IRC Tables R702.1(1) and R702.1(2), gypsum plaster
One column of one table spans a factor of ten. Gypsum plaster on expanded metal lath is five eighths of an inch minimum; gypsum plaster on a veneer base is one sixteenth. Those are two different trades wearing one word, and a quote that does not say which is a quote for nothing. The second table is stranger still: it does not tell you how much plaster to use, it caps how much sand you may stretch it with, and the cap changes between coats.
Plaster volume, and the code's own mix ceiling
ft
ft
sq ft
sq ft
cu ft
%
The code works in 100 pound units of neat plaster and never says how much set plaster that makes, because it is a product figure rather than a code one. Enter the yield off your own bag and the page will use it.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
Finished plaster thickness, every base on one scale
Worked example
A 40 ft by 9 ft wall with 40 sq ft of openings, three coat work on expanded metal lath, damp loose sand, 10% waste.
Area. 360 less 40 = 320 sq ft.
Thickness. Expanded metal lath: 5/8 in minimum, from the table.
Volume. 320 × 5/8 ÷ 12 = 16.67 cu ft.
With waste.18.33 cu ft.
The sand cap. First coat 2 cu ft, second coat 3, per 100 lb.
The catch. One mix all day is outside the table on one of those coats.
Now change the base to a gypsum veneer base and nothing else. The thickness drops from 5/8 to 1/16 and the volume from 16.67 cubic feet to 1.67, on the same wall, out of the same column of the same table. That is not a refinement in the specification, it is a different job, and it is why the base is the first thing to establish and the last thing anybody asks about.
The formula
The volume is exact. The mix is a ceiling rather than a recipe:
V = A × t / 12 aggregate ≤ cap × (neat plaster / 100 lb)
A
area, less openings
t
finished thickness in inches, from Table R702.1(1) for your base
cap
cubic feet of aggregate per 100 lb neat plaster, Table R702.1(2)
≤
the code writes a maximum. Nobody is cited for using too little sand
yield
cubic feet of set plaster per 100 lb, from the product, not the code
The thickness in the table is the total across all coats, not the thickness of each. And the join between the code's 100 pound units and a volume on a wall is the one thing neither table supplies, which is why this page asks for it instead of assuming it.
Why the sand is the regulated part
It reads backwards the first time. A code that cares about plaster performance sets a minimum thickness, which Table R702.1(1) does. Then Table R702.1(2) sets a maximum quantity of sand, which sounds like the wrong end of the problem.
It is the right end. Neat plaster is the expensive component and sand is nearly free, so every commercial pressure runs towards more sand and thinner plaster per square foot. A sanded mix goes further, costs less and is weaker, and nothing about the finished wall shows it until something hits it.
So the code writes the ceiling. It is enforced by testing set plaster rather than by watching the mixer, which is why the table carries a ten per cent tolerance on determining the aggregate afterwards.
And the cap is not one number. It changes with the work, the coat and the base, which means a plasterer running a single mix through a whole three-coat job is outside the table on one coat unless they use the 2.5 cubic feet the footnote permits for exactly that case.
Finished gypsum plaster thickness by base
Plaster base
Finished thickness
Minimum or maximum
Against the thinnest row
Note
Expanded metal lath
5/8 in
minimum
10×
3/4 in minimum where measured from the back plane of the lath
Wire lath
5/8 in
minimum
10×
Gypsum lath
1/2 in
minimum
8×
Masonry walls
1/2 in
minimum
8×
thickness need not be uniform, because masonry varies in plane
Monolithic concrete walls
5/8 in
maximum
10×
a maximum, not a minimum
Monolithic concrete ceilings
3/8 in
maximum
6×
a maximum, and the thinnest row in the table apart from veneer
Gypsum veneer base
1/16 in
minimum
1×
veneer plaster, which is a different trade from the rows above it
Read 2026-09-10. Thicknesses and the minimum or maximum reading are quoted from IRC Table R702.1(1). The ratio column is this page dividing.
