Level tolerance for garden office foundation

[daisy12345]

Hi all, we bought a flatpack timber frame garden office that is about to be delivered and installed. The site is naturally sloped. We've prepared a raised concrete platform as the foundation (mostly built on the original concrete base for a garage). The garden office will sit on top of a wooden frame base of about 4.3m x 3.3m. When we measure the levelness and flatness of the concrete platform using a 3-meter straight timber, there's about 20mm gap to get it completely level (likely due to the site), but only 5-10mm gap to get the timber to sit flat on the platform. Is this something that can be tolerated / easily fixed when installing the wooden frame? Or do we have to get the platform completely level and flat to be able to proceed please? A few photos below. Thanks!

 

[Notch7]

I would suggest you buy a box of plastic shims and level it up, ideally within 5mm.

If you don’t get it pretty level, you might find you create problems with the frames and roof components.

If the concrete base is bigger than the garden office, I would raise the whole floor base up by 25mm using some sort of plastic - otherwise the floor structure will rot.

 

[daisy12345]

I would suggest you buy a box of plastic shims and level it up, ideally within 5mm.

If you don’t get it pretty level, you might find you create problems with the frames and roof components.

If the concrete base is bigger than the garden office, I would raise the whole floor base up by 25mm using some sort of plastic - otherwise the floor structure will rot.

Thank you very much! I was reading an old thread discussing similar issues (https://www.diynot.com/diy/threads/concrete-base-uneven-for-garden-workshop.525258/) and you and others suggested screed as well? We may consider that too.

 

[foxhole]

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[freddiemercurystwin]

Composite decking joists make good waterproof bearers for use under the timber joists. Readily available off ebay.

 

[daisy12345]

Thanks! @foxhole @freddiemercurystwin Though it looks like for our size the plastic base can get quite expensive... The platform is already raised quite high off one side of the ground due to sloped site, so hopefully any water should drain reasonably well? And the wooden frame base coming with the kit is said to be pressure treated. We are trying to avoid raising the base more from here for fear of planning permissions...

 

[freddiemercurystwin]

I would be very wary of siting your timber straight on the concrete, pressure treated r not, it will have a reduced lifespan. You could consider adjustable decking supports, that would create a barrier between the concrete and the deck and would also solve your slope problem. there are plenty of others available if you google them. eg https://www.mulberrydecking.com/pro...-pedestal-28-40mm-10-pack-adjustable-shed-fee

How high is the building you're buying?

 

[daisy12345]

I would be very wary of siting your timber straight on the concrete, pressure treated r not, it will have a reduced lifespan. You could consider adjustable decking supports, that would create a barrier between the concrete and the deck and would also solve your slope problem. there are plenty of others available if you google them. eg https://www.mulberrydecking.com/pro...-pedestal-28-40mm-10-pack-adjustable-shed-fee

How high is the building you're buying?

Thank you! We are looking into those decking risers now. The building itself is 2.48m tall at the ridge (https://dunsterhouse.co.uk/titania-garden-office-w4-3m-x-d3-3m). We are on a sloped site, so think we can get away with the raised platform as it's about level to the highest point of the surrounding ground. Also because there was a pre-existing concrete base where an old garage and an old shed used to sit, before we moved in. We just poured around 6cm of concrete on top:

 

[daisy12345]

Trying to figure out:
1) If we can replace the timber bearers altogether with those adjustable decking risers, and if so, how many risers do we need?
2) If we have to keep the timber bearers, and put the risers under the whole base structure, do we have to place one riser at each crossing point?

Thanks!

 

[Notch7]

Maybe some recycled plastic battens, they are solid and not too expensive.

Recycled Plastic Flexible Batten - 3m x 50mm x 25mm

www.rockwellbuildingplastics.co.uk

The floor joists don’t need full support, short pieces can be used at say 400 centres.

obviously as you have a sloping slab, you will need spacers / wedges to level up.

 

[Notch7]

Thank you! We are looking into those decking risers now. The building itself is 2.48m tall at the ridge (https://dunsterhouse.co.uk/titania-garden-office-w4-3m-x-d3-3m). We are on a sloped site, so think we can get away with the raised platform as it's about level to the highest point of the surrounding ground. Also because there was a pre-existing concrete base where an old garage and an old shed used to sit, before we moved in. We just poured around 6cm of concrete on top:

Looking at that slab, I would say absolutely do not put timber directly onto that - it will rot in a few years.

raising it by an inch will help keep the timber dry massively increasing lifespan.

 

[daisy12345]

Maybe some recycled plastic battens, they are solid and not too expensive.

Recycled Plastic Flexible Batten - 3m x 50mm x 25mm

www.rockwellbuildingplastics.co.uk

The floor joists don’t need full support, short pieces can be used at say 400 centres.

obviously as you have a sloping slab, you will need spacers / wedges to level up.

Thank you again! This solution looks more cost efficient
Spacers / wedges are shims right? like these: https://www.screwfix.com/p/broadfix-assorted-plastic-shims-medium-200-pcs/80408

 

[daisy12345]

Looking at some of those recycled plastic battens and composite decking joists, not sure if they are strong enough to take heavy weights like a whole garden office? And shims don't seem to be able to bear heavy load either? @Notch7 @freddiemercurystwin

If not probably still need to go for the adjustable risers

 

[conny]

The plastic battens look ideal. One length under each joist, including the outer frame ends will raise the base by 24mm.
Cut 1" shorter at each end they will be recessed and unobtrusive. Will certainly help to preserve the wood from rotting by keeping it off the wet/damp concrete. You could always fit some trim around the bottom edge to hide any gaps that may be on show.

 

[Notch7]

Looking at some of those recycled plastic battens and composite decking joists, not sure if they are strong enough to take heavy weights like a whole garden office? And shims don't seem to be able to bear heavy load either? @Notch7 @freddiemercurystwin

If not probably still need to go for the adjustable risers

The load on each shim wouldnt be that great as it would be spread over quite a few.

I built my garden office on a foundation of 3 concrete strips laid with levelled shuttering, however I packed the joists up with upvc 30 x 9 section. I’ve had no problems, it’s a garden office 4.5m x 5m built to building regs standards so a quite heavy structure.