Plaster Thats Crumbling (Due To Damp?)

[HongKongphooey]

Our house was built around 1901. The whole house has had damp proofing which has worked fine,EXCEPT in the back porch. ( Floor is lower here than rest of house) In this area about 5 years ago my partner took up the original tiled floor laid a damp proof membrane and covered it with a concrete floor. About a year ago I noticed that two lots of plaster low down right next to the Upvc back door were crumbling and showing signs of damp. The area on each side is about 8 to 10inches high from ground up. What can I do to fix this.?? We want to put the house on the market and my partner wants this sorting, but typical man won't employ anyone to fix it. A builder friend suggested Alkali primer, but the plaster is crumbling so much it looks like it might need re-plastering. Even bought damp proofing chemical to inject into the wall but that hasn't worked. Any ideas would be gratefully received. Exasperated female DIY-ER!!!!!!!!!!!!!!

[boxtops]

Well - I'd get a man in. We have crumbling plaster (1860 house) and it's way beyond our capabilities to deal with it.

[tonyav]

Hope Builder see's this post, he'll most likely be able to point you in the right direction.

[dekkypoo]

The only permanently effective cure for "damp" is to take the moisture away from the wall. Take the concrete away dig well down and refill with non-porous filling then put suspended floor in. On the outside if possible dig a trench around the porch and fill with non-porous filling (pebbles), I've only done this 8 times, the first one 23 years later, it is still 100% effective!

[buildersmate]

The clue is probably in your line "the floor is lower here than rest of house". Agree with above suggestion, especially on the outside. Is the ground level the same on the outside here? Google the term French drain for more information.

[The Builder]

Perversely, it was probably putting the floor membrane in that made the situation worse.
Damp control using barriers and membranes isn't quite the easy solution that people tend to think it is. In simple terms, moisture given a barrier in one direction will always try to find another, easy, route. In this case... up yer walls ;o(

The "French Drain" is classic. A big trench full of clean stone to "soak up" surrounding moisture. Possibly a little drastic for a mere porch, but would certainly help.

Damp is squeezing up from under the floor membrane and climbing the walls. For a barrier or "tanking" system, the floor membrane should be linked to a tanking membrane on the wall, especially where it's lower than the house floor level. (Jargon alert: "Sub-terranean")

So many ways to go: Either...

Knock off all internal porch plaster ... tank with bitumen or, better still, a cement-based waterproofer such as "Vandex". Then re-plaster.

Or, after removing plaster, NO tanking... just replaster with lime plaster. (Completely breathable, so moisture just evaporates through it.

I won't go on. It's all getting too complicated. It's easy to be too purist about this. In your case, you just want to sell. I would just remove the suspect plaster and replace it with a simple old-fashioned lime mix. Then paint it with a porous NON-vinyl emulsion. Builder's Merchants stock basic "Trade" or "Contract" emulsions for new plaster (porous).