The code gives finished plaster thickness by what it is going on. Read the gypsum plaster column down and the range is not a range, it is two different jobs:
IRC Table R702.1(1), gypsum plaster column
Plaster base
Finished thickness
Minimum or maximum
Against the thinnest row
Note
Expanded metal lath
5/8 in
minimum
10×
3/4 in minimum where measured from the back plane of the lath
Wire lath
5/8 in
minimum
10×
Gypsum lath
1/2 in
minimum
8×
Masonry walls
1/2 in
minimum
8×
thickness need not be uniform, because masonry varies in plane
Monolithic concrete walls
5/8 in
maximum
10×
a maximum, not a minimum
Monolithic concrete ceilings
3/8 in
maximum
6×
a maximum, and the thinnest row in the table apart from veneer
Gypsum veneer base
1/16 in
minimum
1×
veneer plaster, which is a different trade from the rows above it
5/8 of an inch against 1/16, a factor of 10, in the same column of the same table, for a material called plaster in both rows.
Those are two trades. Conventional plaster is a wet build-up in coats over lath: base coats, a finish coat, curing time and a mix that changes between them. Veneer plaster is a thin skim over a purpose-made board, applied in a day, and it is what most modern interior plaster actually is.
So a quote for plastering that does not say which one is not a quote for anything. The material quantity differs by a factor of ten and the labour by more.
Two rows also read the opposite way to the rest: monolithic concrete walls and ceilings are maxima, because the concrete is already the wall and the plaster is a finish on it. Everything above them is a minimum. The cement plaster column of the same table belongs to the stucco calculator, which sets out the same trap on the exterior side.
The code caps the sand, not the plaster
The second table is the one that surprises people. It does not tell you how much plaster to use. It sets a maximum volume of aggregate per 100 pounds of neat plaster, and it changes coat by coat:
IRC Table R702.1(2), gypsum plaster proportions, maximum cubic feet of aggregate per 100 lb of neat plaster
Work
Coat
Base or lath
Damp loose sand
Perlite or vermiculite
Note
Two-coat work
Base coat
Gypsum lath
2.5 cu ft
2 cu ft
Two-coat work
Base coat
Masonry
3 cu ft
3 cu ft
Three-coat work
First coat
Lath
2 cu ft
2 cu ft
2.5 permitted if the same mix is used for both first and second coats
Three-coat work
Second coat
Lath
3 cu ft
2 cu ft
perlite may go to 3.0 where the plaster is 1 in or more in total thickness
Three-coat work
First and second coats
Masonry
3 cu ft
3 cu ft
Sand is the cheap part. Adding more of it stretches the plaster over more wall and weakens it, so the figure the code writes down is a ceiling rather than a recipe. Nobody is ever cited for using too little sand.
And it changes between coats. On three-coat work over lath the first coat is capped at 2 cubic feet and the second at 3, so a plasterer running one mix all day is outside the table on one of them. The code anticipates exactly that and writes the compromise out:
If used for both first and second coats, the volume of aggregate shall be permitted to be 2.5 cubic feet.
Two and a half, which is neither figure and is legal for both. That is a rare thing to find in a code: an explicit allowance for how the work is actually done rather than how it is drawn.
Lightweight aggregate is capped lower than sand on everything except masonry, and there is a further allowance:
Where plaster is 1 inch or more in total thickness, the proportions for the second coat may be increased to 3 cubic feet.
Veneer plaster is not thin conventional plaster
The two ends of that column are not the same product applied differently. They are different systems.
Conventional plaster is a wet build-up over lath. A scratch coat keyed into metal or wire lath, a brown coat to build thickness and plane, a finish coat, and time between them. It is heavy, it is slow, and it is why old houses have solid-feeling walls.
Veneer plaster is a skim over a purpose-made board. The board does the work the lath and base coats used to do, and the plaster is a hard finish on top of it, one sixteenth of an inch and up. It goes on in a day.
