foundation Dropped floor level for headroom in garden gym – good practice?

[seanious]

 

Afternoon all,

I’m taking on a project I’ve wanted to do for a very long time, and I’ve finally got the time to tackle it. I’m planning to build a summerhouse / workshop / gym (approximately 4.2 m x 7 m) at the bottom of my garden.

The plan is to stay within permitted development, which (from my understanding) limits the overall height to 2.5 m at the highest point. The building will be single storey, constructed on trench foundations, with brick and block walls and a single-pitch (hybrid) roof.

I’ve recently realised that once I factor in a treadmill and the deck height being around 300 mm above floor level, I don’t quite have enough internal height to run without banging my head on the ceiling. Ideally, I’d like to position the treadmill at the lowest point of the roof, as I have other plans for the higher end of the space. However, even placing it at the highest point still appears to leave me short on headroom.

I’m not entirely confident my calculations are correct — most of my information has come from online resources.

My main question is:
Is it acceptable / normal practice to lower the finished floor level, as shown in the diagrams above?  Doing so would likely mean incorporating one or two internal steps, which I’d prefer to avoid if possible, as I don’t think it would look particularly good.

I’ve put together two very basic diagrams in Excel (I’m not sure what other software would be better — suggestions welcome). The first diagram shows my initial attempt, where the floor level is slightly higher than the external ground level. The second shows the same building with the floor dropped. I could potentially go lower if required, but before doing that I’d really appreciate some professional input.

I’m particularly concerned about damp, based on some reading I’ve done online. I’ve yet to finalise the roof design, but again from what I’ve read, a hybrid roof seems to offer a bit more internal headroom.

For context, I’ve never taken on a project like this before and I’ve never laid a brick — but I’m keen to learn, realistic about my limits, and determined to do things properly.
I’m mainly looking for advice on:
•    Any pitfalls with lowering the floor
•    Whether this approach is structurally and practically sound
•    Anything obvious I might be missing at this stage
•    Whether 1000 mm concrete footings plus 225 mm trench blocks would be sufficient in Lincolnshire
The diagrams are basic, but hopefully clear enough to illustrate what I’m trying to achieve.
Any advice would be very much appreciated.
 

 

 

Edited Monday at 17:54 by seanious
Put text in

[Nickfromwales]

First obvious one, just do a flat roof. You're losing head height for no reason. An EDPM 'rubber' membrane can be flat as a witches tit, or GRP same, with either just needing a whiff of a fall off to one side to function if preserving head height internally is a big box to tick.

 

Sketchup is what a few folk on here use for detailed DIY drawings  Lots of other stuff available, others will comment I'm sure.

 

Going below grade outside renders you being at risk of of flash flood / excessive storm rainwater finding its way in, so you'd need to understand the permeability of the ground and where the water table resides before even considering dropping down.

 

Your foundations are huge, so just ditch the masonry and go for a lightweight twin-wall timber frame arrangement, well insulated, and whack some cheap AC in there for heat / cool. Zero need for the masonry walls, but you probably do need to have a solid floor / slab. You could incorporate a recessed area aka sump in the centre of a poured slab where the treadmill would then live (a bit like a pit in a car mechanics garage), to get you another 200-300mm of useful head-height when in use. I'd prefer my chances tanking and waterproofing that than dropping the whole slab that far down.

[seanious]

Thanks Nick,

 

I did wonder about a flat roof, but was concerned of pooling water but as you suggest, a very slight tilt should be enough giving me enough headroom. One of the reasons I want to go down the brick and block route is so I can hang heavy things (50-60kg) on the wall, since this will also be a workshop.  I also didn't consider a pit for the running machine either, that's a really good idea.

 

I do have the opportunity to place the workshop / gym / summerhouse in a different place in the garden which would actually be 2 meters away from the boundary, and from my understanding that means I could have a dual pitched roof meaning I'd have plenty of space for the running machine.  All I need to do now is wait a few weeks until I'm home to double check the practicality of moving the planned building.

 

I'll have a look at Sketchup and see what that's like. I don't mind paying for software if it does the job and is easy to use.

 

Thanks again

Sean