Backup of: https://grantpark.org/historic-home-care
Repointing Bricks
Check out this Preservation Brief from the National Park Service.
View examples of damage: The Mason You Hire Can Destroy Your House
Most homes built prior to 1930 need softer N, O or K mortar mixes. (Capital letters go from hard to soft in MaSoN wOrK.) The extra lime & sand in these softer mortars allow for a greater amount of expansion & contraction in the damper crawl space (think of it as soft, healthy gums for your teeth)....do not use straight Portland cement (sidewalk cement, quick dry) as mortar - placing a mortar that is too hard between the softer bricks will cause them to crack and crumble as the seasons change.
Your existing mortar will likely differ from a premixed formula. The main priority is to make sure it's softer than the standard Portland Cement mix is - with a greater lime content to self-heal small cracks. Here's how to visually inspect your mortar.
Type O is NOT sold in Atlanta at Home Depot or Lowe's.
Even Type O Tuck Pointing Mortar (AMX 420) is NOT carried by the local NewCastle vendor georgiamasonrysupply.com in Lawrenceville.
Here's how to mix your own Type O from Type N Mortar (Not for Type N Masonry Cement)
Type N mortar is already pre-mixed as 1-1-6 (1 portland cement to 1 lime to 6 sand)
Type O would be 1-2-8.
So for a 60 lb bag of Type N Mortar, each of the 8 parts is 7.5lbs (with 1 already lime and 6 already sand).
So to create Type O, add 1 more part hydrated lime (7.5 lbs) and 2 more parts sand (15 lbs).
60 lbs (8 parts) - Type N Mortar (not cement)
7.5 lbs (1 part) - Lime
15 lbs (2 parts) - Sand
Pigment
We're trying a mix 3 parts brown to 1 part charcoal
The original mortar in your chimney probably has a slight yellow tinge or a buff color typical of older mortars. You can optionally add a little buff-shaded masonry cement color to tone it out, but not too much buff since it may look orange.
Alternative to buff: The BROWN UMBER Collection - Pack of 3 Natural Mineral Pigment Pack (150 ml|5OZ EA): Natural Umber FL | Burnt Umber France | Natural Umber France
If you are starting with Type N Masonry Cement (Not Type N Mortar)
With a ratio of portland cement to lime to sand:
The bag states that the Type N mix is one 70 lb bag plus 2.5 to 3 parts sand.
So with 50 lbs bags of sand:
2.5 parts = 175 lbs sand = 3.5 bags
3 parts = 210 lbs sand = 4.2 bags
So if we increase from 4.2 bags sand to 6 or 7 bags of sand.
We can add an amount of hydrated lime equal to half of the additional sand.
1.5 x 50 lbs bags of hydrated lime (since we are adding 3 extra bags of sand, for a total of 7 bags of sand.)
1 - 70 lb bag Type N Masonry Cement
1.5 - 50 lb bags lime to soften
7 - 50 lb bags sand (Type N bag says to add 2.5 to 3 parts sand which would be 4 50 lb bags of sand, so we use 7 to soften)
When the bricks seem newer, and you're worried about erosion of the mortar, we might do:
1 - 70 lb bag Type N Masonry Cement
1 - 50 lb bags lime to soften
6 - 50 lb bags sand
In the old days, the ratio was 1:3 quicklime/sand
Hydraulic Lime as alternative to Hydrated Lime - Still investigating, reddit
Felt this was not necessary since Type N mortar contains lime, then added Hydrated lime from Amazon.
https://www.limeworks.us/product/st-astier-naturally-hydraulic-lime/#1632154263725-f24f3280-bb9e
https://www.traditionalbuilding.com/opinions/hydrated-vs-hydraulic-lime
How to Apply
Clear to 1 inch using chisel or angle grinder.
Wet (drench) the existing bricks so they do not pull moisture out of the drying mortar.
If you want an exact match for the consistancy or color or your mortar, check out the Mortar Analysis and Matching Service at USHeritage.com
This article provides a formula for making Type O mortar from portland cement (not from portland mortar)
1 part Portland Cement Type-N NOT MORTAR (Careful, avoid buying "Mortar" since it already has sand mixed in.)
2 parts Mason's Hydrated Lime Type-S (Amazon) - Mutual Industries Western Hydrated Lime for Masonry Work. NOT sols at Home Depot or Lowe's.
8 or 9 parts washed building sand (9 for larger grains)
Type S or "Special" hydrated lime products can be used after a short soak period with excellent workability.
Don't let a mason water seal or treat the bricks (bricks DON'T need it). Avoid applying anything that cannot be reversed.
The original historic mixes ranged from 1:2 to 1:3 lime to sand ratio and tended to be closer to 1:2.
Since we are usign 1 part portland cemend as 1 of 3 parts of the lime side of the equation, a 3:8 ratio puts us between 1:2 and 1:3.
1:3 suits a general building sands, Whereas 1:1½ suits a fine local sand. Source
Avoid Over Vapor Barrier-izing
Be weary of over vapor barrier-izing your ground. This can focus "rising damp" into your piers where they touch the ground and create a faster capillary action of ground water concentrated right up into your wood structural members.