10-year old facade completely rotted on one side

[Prestressed Guy]

I was called today by a client that is having the roof on one of her buildings re-roofed. When the contractor cut back the membrane that is covering the roof side of an EFIS parapet that was build in 2007 all of the roof side sheathing was completely dry-rotted. About 2" of each stud was also dry-rotted but all of the rest of the wood was as bright and yellow as the day it was built.
The clean side is covered with EFIS and there does not appear to be any ventilation of the stud cavities. My thinking is that the EFIS formed insulated vapor barrier on the street side and kept it cool. The roof side of the parapet had the roof membrane wrapped up the wall and over the top of the top plate which created a non-insulated vapor barrier on the warm side. With vapor barriers on both sides of the wall any moister that got in had nowhere to go and condensed on the warm face and rotted it while leaving the cool side unscathed.

Seem reasonable?

 

[StructDave]

Almost reasonable. Are you in a cold climate? It seems more likely moisture would have condensed there when the roof-side was cold. Same result.

If moisture were trapped in the space when the roof-side was hot, wouldn't it remain in vapour-form, at least until cooler evening hours?

 

[dik]

High humidity area? There were a lot of EIFS failures in Vancouver, Canada several years back. In most instances, faulty installation. There are a lot of standard details for EIFS that help eliminate the issue. Ron may be able to fill in some of the blanks.

Dik

 

[Prestressed Guy]

This is a parapet wall so the only thermal driver is the outside temperature and sun on the face of wall.
The construction is 2x12 wood studs with plywood sheathing on both sides. On one side (side facing the street) plywood has foam board insulation and EFIS. On the other side (side facing the roof) the plywood is covered by a PVC roof membrane that wraps up over the top of the parapet wall. The side with EFIS has insulation so it will change temperature more slowly so it will be cooler as the temperature rises and warmer as the temperature drops.

 

[Prestressed Guy]

Yes, high humidity Pacific Northwest. Also note that all of the wood framed wall is above the roof line. The wood stud framed parapet is setting on top of a CMU gable parapet that extends 12” above the roof. The wood stud wall extends the front façade to a level top so it is basically a couple of triangle shaped walls extending each way from the ridge.
Anyone have any suggestions as to how to repair and keep from happening again?
For repair I plan the following: My main concern is how to prevent this from happening again. Any thoughts or suggestions for item 9 below?
Note: all repairs will be done from the roof side of parapet. EFIS side of parapet to remain undamaged and unmodified.
1. Remove metal top cap flashing.
2. Remove damaged plywood to expose damaged wood framing.
3. Remove top plate.
4. Scrape or cut away all rotted wood from studs back to sound wood.
5. Treat remaining wood with an anti-microbial treatment such as Concrobium.
6. Sister the studs with new 2x12's to bring it back to the correct thickness.
7. Add new top plate to parapet
8. Install new plywood sheathing to roof side of parapet.
9. Install continuous or individual stud cavity vents at top of wall with wall cap flashing to cover for added rain protection to allow moisture to escape parapet.
10. Install new flashing to roof side of parapet wall above roof.

 

[Prestressed Guy]

Anyone have any thoughts about the proposed procedure?

 

[DBronson]

Haydenwse:

Take a look at the Cor-a-vent Soffit vent material. Should work well in your application. It will definitely need a counterflashing.

Link

Regards,

DB

 

[Prestressed Guy]

Thanks for the heads up on the Cor-a-vent. That might do what I need.

One other thought is put a typical siding on the roof side of the wall instead of running the membrane up over the top of the parapet. The roof membrane can be flashed to the concrete parapet and the siding would terminate on the top of the concrete parapet with a "Z" flashing.

 

[BridgeSmith]

"...put a typical siding on the roof side of the wall instead of running the membrane up over the top of the parapet. The roof membrane can be flashed to the concrete parapet..."

I'd be cautious about that if there's a chance of water backing up higher than the concrete parapet or no snow drifting higher and melting against it. So long as it's vented, running the membrane to the top (but maybe not over the top) shouldn't cause a problem.

 

[Prestressed Guy]

Here is sort of what I am looking at. It seems to me that the problem is that the parapet is sealed up too well from both sides. In this sketch, the roof side of the parapet is sheathed with T-111 which would be rain tight but still let vapor escape.

any suggestions for improvement?

 

[Prestressed Guy]

Another thought would be to put metal siding on the roof side.
This project is in the coastal region of Washington state. Given that we are not subject to heavy snow or particularly heavy rain. 1-2 inches in 24 hours is about all we get. Slow and steady wins the race. Annual rainfall is about 21 inch. It is damp for 9 months of the year.

 

[msquared48]

Venting was my first thought, so make sure that happens.

Mike McCann, PE, SE (WA)

 

[Prestressed Guy]

Any suggestions on how to vent. My thinking with the wood or steel siding is that neither are vapor tight so would inherently provide ventilation. It would be the same scenario as a typical wood framed wall. In cold climate we put a vapor barrier in the inside to minimize the amount of moisture that is drawn into the wall cavity from the warm moist interior of the house. On the outside of the wall we would never put another vapor barrier and instead use Tyvek which is a breathable water shield. It stops water droplets from getting on the plywood sheathing but lets the water VAPOR escape.

 

[dik]

You may want to modify the flashing as shown. Overhang from the face should be 1" min., 1-1/2" is even better. Research shows this is a real improvement from 1/2" or 3/4".

Dik

 

[Prestressed Guy]

Thanks Dik.

Any thoughts about using either a wood or steel siding to provide the ventilation to the stud cavity rather than running the PVC roofing over the top of the parapet and then adding vents to each cavity?

 

[dik]

Either work, if done properly, and as you've seen... neither works if not. I think you are on the right track Because of pressure differentials, it's difficult to keep something 'waterproof' when located out of doors. the reason for ventilation is to equalise the pressure. Just have to keep the water from the vents... and if water can get in there has to be a means for it to leave.

Dik