Garden Office Foundation - Concrete Base

[chrisgm]

I'm about to start a self build garden home office of 3.5m x 4m - For the foundation, before it there was a large outdoor summer house there on a concrete base. The concrete is not a smooth finish, but a rough edged finish to the surface. My question is, I know the concrete is suitable and strong but as the surface is a rough finish whats the best way to put my wooden base frame down on it? Will it be ok to set it on there or should I still look to raise it? My thought process was water sitting on the concrete and the wooden base just sat in it?

[Russell griffiths]

Row of blocks laid down, then build the timberframe on top of this, that will allow you to insulate the floor inside. 

 

You cannot build it straight of that slab if you are considering using it for anytime apart from the middle of summer as it will be like Siberia in there. 

[Nickfromwales]

23 minutes ago, chrisgm said:

I'm about to start a self build garden home office of 3.5m x 4m - For the foundation, before it there was a large outdoor summer house there on a concrete base. The concrete is not a smooth finish, but a rough edged finish to the surface. My question is, I know the concrete is suitable and strong but as the surface is a rough finish whats the best way to put my wooden base frame down on it? Will it be ok to set it on there or should I still look to raise it? My thought process was water sitting on the concrete and the wooden base just sat in it?

Is the existing concrete base the correct size for your new office? If so simply lay a course of thermalite block around the perimeter, cutting them at around 70mm thick as a thermal break. Bed these down in 15-20mm of cement and tap down accordingly to get a nice straight ( flat and level ) top edge using the cement taking up any undulation. Use that then as an insulated footer for the stud walls with the thermalite blocks providing at least some thermal break.

Then you can blind the remaining slab surface with sharp sand and lay 50mm or 75mm of EPS ( polystyrene ) insulation directly onto the slab. Put two layers of 18mm OSB3 atop the insulation, with the second layer perpendicular to the first and you'll then have a nice start point to get the stud frame walls up and have a pretty decent separation from the cold of mother earth.

DPM goes over the thermalite and the EPS to make the damp barrier, so try to match that level as best you can by deciding on the thickness of insulation you desire and then cutting the thermalite blocks to suit. Leave the DPM overhang the thermalite by a good 150mm and then clad over it when sheathing the outside walls with your chosen rain barrier to keep it down-turned over the thermalite ( as that will attempt to bridge damp up / outward so cannot touch anything porous ).

I've done a few 'garage conversions' with this method, and the double layer of OSB ( tongue and groove 2400x600mm sheets ) over the loose insulation feels rock solid underfoot. You will deffo need to blind the slab with sharp sand first ( after laying the perimeter blocks ) and get that nice and uniform for this to have little / no 'wobble' possible. Invest some time there as it'll pay dividends later on  

3x2" timber at 400mm centres vertically for stud walls, with 25mm EPS across the whole of the internal walls and then screw 11mm OSB3 through the EPS and into the studs for final internal finish. Looks fine when painted, but you could plasterboard if you need a 'posh' finish.

Monopitch roof is a doddle, and again, just timber for simplicity. Make the walls all level and fit some furring pieces on top to create a downward slope, from front to back, so the guttering is at the rear. 6x2 pearlings at 600mm centres atop the furring pieces, from left to right as you look at the front, and 18mm OSB3 atop the pearlings and whatever you want on top of that ( EDPM / felt / GRP etc ) but beware metal sheets as I found the rain made it nigh on impossible to have a conversation when it was lashing down. Solution will be; acoustic insulation and SB plasterboard most prob, but will be a job for a rainy day   Depending upon your choice of roof material you can just get away with insulating the roof void with cheap attic roll in the depth of the 6x2"s and covering with green plasterboard.

[chrisgm]

11 hours ago, Nickfromwales said:

Is the existing concrete base the correct size for your new office? If so simply lay a course of thermalite block around the perimeter, cutting them at around 70mm thick as a thermal break. Bed these down in 15-20mm of cement and tap down accordingly to get a nice straight ( flat and level ) top edge using the cement taking up any undulation. Use that then as an insulated footer for the stud walls with the thermalite blocks providing at least some thermal break.

Then you can blind the remaining slab surface with sharp sand and lay 50mm or 75mm of EPS ( polystyrene ) insulation directly onto the slab. Put two layers of 18mm OSB3 atop the insulation, with the second layer perpendicular to the first and you'll then have a nice start point to get the stud frame walls up and have a pretty decent separation from the cold of mother earth.

DPM goes over the thermalite and the EPS to make the damp barrier, so try to match that level as best you can by deciding on the thickness of insulation you desire and then cutting the thermalite blocks to suit. Leave the DPM overhang the thermalite by a good 150mm and then clad over it when sheathing the outside walls with your chosen rain barrier to keep it down-turned over the thermalite ( as that will attempt to bridge damp up / outward so cannot touch anything porous ).

I've done a few 'garage conversions' with this method, and the double layer of OSB ( tongue and groove 2400x600mm sheets ) over the loose insulation feels rock solid underfoot. You will deffo need to blind the slab with sharp sand first ( after laying the perimeter blocks ) and get that nice and uniform for this to have little / no 'wobble' possible. Invest some time there as it'll pay dividends later on  

3x2" timber at 400mm centres vertically for stud walls, with 25mm EPS across the whole of the internal walls and then screw 11mm OSB3 through the EPS and into the studs for final internal finish. Looks fine when painted, but you could plasterboard if you need a 'posh' finish.

Monopitch roof is a doddle, and again, just timber for simplicity. Make the walls all level and fit some furring pieces on top to create a downward slope, from front to back, so the guttering is at the rear. 6x2 pearlings at 600mm centres atop the furring pieces, from left to right as you look at the front, and 18mm OSB3 atop the pearlings and whatever you want on top of that ( EDPM / felt / GRP etc ) but beware metal sheets as I found the rain made it nigh on impossible to have a conversation when it was lashing down. Solution will be; acoustic insulation and SB plasterboard most prob, but will be a job for a rainy day   Depending upon your choice of roof material you can just get away with insulating the roof void with cheap attic roll in the depth of the 6x2"s and covering with green plasterboard.

Super super detailed thanks very much - Luckily the concrete base there is the same size as office I'm going to build. I will definitely review as I go through each stage. the main bit I wanted to make sure was having air go through underneath and the base not just sat on the concrete with rain water near it.I'm actually following Ali Dymock on youtubes videos as they are very similar to what I'm looking to do.

[Temp]

For my outbuilding I arranged for the top of the slab to be above ground level and slightly smaller than the building so rain running down the cladding would drop onto the ground not the top of the slab. I also raised the frame on a course of bricks and a DPC so no way the frame can sit in water.