The base is what decides which one you are doing, which is why it is the first input here. And the code notes that where gypsum veneer base is the plaster base, its attachment is by Table R702.3.5, which is the drywall fastening table, because at that point it is a board.
Neither is drywall with a texture on it, and neither is the other.
The two rows that are maxima, and why
Every row of the thickness table is a minimum except two. Monolithic concrete walls are 5/8 of an inch maximum and monolithic concrete ceilings are 3/8 maximum.
The reason is that the concrete is already the wall. On lath the plaster is the wall surface and its thickness is what makes it durable, so the code sets a floor. On concrete the structure is behind it and the plaster is a finish, so the code sets a ceiling instead, and thick plaster on concrete is mass that can fall off rather than performance.
The table adds a note that softens it in one direction:
Because masonry and concrete surfaces vary in plane, thickness of plaster need not be uniform.
Which is a practical exemption, not a loophole. A concrete wall that is out of plane by half an inch cannot be plastered to a uniform thickness and be flat, and the code chooses flat.
The same reversal happens in the cement plaster column beside it, and the stucco calculator covers that side.
The tolerance, and the one mix that is capped much harder
Two footnotes to the proportions table are worth reading on their own.
Where determining the amount of aggregate in set plaster, a tolerance of 10 percent shall be allowed.
10 per cent, on determining the aggregate in set plaster. Which tells you something about how this is enforced: not by watching the mixer, but by testing what is on the wall afterwards, and with an allowance because that test is not exact.
Wood-fibered gypsum plaster shall be mixed in the proportions of 100 pounds of gypsum to not more than 1 cubic foot of sand where applied on masonry or concrete.
One cubic foot, against two to three everywhere else in the table. Wood-fibered gypsum plaster on masonry or concrete is capped at a third of the sand of ordinary work, because the wood fibre is already doing the job the aggregate would do and the mix has no room for both.
And one more, which is the only place in the table that permits mixing aggregates at all:
Combinations of sand and lightweight aggregate shall be permitted to be used, provided the volume and weight relationship of the combined aggregate to gypsum plaster is maintained.
Permitted, provided the volume and weight relationship is maintained, which is a harder condition than it looks: perlite and sand differ by a factor of several in density, so a blend that satisfies the volume cap can fail the weight one.
Why this page asks for a yield instead of printing one
The volume of plaster on a wall is exact: area times finished thickness, and the thickness is in the table. Turning that into bags is not, and the code does not help.
The code works in 100 pound units of neat plaster and says how much sand may go with it. What it never says is how much set plaster that makes, because that depends on the product, the aggregate and how much water goes in, and it is a manufacturer figure rather than a code one.
So the calculator asks for the yield off your own bag and does the arithmetic around it, the same way the blown-in insulation page asks for the coverage chart. Where you have not got it, the page gives you the volume and the code's own ratio and stops there rather than inventing the join.
Two practical notes it will not calculate. Plaster is mixed in batches sized to what can be applied before it sets, and the set is a chemical reaction rather than drying, so a batch that goes off in the tub is gone. And the finished thickness in the table is the total across all coats, not the thickness of each one.
The source, and what is next door
Read 2026-09-10. Seattle Residential Code, Chapter 7, Tables R702.1(1) and R702.1(2), City of Seattle, adopting the 2018 International Residential Code. Prints the finished plaster thickness by base, for gypsum and cement plaster in adjacent columns, and the gypsum plaster proportions as a maximum volume of aggregate per 100 pounds of neat plaster, coat by coat.
Seattle is used because codes.iccsafe.org returns HTTP 403 and cannot be fetched, and because a city PDF states its edition and jurisdiction on the document. Your jurisdiction may amend Chapter 7.
The cement plaster half of these tables is on another page. Cement plaster thickness, the number of coats, the curing schedule and the weep screed clearance are on the stucco calculator, which owns that material. One page owns each figure here, so this one stays on the gypsum side and links rather than repeating.
