Garage Conversion smells musty when it rains - please help me figure out the cause?!

[ipsofatto999]

We had a garage conversion done 2 years ago and very happy with it except for one issue. Whenever it rains, there is a rising, strong, musty, earthy smell like damp soil that fills the whole room. It seems to be coming from the walls but it's so hard to tell. There are no stains, visible leaks, mould, mildew or anything else like that. THere's a pitched roof. The gutters are not leaking. The floor is laminate and the floor does feel very cold in the winter but we're wondering if there's some kind of drainage issue that could be causing the smell. Is that possible and if so, is that fixable?! The plans give the following info for the floor: "Ground Floor - Solid - Insulation over slab P/A 0.8
65mm Screed, over 500g Polythene separating layer, over 75mm Celotex FR5000 over 1200g Polythene DPM over 100mm Concrete oversite, over Sand Blinded Hardcore"

Can anyone here help me figure this out? The builder who did the original conversion is coming round tomorrow afternoon and it would be really good to have some ideas or potential causes to discuss. I just don't want to be fobbed off and want to get this sorted out..

You need @PigletJohn
He’s always very knowledgeable about house stuff.

where does the downpipe from the gutters go?

can you see the dpc?

I suspect the floor is damp.

@pigletjohn Thank you so much for your help - the downpipe from the gutter goes right down in front of the garage conversion and into the ground in front of it. I can't see the DPC. If we pull up some of the laminate would that let us see if there's damp - maybe particularly around the area near the gutter downpipes? Any advice on how to raise this constructively with the builder would be very welcome - feel totally out of my depth on this.

yes, I think you need to look under the laminate.

Thank you - we'll do that. Do you know how "fixable" those kind of drainage issues are ? Hoping it's remediable without too much major upheaval... Thanks for your help

Did you take regular photos while the work was going on? It would be worth having a look back if you did (post them here if you don't know what you're looking at!!).

Thanks canyousewcushions - we took a few but not regularly and I definitely don't know what I'm looking at! Unfortunately none of them seem to show much of the floor...but I've attached two. Do these help show anything relevant at all?

I see the DPM is turned up at the edges, which is normal, it's probably lapped to the DPC in the walls.

However, if there was a defect, such as water overflowing from a downpipe or leaking at the edge of the roof, and water reached the inside surface of the wall, it would collect in the DPM tray and soak into the floor. You might not be aware of damp walls, the plasterboard may well have a metal foil face on the side away from you so it would not show inside. Have a walk round the outside looking for any possible damp marks.

it looks to me like the dry lining is standing on a chipboard floor, which is probably on insulation on DPM on concrete.

I should have said, the DPM turned up at the edges and lapped into the DPC is normal when a concrete floor is laid in a house, and the DPM is lapped into the inner leaf of a cavity wall.

I don't know that it is normal on the single leaf of a (former) garage wall.