Hot Topics: Waterproofing Stucco Window Sills

A handyman painting a stucco house.

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A new homeowner wonders how much work it will take to waterproof a windowsill showing signs of damage. The forum has a few suggestions for them.

Original Post: Waterproofing stucco window sills

Newbie Member

Just bought a house that was built in the '80s. The builder decided to get fancy and used those hard foam forms to build window sills. They pretty much all failed and I am sure are causing some water intrusion. I'm going to repaint the house possibly using elastomeric paint to keep it waterproof, but what can I use to fix all the window sills? I doubt they are really capable of supporting a layer of stucco on top of them. Maybe a thinset?

chandler Forum Topic Moderator

It's possible that the foam forms can be cut out and normal PVC sill nosing installed, but we can't advise adding stucco to failed stucco.

marksr Forum Topic Moderator

Once you've got the repairs done, caulking any cracks/joints and using an elastomeric paint will waterproof it about as good as you can. Is this a block house or wood framed?

Newbie Member

The house is wood framed, full stucco. It doesn't rain here too often, but when it rains it pours—that was the case this spring.

marksr Forum Topic Moderator

Do you know what type of construction it is? Some stucco houses are stucco over block and sometimes foam added for detail. Others are wood frame with the stucco applied to wire lath.

Newbie Member

I think it's the wood frame with stucco applied to wire. Most homes here are built that way and foam is added for detail.

marksr Forum Topic Moderator

I'm a painter, not a stucco guy, but I'd think you could just re-stucco those areas. It's possible painting the area with a bonding agent might be best. We have a pro plaster guy who's a member—hopefully he'll have time to chime in with expert advice for you.

Newbie Member

I was thinking of some kind of membrane on top of the sill and then waterproofing it with something like Henry Tropi-Cool coating, but not sure if it's worth it or even possible to attach any membrane to this product. I'm wondering what the bonding agent is and if thinset is out of the question.

tightcoat Member

Where in California are you? Around here, the house has the windows flashed and then the wire lath is installed and the house is scratched and browned.

I guess I should ask at this point if the house has traditional stucco—three-coat work about 7/8" thick. If so, then the house is browned, the second stucco coat applied, and then shapes were installed. If the house is properly flashed and lathed, then it's unlikely that you have a serious moisture problem. The water that does get behind anything will just drain down the stucco, or if it gets behind the mortar of the stucco it will drain down the moisture-resistant paper and out the weep screed.

If your house is thin-coat stucco over foam, it's still flashed. The moisture-resistant paper is installed behind the foam and the shapes are added to the foam and either stuccoed or else added after the brown coat. So, to fix the shape's exterior thinset mortar is a pretty good choice. If there is a stucco supplier in your area, they will likely have the material that was used originally.

No matter what material is used, the bond of mortar to the foam is not great. If the foam has been exposed to the elements for a few weeks, you should abrade the deteriorated foam down to good white, sound foam. Using a bonding agent before your mortar will help, but the proper mortar for the job has adhesive included in the mix. Talking to the local stucco supplier is a good next step.

Newbie Member

I did check. There is paper behind the scratch coat—at least, I hope it's a scratch coat. The whole house is built weird. The drywall is applied over some plaster material, the bathroom has "open" can lights, travertine is used as kitchen get the picture. I'll probably talk to the stucco supplier tomorrow to see if they have any ideas for me.

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