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I need to build a small door between my garage and my attic. The rest of the wall is plaster on button board and is about the equivalent of 5/8" sheetrock. If I doubled up two sheets of 3/4" plywood, would it offer the same or better fire protection?

  • This reads like more of a DIY.SE question since just wood, engineered or not, should not be considered fire protection. That and you are just really asking about making a fire protected door.. – Matt Dec 14 '15 at 17:43
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    This was an interesting read on the subject: homeinspectionservices.org/inspection-blog/… – Matt Dec 14 '15 at 18:08
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    Note that this question might get better answers (or at least differenr answers, with code citations) on the Home Improvement area (misnamed diy.stackexchange.com) – keshlam Dec 14 '15 at 18:17
  • Well you could add a layer of PEEK (polyether-etherketon) on top of the plywood... low thermal transmission, ignition temp of around 600°C which is about twice the ignition temp of wood, and self-extinguishing. That'll greatly improve the overall resistance to fire. – Damon Dec 15 '15 at 10:26
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To lead from a comment you made on another answer

In a hospital I used to work in, the fire doors were made out of wood and were rated at 1.5 or 2 hours of protection as a function of their thickness.

I think I get it now. The doors you speak of do not stop fire dead. It's not about fire protection but more about giving time for people to be able to leave the premises. Fire beats wood. That is a fact. It does however take time. A basic definition from a Quora post:

Fire Door specifications

Fire doors are not just the door itself, it includes the frame, ironmongery, glazing and smoke seal.

Point to take from that is just having wood alone does not constitute fire protection. There is a reason that these types of things have complete codes to define their production, testing, use and safety thresholds.

2 sheets of plywood alone I do not think are good fire protection by any means. The thickness of the wood is not the sole reason it suffices in other fire doors.

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  • When I checked at a local lumber yard, they had fire resistance treated plywood. I would only need about half the sheet to make up the door. I was hoping conventional plywood would suffice and I could use the rest of the sheet for something else. Regardless of what I do for the door, there are air gaps in the skip sheathing on the rafters so any fire would easily make its way through them. The house was built in 1949. – Curt Dec 14 '15 at 18:57
  • _ Regardless of what I do for the door, there are air gaps in the skip sheathing on the rafters so any fire would easily make its way through them_. That was the point I hope you would make. – Matt Dec 14 '15 at 19:22
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It would be a better idea to screw a sheet of steel to it on both sides.

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    Polished (at least not painted) steel reflects heat. The same way that the steel that is on the fire side reflects heat, the one on the other helps the heat stay inside the door. Which leads us to heat conduction; the same steel that helps lead heat away from the door also helps it into the wood in the door. The situation is garage/attic so we can guess the fire is in the garage. A reflective piece of steel on the garage side and none or conductive on the attic side. It is beginning to get complicated... That is why we have drywall. And treated plywood. FWIW – LosManos Dec 15 '15 at 10:02
  • An important benefit of sheathing is that it keeps air away from the wood, so no flame can develop. If you think steel conducting heat to the wood is significant, consider the effect of a flame on the surface of wood. – WhatRoughBeast Dec 28 '15 at 4:40
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If I doubled up two sheets of 3/4" plywood, would it offer the same or better fire protection?

Unless it's plywood treated with flame-retardant, no. Regular plywood has basically zero fire rating. Type X 5/8" gypsum drywall has a 1-hour fire rating.

Why would you want to build a fire barrier out of something flammable?

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    In a hospital I used to work in, the fire doors were made out of wood and were rated at 1.5 or 2 hours of protection as a function of their thickness. I used a couple of the discarded doors to make a workbench top. I far as I could tell they were just untreated wood. – Curt Dec 14 '15 at 17:34
  • @curt, are they solid wood? I thought fire doors usually had some sort of fire-retardant fill. – grfrazee Dec 14 '15 at 17:36
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    they are solid core with a veneer face. The core looks just like normal wood. It's possible it was treated if the treatment doesn't give off and odor or discolors the wood. The two hour door is 1 13/16's inch thick. – Curt Dec 14 '15 at 17:53
  • There is a basic misunderstanding regarding fire protection in this answer and comments. Matt hit it on the head in his answer. Fire protection is not intended to stop a fire forever, just long enough for evacuation. Drywall, plaster and even steel will not stop a good fire, but all will slow it down, including wood. Some fire rated doors will be solid wood, others may have other fillers. – Ashlar Feb 17 '16 at 2:52
  • All construction assemblies requiring a fire rating by code are tested in laboratories and rated according to test results. A 2-hour door will stop a fire for at least that long. Incidentally. the drywall on wood joists would be rated about 20-30 minutes provided the drywall is fire rated. – Ashlar Feb 17 '16 at 2:52

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