Improving a badly built extension

[Mike Clarke]

I'm thinking about doing some major improvements to an existing flat roofed extension to our bungalow. I'd welcome any comments on the best approach.

The extension along the back of the house is about 30 feet long and varies between 6 to 9 feet wide. Most of the exterior wall is rendered single leaf brick or blockwork. A lane runs along the back of the extension about 2 feet above the interior floor level. The lane only serves 4 other houses so doesn't get much traffic.

The extension was added many years before we bought he house and was built to rather poor standards. Needless to say the thermal insulation is _very_ poor, especially with the extension being on the north side of the house. I also have a suspicion that we're getting some dampness coming in through the bottom of the single leaf wall.

The plan looks something like this (needs a fixed width font):

|------------------------------------------| | Extension | | | | ################################ | # # |################# # # # # # # # # House # # # # # # # # # # # # # ################################

| and - are single leaf walls and # are cavity

And a section through it:

\ \ \ \ \ -------------------- \ # # Flat roof # ------------------- # | C # | a # |

[Hugo Nebula]

Why are my ears burning..?

In terms of what you have to do to comply with the B/Regs; replacing a flat roof with a pitched one is a structural alteration, so that comes under Building Control straight away, and the work you do should comply with the applicable requirements (i.e., structure, ventilation, insulation, etc.).

Upgrading the insulation to the wall comes under Building Control as well. You you need to achieve a U-value of 0.3W/m^2K or less, which, according to the Celotex website, needs 55mm PIR insulation.

As the work is not a change of use or an extension (and if it's been built more than two years, then no Building Regulations enforcement can be taken), then you are not required to improve the damp-proofing nor the retaining wall, but it would be short-sighted not to. If you did rebuild the wall, that would be a structural alteration requiring an application.

I would be concerned if a retaining wall is only 100mm thick. It should be 1/3rd of the retained height, so 200mm. Does the wall above ground have piers at 3m centres? If not, then it's not just the retaining wall that could be a problem.

The insulation should be installed between the studs leaving a small cavity. Some kind of cavity tray should be installed above the adjoining ground level falling outwards. If it's a masonry wall, it should be off a foundation, not just off a floor slab, which would have to be done in sections like underpinning.

[js.b1]

Does it not come under "retained" and thus a lesser figure?

Wonder if they have turned the blocks on their side to make it thicker below ground... well worth taking some plasterboard off and seeing what is going on in there.

If it specs like a shed... it probably is a shed... except it is stuck on a house :-)

[Hugo Nebula]

That is the required U-value for a 'retained' solid wall from October this year; it used to be 0.35. An existing cavity wall can have a U-value of 0.55W/m^2K, so that filling the cavity will comply, and there is an allowable reduction in the thickness of the wall insulation if it would reduce the floor area by more than 5%.

Those were my thoughts too. I suspect the building may have originally been a garage or outbuilding that has been converted to habitable accomodation.

[Mike Clarke]

[snip]

... and changing the roof line will require planning permission too. We're in a National Park so I expect this could end up being more tedious than elsewhere :-(

OTOH if we stick with a flat roof (which would need re-felting and possibly other repairs anyway) we'd probably be improving the insulation at the same time so I imagine it would still come under Building Control.

Well it's been used as a habitable room for at least 15 years and probably over 40 years (the electrical fittings look like 60's to early 70's) so there's no problem there.

My sentiments too. With the other changes we're planning to make we might as well go a bit further and avoid having to rip it apart again later if problems eventually develop.

No piers visible but that got me to thinking that the generous space behind the plasterboard could be hiding some on the inside of the wall. Some further probing through the plasterboard failed to reveal any but was a bit more revealing in other ways though. What we now appear to have is:

A B C ?###################-----------------------| ? Extension | ? | ? ################################ ? # # ?################# # # # # # # = double leaf # # - & | = single leaf # House # ? = unknown # # # # # # # # # # # # ################################

I know that this was built in two distinct stages with a cloakroom/toilet and entrance porch in the section B-C being much older than the rest of the extension and I'm guessing that A-B might have been an outhouse or coal store of the same era, So perhaps the "newer" part of the extension was built a bit better than the earlier part. I know for certain that B-C is definitely single leaf because there are windows in this section so we can see how thick the wall is but the left hand end of the wall is solid - the only glazing at that end is the patio doors on the south side of the extension.

[js.b1]

That 5% figure catches many "lean to" kitchens with 3 outside solid walls (damp walls with wet verges).

Aside: radiators do not work well, but floor level IPx4 electric fan heater or electric hydronic plinth heaters work brilliantly.

Hence the door requirement on conservatories, a deliberate delineation some do not understand :-)

Will be interesting to see what they have done re retaining the path...

[js.b1]

If the roof is single storey and accessible, you could use EPDM rubber sheet.

It is a reliable roofing material, very tough, simply applied with a contact adhesive, use OSB3 or OSB4 underneath or whatever they recommend.