Raised deck and dampness

[Elieza]

I’ve built a raised deck which is about 1.5 feet high next to the house.

It’s 2 courses of bricks above the air bricks (small air bricks about half an inch wide by the height of the course of bricks) and is level with the back door (did away with step up to back door). It’s supported on blocks of wood about 2 feet apart and is 12ft square. There is plenty air circulation room but next to the house the blocks are only 2 inches away from the house wall. I don’t know if that’s too close.

There’s now a smell in my house. Not sure if the deck could be trapping moisture and creating damp. It’s been there at least a year but the smells only appeared now. It happened once before and vanished after three weeks for no apparent reason.

The lower internal walls do not feel damp.

I’m also concerned the smell could be something dead under the house (field mice or something small, perhaps lots of them as neighbours have put poison down).

There is a small hole in the wall under the door which seems to be missing a bit of brick so something could have crawled in and succumbed to poison.

Anyone know how far decks should be away from a house wall if built above the air bricks, and what dead things under the house smell like!? Thanks!

Found answers about decking over air bricks on other forums

www.mybuilder.com/questions/v/45845/decking-and-air-brick

ultimatehandyman.co.uk/forum1/viewtopic.php?t=30529

www.ultimatehandyman.co.uk/how-to/decking/decking-basics

This stands out
Ensure that any air vents for the property are not blocked by the ledger as this will cause the timber on the inside of the house to rot

Along with
Elevated decks that are more than 600mm from the floor should be designed by a structural engineer or builder. Elevated decks may require planning permission

Thanks. I had a look and there seems to be lots of people who have followed the rules of keeping below air bricks, and plenty more who have not - without any issues arising so far.

I’m still confused. And I’ve no idea what a ‘ledger’ is!

I just saw the couple of posters who had gone over air bricks and seemed to have problems.

There’s photos on the last link which were quite good and shows the gaps needed in the ledger where the air bricks is.

“Deck ledger
Ledger board decking is used when the decked area is up against an existing building to which the ledger board is then bolted to keep the decking securely in place. Typically this would be where a door opens up into the garden space“

decking often has rats under it. They may be dead or alive, and may drag things in to eat. my BIL had some drains completely blocked the bones that rats had dragged in.

A ledger board is a sort of joist fixed to the side of your house, that you might use as a support for a lean-to roof, or decking.

Obstructing your airbricks can be expected to make the subfloor damp.

There needs to be a good airflow where it blows in one side and out the other. A builder could put additional ones in, maybe on the side of the house. Old houses often have too few.

It is possible to fit a "periscope vent" where the outside is higher than the inside, but this is extra work and cost.

So there are a couple of possibilities here. Does the deck attach to the house or visit completely freestanding? Have you had a lot of rain recently?
If any part of the deck contacts the building both above and below the Damp Proof Course then damp can wick up and make the wall damp. If the deck is near or above the DPC rain can bounce up and hit the wall. If there is no fall on the deck then rain could be running back at the house wall. Alternatively it may be nothing to do with the deck!

The deck is freestanding. The bit which is nearest to the doorway is butted up 5mm away from the door so a stiletto heel can’t fall through the gap! There is a bigger gap of a couple of inches all round the rest of the deck where it is next to the house wall. I can see under. No rats nests or dead vermin.

There is an small overhang above the door too which protects it a bit from rain or drips. It’s usually always dry there on that bit of deck.

The house faces the direction of least wind/rain and it is generally dry on the walls at that side.

The bits of deck next to the house are all dry just now for about an inch so I’m not suspecting it’s rain getting through or down anywhere. It’s heavy rain here today.

I just don’t know how to tell if air circulation is sufficient or not. What causes damp, presumably static damp air?

Sadly it could well be a dead rat or mouse under the floor. Apparently they take two weeks to decompose. During which time they smell like sewage. I had the plumber out already but found no sewage related issues and my pipes are flowing freely.

Just debating whether to ditch the deck or if it’s totally fine and it’s a rat problem! Thanks.

in the underfloor void, damp from the ground beneath evaporates off the soil or oversite concrete.

More modern houses may have a DPM under the concrete, but are less likely to have a ventilated void. You need quite a breeze to take away all the water vapour and keep the void dry. Blocked airbricks are a very common cause of damp and rot.

Thanks. It’s a solid concrete foundation (new build 20 years ago in an area of subsidence apparently) with one course of bricks on top of the foundation, then a dpc, then another course of bricks above the dpc with air bricks spaced out here and there.

relatively modern, so the airbricks as built should have been enough to ventilate it. But if some have been blocked, you need to improve it again.

As you say, a mouse might have got in. If the hole is in brickwork, you can mix up a tiny bit of sand and cement mortar in a butter tub or similar, and push it in to fill the hole deeply. If you see someone bricklaying, they might give you a handful. It stays workable for at least an hour.

Thanks. What constitutes them being blocked? Wooden pillars holding up the deck 2inches away from the air vents? Or is that enough space? How will I know if dampness is building up? Thanks

Good idea re the holes. Thanks.

Won’t block them up quite yet in case I need to get a snake camera thing in to see what’s under the house as it’s an easy entry point.

Although I have no idea what company would do such a thing! I’m just planning ahead.

I’m not convinced the vents you describe are air vents as such, they sound more like weep vents for the cavity wall?

I would temporarily block the holes as soon as possible, mice and rats are on the hunt for cozy winter nesting areas at this time of year and will be a nightmare to get rid of.

I googled ‘weep vents’ and these do look like that! Thin rectangular things.

The house is a timber frame inside with bricks on the outside if that makes any difference to anything.

There are no other vents anywhere else in the whole property.

I’m guessing you have solid floors hence no air bricks? Given the fact that your neighbour has been using poison there’s a strong possibility that a rodent has already entered through one of the holes and has died in the cavity and is causing the smell, if that’s the case there’s no much you can do other than wait for it to disappear unless you want to start dismantling your walls. In the meantime you really need to block the holes to prevent it being an ongoing issue. You mentioned previously a similar smell had lasted 3 weeks which I believe would be about the time it takes for a dead mouse to decompose enough for the smell to disappear.

From your description the deck is in close proximity to the wall but not touching it but the deck is above the DPC. It could be rain bouncing off the deck and soaking the wall - have you had lots of rain?

The house wall doesn’t get much rain on that side of the house and the overhang of the roof protects it a bit, however there has been wind and unusually heavy rain recently in this direction which could account for it but if that’s the cause it’s not happened before and the decks been up at least a year and there have been storms before during this time which havent caused this issue.

It’s a mystery!