I own a house that was built in 1890. The area of my foundation that I need to work on is an exterior section that is above ground. It is the part between my driveway and where the siding starts. I believe it is fieldstone, but am unsure. I have contacted Sakrete to as what type of mortar they would recommend and they said Amerimix Tuck pointing mortar, which is only available a few hours from my house and is Type O mortar I believe. Is Type N a bad idea to use? The reason I ask is because it seems to be more readily available in my area, but I've read posts where people warn against cracking their stones as Type N has a higher psi than Type O.

We have had problems recently with mice and I would like to dig out the old mortar and fill in with the new to decrease the amount of mice getting in and to help maintain the exterior foundation. Any guidance on if this is actually fieldstone and the type of mortar that is acceptable would be greatly appreciated. Thanks!

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  • 1
    I presume the orange is spray foam that someone put in there to try to fill the gaps & keep the critters out? Can you confirm that the original foundation was actually mortared and wasn't dry set? Those are very deep recesses, and what little bits of crumble that are visible in that pic could be just dust that's settled, not the remnants of old mortar.
    – FreeMan
    Jun 23, 2020 at 15:51
  • That's correct, the orange is spray foam to keep the mice out and to fill in the gaps. How would I go about confirming that the original foundation was mortared and not dry set? The inside is a smooth wall without gaps, it's just at the top of the foundation on both the inside and outside there are gaps where it looks like it may be crumbling.
    – Tim
    Jun 23, 2020 at 16:10
  • Shine a light into one of the larger gaps and get a clear photo of the interior of the gap. Someone will be able to identify if it has/had mortar or if was dry set. If you can see the inside of the foundation wall, but it's smooth with no sign of the stones themselves, then the inside was parged or otherwise covered to create that smooth surface. If the wall is dry set, your best bet may be to fix/finish the parge coat on the interior.
    – FreeMan
    Jun 23, 2020 at 16:36
  • The view from inside the basement looks the same except that the stones don't have any decaying mortar in between them. It almost looks like someone repointed the interior but not the exterior. I will try to add a photo of the interior of the gap. I've also edited my answer to include a photo from the inside of the basement. --- Edit: trying to attach a second photo isn't going so well as I'm new to stackexchange
    – Tim
    Jun 23, 2020 at 16:56
  • Even I can see plenty of mortar there, it won't take an expert to see this wasn't dry set! :)
    – FreeMan
    Jun 23, 2020 at 17:37

1 Answer 1


Type “O” mortar is often used because it is not very strong and it’s somewhat flexible. We generally use it for “sandstone” not fieldstone and interior use only. See below:


I’d use Type N for exterior use.

  • also, just thought of this, but is it possible that the stones in my photo are softer stones and as such would need a softer mortar? Just wondering since I don't know much about stone types and have also seen some situations where people mix sand into Type N mortar to decrease the PSI.
    – Tim
    Jun 24, 2020 at 2:25
  • If you don’t know for sure or if you don’t have experience identifying the stones, it’s better to be safe than sorry. (Please note that the article lists the amount of sand for each Type. )
    – Lee Sam
    Jun 24, 2020 at 3:34

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