Relaid patio now above some vents, water pooling... what to do?

[TimD]

Recently had a decaying bit of back patio behind our 1890s Edinburgh tenement taken up and relaid.  Apparently there wasn't much under it before and by the time the guy (recommended by some gardeners we use) had put down a base he was happy to put it all back on, the whole thing was an inch or two higher than it had been before.

 

Unfortunately that means there's now a dip around a couple of low vents in the back wall.  One of them (on the right in the photos) is from the bathroom extractor (it runs down and under the kitchen floor behind this wall); I've no idea what the other one is.  I noticed the dip had collected pooled water earlier this morning from heavy rain last night, although it had all disappeared (I hate to think where!) by the time I got round to taking the attached photos a few hours later.  (The spirit level in the second photo is just something to make it easier to see how deep it is.)

 

Any thoughts on how to deal with this?  My first idea is to put some sort of little "hut" over it (with enough louvre to allow the vents to still vent) and make sure it's sealed enough around the sides to not allow water to flow in under it or down the walls.  Open to other suggestions though.  (And my Google-fu is not having much luck finding where I could buy such a thing ready made if they exist... just endless meter boxes and simple vent covers).

 

Thanks for any ideas

[Mr Punter]

The patio is too high.  Raising it has made it even worse and he should have known better.  The wall looks very damp.  Could you form a channel next to the wall around 120mm wide x 150mm deep to act as a French drain?

[Redbeard]

1 hour ago, Mr Punter said:

Could you form a channel next to the wall around 120mm wide x 150mm deep to act as a French drain?

...and is there somewhere for the water collected by the French Drain to go? A main drain? I'm guessing a soakaway would be too close to the building (?), though I have no idea of Scottish regs.

[Iceverge]

Get a consaw and cut the patio back 400mm from the wall. 

 

Like redbeard says, Assuming you have a lower level to drain to, I would install a french drain. 

 

As deep as you can get the water to flow away from.

 

Back fill with something like 20mm chip to a level no higher than 150mm below the bottom of the vent. 

 

Alternatively assuming that you can't drain to a lower plane I would look into something like an external periscope vent. 

 

The wall will remain damp unfortunately though in this instance. 

Edited yesterday at 11:08 by Iceverge

[JohnMo]

Patio looks to above the damp course level.  Your next issue will be damp internal walls etc. it is more normal to have the ground level 150mm below this, so your patio is way too high.

[TimD]

Thanks for all the input folks.  Wish I knew more about the construction of these old buildings!  Got some full-time patio pros who did an excellent job at a similar property down the road coming to take a look (should have used them in the first place rather than someone who seemed to be more a "landscaper"; ho hum you live and learn).

 

It's basically the same patio as was there before but the slabs were pretty much just laid on earth as a quick-fix for a grassless back plot (think some sand or gravel went down, but no membrane) about 20 years ago.  It was grouted but I think a lot of water just drained away down through cracks.

 

It's not so dissimilar from what neighbours have.... there's certainly no "French drains" up against the building.  But the last time we had damp problems it around 15 years ago when a storm ripped the gutters off the back wall roof above it (and most of the roof felt too!) and by the time that was properly fixed (it took months) the back wall had gotten quite damp and everyone (there's 4 floors above us) needed plaster stripping and dehumidifiers running.  Keen not to have a repeat of anything like that though, so want to get on top of this pronto.

 

BTW, received wisdom is that sandstone construction tenements of this age do not normally have a damp course; supposedly the sandstone's breathability enables it to deal with it: see e.g https://mysweethomes.co.uk/tenement-damp-why-the-standard-answer-is-usually-wrong/ and https://dampdetect.com/rising-damp/edinburgh .  Fortunately we've never seen anything like the "rising damp tide-marks on ground-floor flats and stair walls" one of those mentions as a common issue.

[Mike]

3 hours ago, TimD said:

BTW, received wisdom is that sandstone construction tenements of this age do not normally have a damp course; supposedly the sandstone's breathability enables it to deal with it

It's true that buildings without a DPC rely on moisture evaporating from the wall. However some height difference is needed between external ground level and internal floor level, so that enough moisture has evaporated before it reaches that floor level. If the floor is timber, then you also need ventilation beneath the floor, from one side to the other, to prevent excessive moisture build-up. If there's now a concrete floor with a DPM, that will block the evaporation towards the inside - in which case, unless the wall can evaporate the extra moisture to the outside, it's likely to cause problems on the inside above floor level, making that height difference even more important.

 

In other words, sandstone can only deal it in the right conditions. So definitely a good idea to do some remedial works as suggested above.

 

Edited 2 hours ago by Mike