And the fastening of the base is a third table again. Where gypsum veneer base is the plaster base, attachment is by Table R702.3.5, which the drywall calculator sets out.
Frequently asked questions
How thick should plaster be?
It depends entirely on the base. IRC Table R702.1(1) gives 5/8 in minimum on expanded metal or wire lath, 1/2 in minimum on gypsum lath or masonry, 5/8 in maximum on monolithic concrete walls, 3/8 in maximum on concrete ceilings, and 1/16 in minimum on a gypsum veneer base. That last figure is a tenth of the first.
How much sand goes in plaster?
The code sets a maximum rather than a recipe: cubic feet of aggregate per 100 pounds of neat plaster, from Table R702.1(2). Three-coat work over lath is capped at 2 cubic feet on the first coat and 3 on the second. Sand is the cheap component, so the pressure is always towards more of it, which is why the number is a ceiling.
Can I use one mix for the whole job?
On three-coat work over lath the caps differ between coats, so a single mix is outside the table on one of them. The code anticipates this and allows 2.5 cubic feet where the same mix is used for both the first and second coats, which is neither printed figure and is legal for both.
What is veneer plaster?
A thin hard plaster finish over a purpose-made gypsum veneer base board, 1/16 in and up. The board does the job the lath and base coats used to do. It is a different system from conventional plaster over lath, applied in a day rather than over several, and the code lists both in the same column of the same table.
Is perlite or sand better in plaster?
They are capped differently, which tells you something. Perlite and vermiculite are limited to 2 cubic feet where sand is allowed 2.5 or 3 on most rows, and the same on masonry. Lightweight aggregate is lighter and less conductive; sand is harder and cheaper. Blends are permitted provided the volume and weight relationship of the combined aggregate is maintained.
Why is the concrete row a maximum?
Because on concrete the structure is already there and the plaster is only a finish, so thickness is mass that can come off rather than durability that is earned. On lath the plaster is the wall surface itself, so the code sets a minimum. Two rows of the same table read in opposite directions and it is easy to miss.
How many bags of plaster do I need?
That needs a figure the code does not supply. It works in 100 pound units of neat plaster and caps the aggregate, but never states how much set plaster a unit makes, because that depends on the product, the aggregate and the water. Read the yield off your bag and enter it here; the volume this page calculates is exact.
Is plaster the same as drywall mud?
No. Plaster sets by a chemical reaction and joint compound dries by evaporation, which is why a batch of plaster that goes off in the tub is gone and a bucket of compound is not. That also sets the batch size: plaster is mixed to what can be applied before it sets.
Check these numbers yourself
Seattle Residential Code, Chapter 7, Tables R702.1(1) and R702.1(2)City of Seattle, adopting the 2018 International Residential Code, read 2026-09-10. Table R702.1(1) gives finished plaster thickness by base for gypsum and cement plaster in adjacent columns, with the monolithic concrete rows written as maxima where every other row is a minimum. Table R702.1(2) gives gypsum plaster proportions as a maximum volume of aggregate per 100 pounds of neat plaster, coat by coat, with footnotes permitting 2.5 cubic feet where one mix serves two coats and capping wood-fibered plaster at 1 cubic foot on masonry or concrete. Seattle is used because codes.iccsafe.org returns HTTP 403.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-10. Codes are amended locally; confirm against the edition your jurisdiction enforces.
The thicknesses and the aggregate caps are the code's, quoted from the Seattle Residential Code as read on 10 September 2026, and your jurisdiction may amend Chapter 7. The thickness in the table is the total across all coats, not per coat, and two rows are maxima rather than minima. The aggregate figures are ceilings: the code carries a 10 per cent tolerance on determining aggregate in set plaster. The yield per 100 pounds of neat plaster is a product figure that the code does not supply and this page will not invent. The waste allowance is this page's own and is an input. Cement plaster is covered on the stucco calculator.