Timber Frame Bases for Garden Room Kits on Adjustable Posts
Published on January 21, 2026
Garden room kits include floor joists or base frame components in their assembly instructions. These components are designed to rest on solid, level surfaces—typically concrete slabs or continuous timber platforms.
When installing on adjustable posts instead of slabs, a structural timber base frame is required first. The kit's floor joists then install on top of this frame.
What Manufacturers Specify
Garden room kit instructions typically state:
"Place foundation joists on level foundation surface."
"Install damp-proof membrane between joists and foundation."
These instructions assume a continuous level surface already exists—either poured concrete or a constructed platform. The kit's floor joists are not engineered to span between isolated support points or carry structural loads independently.
Installing kit floor joists directly on adjustable posts without an intermediate structural frame results in inadequate support. Floor components flex between posts, causing deflection that affects wall alignment and door operation.
Structural Base Frame Requirements
To install garden room kits on adjustable posts, construct a structural timber base frame that:
- Provides continuous support for kit floor joists
- Distributes building loads to post locations
- Maintains rigidity independent of kit floor system
Frame specifications:
- Timber dimensions: 50×100mm treated lumber minimum (increase for larger structures)
- Joist spacing: 600-800mm centers
- Assembly: Square corners, rigid connections using structural screws or metal brackets
- Post spacing: 1200-1500mm intervals under primary beams
Installation sequence:
- Construct timber frame to building dimensions
- Position adjustable posts at specified intervals beneath frame
- Either:
- Lift completed frame and attach posts from below, or
- Build frame inverted, attach posts, then flip assembly upright
- Adjust post heights until frame is level
- Install kit floor joists on top of leveled structural frame per manufacturer instructions
Why This Configuration Works
The structural base frame:
- Distributes building weight across multiple post locations
- Provides continuous level surface for kit floor joists as manufacturer intended
- Allows post height adjustment to compensate for ground variation
- Elevates structure for drainage and ventilation
Common Questions
Why can't kit floor joists rest directly on posts?
Kit floor joists are engineered for installation on continuous surfaces, not for spanning between isolated supports. Adding more posts does not address this—the joists themselves lack the structural capacity for point loading.
How many posts does the structural frame require?
Typical spacing: one post every 1200-1500mm under primary frame beams.
Example quantities:
- 3×4m structure: 6-8 posts
- 4×5m structure: 10-12 posts
Can the frame be relocated?
Yes. Both adjustable posts and timber frame can be disassembled, transported, and reinstalled at new locations. Frame remains reusable.
Summary
- Garden room kit floor joists are designed for continuous surface installation
- Installing on adjustable posts requires structural base frame first
- Frame specifications: 50×100mm treated timber, 600-800mm joist spacing
- Post spacing: 1200-1500mm intervals
- Installation: Build frame, attach posts, level, then install kit floor system on top
- After kit installation, verify level and readjust posts if necessary
The structural base frame enables garden room kit installation on adjustable posts. Frame provides load distribution and continuous support while posts compensate for ground variation and allow height adjustment.