DPC - new porch build

[cavair]

My son in law and I have started to build blockwork for a replacement and extended porch..
there is DPC under old concrete base, but we laid extended base without yet putting DPC. There is a 2" leeway for putting DPC sheet in with scree on top up to level of door step. We are however putting large tiles with waterproof adhesive down - will this be sufficient or can i put tiles down on top of membrane.? as there wont be enough height for both scree and tiles.
the new concrete base was laid on top of concrete path already in existance, not bare ground.

also we forgot to put DPC in blockwork till after 3rd course! How much risk is there that damp could rise to damage internal plaster and ext render in the future.? Is there anything i can treat heavy duty, dense concrete blocks weve used (not breeze blocks)

there will be 4' french doors at either end and sloping roof with cedar shingles.

cheers for any advice!

 

[Richard C]

Unless it’s one of the very expensive 2 part epoxy products, waterproof tile adhesive & grout is only waterproof in the sense it won’t degrade when wet, it’s not impervious to water; basically its a cementitious powder product. You could use a liquid membrane such as Aquaseal or probably a better option is to tank it using this & tile over that;
http://www.bal-adhesives.co.uk/products/wp1-coating

Not good that you did not put a DPC in bottom course of the block work; you may be able to use a one of the silicone injection DPC’s but I’ve no experience with this & not sure if it’s a DIY option. Also, if you’ve only used a single skin construction, damp is likely to permeate through from outside; if you planned just to plaster the internal walls, plaster doesn’t like damp & I’m afraid there is every chance it will fall off before too long. One way of doing it would be batten the wall & fix Duplex (with a foil backed membrane) plasterboard & plaster that. You may be able to waterproof the external render, here’s a link to a recent thread which may help but I don’t know how relevant it is as haven’t read al of it.
//www.diynot.com/forums/viewtopic.php?t=182171

With 4ft French doors either end it sounds like a pretty large & strange sort of porch; if it’s more than 3 sq/m external floor area it requires Planning Permission & unless you keep the original exterior door in place it will also need to comply with Building Regulations;
http://www.planningportal.gov.uk/england/genpub/en/1115315235275.html

 

[noseall]

If it wasn't for the fact the building is a porch then i would say knock it down and start again.

You may have issues with condensation and damp, but it is just a porch.

We all live and learn.

 

[cavair]

we checked with planning by sending in a drawing etc and they said we didnt need permission, just to keep outside wall in keeping with existing house i.e rendered. even tho it is over 3sq m! just under 6!

it may sound strange with 2 sets of doors but its because weve widened the size of the original porch to include the side path access, hence 2 sets to facilitate access to rear when needed! the main door to house is in original place.
we used one skin of dense blocks instead of breeze to ensure it would support the roof ok, even tho we plan on using cedar shingles instead of normal concrete tiles. Its 1.7m wide with the doors 1.2 so not really enough, or need, to have double skin.

FYI, after reading your links about silicone etc Ive found a DIY DPC injection cream to inject into holes drilled in the mortar layer:
http://www.twistfix.co.uk/2005_shop/product.asp?productID=242&categoryID=169
Hopefully it will save knocking down and starting again!