I want to repaint my ceiling, but I can’t quite tell what’s going on. Is there a layer of drywall? Plaster?
What should I do if I just want to repaint?
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From the big chunk that is missing I would say plaster, but not the first time as it looks like there is another level under that one. In the 70’s my dad remodeled many Victorians in the SF north Bay Area of CA. Even featured in sunset magazine not sure of the year, we would strip the old woodlath and plaster to the studs and Sheetrock then heavy mud to look like old plaster. The tough ones were from the 30-40’s with expanded metal and mortar but these crack but never drop a section as your photo shows. Filling cracks will give some time but it will crack again unless not reenforced with mesh paper tape won’t last as well in my experience. Depending on the area I have found 3 types of studs from your era, Redwood , no rot and easy to rip and strip old plaster, Fir mid level nails pull out most of the time but break off close to the coast, not two bad, Oak framing , this is just about impossible to pull a old nail or drive a new one or screw without a pilot hole being drilled first. They each have pros and cons and depending on the zone the home was built can be great advantages at this age, if 2 layers there ways to fill the hole but cracks will come back from what I have seen unless structural and multi layer problems are repaired.
You have two main issues: 1) crack length of room, and 2) top coat and brown coat separating from scratch coat.
1) When we see cracks running in the middle of a room, it’s usually from undersized ceiling framing. Lumber bends most at the center of a span. If the lumber is undersized, it will deflect excessively and thus cause a crack in brittle materials like: plaster, ceramic tile, etc. You’ll probably need to sister an additional joist next to the existing joists.)
Before you fix the crack and paint, I’d suggest you verify the ceiling framing is adequate. (There are span tables on the internet that can tell you what size lumber should be used to carry the existing plaster plus any additional plaster (or gypsum board) repairs...or you can ask on this forum giving span, spacing of existing joists, any loads above ceiling, etc.)
2) Plaster is generally a 3 step process. Each layer requires the new layer to bond to the previous layer. Looking at the missing piece, it appears the plaster separating is about 3/8” to 1/2” thick. That means the brown coat (about 3/8” thick) and finish coat (about 1/8” thick) is separating from the scratch coat.
There is no easy way to repair this problem without removing the brown coat and finish coat. (You can’t just nail it back up because it’ll continue cracking and separating and it will be very uneven.)
Often people choose to remove the worst areas and then install gypsum board over the entire ceiling. However, that adds a great deal of load, but you may need to reinforce the existing ceiling joists anyway.