read the entire page.
How to interpret the results
- L/360 or higher → typically acceptable for ceramic/porcelain (joist deflection).
- L/720 or higher → typically required for natural stone (joist deflection).
If you’re below L/360, tile can still be done — but it usually means you need structural stiffening (shorter span, sistering, beam, etc.) before you touch underlayment.
Warnings and limitations
Engineered I-joists and trusses
This calculator is intended for solid sawn lumber joists (2×8, 2×10, etc.). If you have engineered I-joists or floor trusses, don’t guess. Use the manufacturer span tables, stamp info, or an engineer. (Those systems vary wildly.)
Joist deflection is only part of it.
You can pass L/360 and still have a tile floor that fails if:
- subfloor is too thin
- seams are unsupported
- panels are damaged/waterlogged
- wrong underlayment is used
Joists are only half the story.
Old houses / unknown framing
Notched joists, bored joists, rot, cracks, over-spans, and previous remodel “creativity” can invalidate any calculator result. If your framing looks questionable, fix the framing first.
Subfloor rules
Minimum subfloor basics (general guidance)
Your exact build depends on joist spacing and your tile/underlayment, but the big principle is:
Tile needs a stiff, stable subfloor. Underlayment doesn’t “fix” a floppy floor.
Double-layer plywood (the part most people screw up)
When you install a second layer of plywood, the second layer should be fastened ONLY to the first layer, not into the joists.
You want to decouple the top layer from joist movement. Fastening the second layer into the joists transfers movement and defeats a big reason you added that second layer in the first place.
General rules:
- Stagger seams from the layer below
- Keep edges properly supported
- Use the right fastener length so it doesn’t reach joists
- Don’t glue unless you’re doing a full-spread glue method.
Quick FAQ
Does this tell me if I can tile directly over my subfloor?
No. It only estimates joist stiffness. Subfloor thickness/condition and the correct underlayment still matter.
Why does stone need L/720?
Stone is less forgiving and fails more readily from flex. The standard is stricter for a reason.
My floor “feels fine.” Can I ignore this?
No. Floors can feel fine and still flex enough to crack tile over time. Tile is not forgiving.
What if I’m close (like L/340)?
That’s a warning sign. “Almost” can still fail. Structural fixes are usually easier before the tile is installed.
***Unfortunately the John Bridge forum went offline last year. This tool was one of the key parts of the site. This calculator does the same thing. In 2012 I developed the Deflecto app based on the online calculator, the math was directly from the Deflecto.
This calculator utilizes the same formula. The math is identical.
