Laying loft boards directly onto joists is possible in some cases, but there are a few important factors to consider first. The main concern is whether the joists can structurally support the additional weight of the loft boards and anything stored on them over time. Here are some quick answers to key questions about laying loft boards directly on joists:
Quick Answers
- Laying loft boards directly on joists is possible if the joists are strong enough to hold the weight.
- Solid timber joists are generally strong enough to take the weight without extra support.
- engineered joists may require extra noggins between the joists to spread the load.
- The maximum joist spacing for overlaying boards is around 400-450mm for domestic floor joists.
- 18mm thick OSB3 boards are a popular choice for loft boarding.
- Boards should be fixed with screws into the joists at 300-450mm spacings.
- Boards may need cutting around access points like loft hatches.
- Leave a small expansion gap between boards and perimeter walls.
- Joists and boards should be checked for level before fixing the boards.
- Insulation can be laid over the top of the boarded loft floor.
Are the joists strong enough?
The key factor in determining if loft boards can be laid directly onto joists is whether the joists are strong enough to take the additional weight.
Solid timber joists are generally robust enough to support loft boards and storage without any extra support. However, some engineered timber joists, like I-beams, may require additional strengthening between the joists before boards can be safely overlaid.
Domestic floor joist spacing
For domestic floor joists in a loft space, the maximum spacing between joists that boards can span without extra support is typically around 400-450mm. If you have joists set 600mm apart or more, you would need to install extra noggins between them to reduce the span before overlaying boards across the whole loft space.
Calculating joist capacities
To check more precisely, the capacity of the existing joists should be calculated to ensure they can support the estimated loads. This would take into account the span and spacing of the joists, their material, depth and profile.
Solid softwood joists tend to be capable of supporting loads of around 20 to 40 kg per square metre. But smaller or engineered joists may have lower capacities, so always check manufactures specifications.
As a guide, 18mm OSB3 tongue and groove boards weigh around 15kg per square metre. So with items stored on top, the total load could be around 25-50kg per metre squared.
Choosing and fixing loft boards
Once you have confirmed the joists can take the expected loading, the next step is choosing and fixing suitable loft boards.
Board options
OSB and plywood boards are commonly used for boarding loft spaces:
- OSB – Oriented Strand Board is a popular loft boarding material made from compressed wood chips and resin. 18mm thick OSB3 boards with T&G edges are readily available.
- Plywood – Structural plywood also provides a robust loft flooring substrate. 18mm thick plywood boards are suitable.
- Chipboard – Basic chipboard is weaker and tends to sag, so OSB or plywood would be better options.
OSB3 boards are typically a cost-effective option that provide sufficient strength for domestic loft boarding.
How to fix the boards
Boards are fixed by screwing down into the joists below. Here are some tips on fixing them:
- Boards should be screwed at 300-450mm spacings along the joists.
- Ensure screws are long enough to penetrate the joists by at least 30mm.
- Predrill clearance holes in the boarding to prevent splitting.
- Use countersunk head wood screws for a flush finish.
- Stagger boards so joints do not all align over the same joists.
Proper fixing is important to prevent creaking and movement later on. Take care around any access points like loft hatches and ensure boards are cut neatly around the openings.
Preparing for boarding
Before starting to fix down the loft boards, it’s important to do some preparatory work:
Check for level
The joists and perimeter walls should be checked with a spirit level to identify any uneven areas. Pack out any dips in the joists with wooden shims before fixing boards to ensure a flat, level surface.
Allow for expansion gaps
Leave a small 5-10mm gap between the edges of the boards and perimeter walls. This allows natural expansion and contraction of the boards through temperature/humidity changes without causing distortion. Fit skirting after to cover the gap.
Plan the layout
Dry lay the intended boards across the joists first to determine the optimal layout and identify where any cuts are needed. Number and mark the boards to keep track.
Install vapour barrier
For a boarded loft space to remain comfortable, it’s advisable to install a vapour barrier like polythene sheeting on the underside of the joists. This prevents damp air leaking through from below.
Finishing touches for boarded lofts
Once the core boarding is complete, there are some finishing jobs that can further enhance the boarded loft space:
Fit skirting
Install timber skirting boards around the perimeter walls using shims packed off the wall to allow for the expansion gap behind. This gives a neat finish concealing the edges of the boards.
Install insulation
Insulation can be installed over the top of the boarded floor to improve thermal comfort and energy efficiency. Use mineral wool between the joists with a cover board overlaid.
Fit knee boards
Optional knee boards create a smooth surface and allow easier movement around near parapet walls. They are fitted between the wall and joists.
Lay floor coverings
To give a warmer, more comfortable feel underfoot, the boards can be overlaid with foam underlay and flooring such as carpet, vinyl or laminate flooring.
Conclusion
It is possible to install loft boards directly over joists, provided some key considerations are addressed:
- The joists must be strong enough to support the additional loads.
- Solid timber joists are normally suitable, but engineered joists may require extra support.
- Boards should be properly fixed with screws at regular spacing into joists.
- Joists and boards should be levelled, and allowance made for expansion gaps.
- Insulation and vapour barriers will help control humidity and prevent condensation issues.
With appropriate joists and a professional installation approach, laying loft boards directly onto the joists can be a cost-effective and practical solution for boarded loft spaces. Always check the specific joists used in your project before overlaying boards across them.
FAQs
Is OSB strong enough for loft boards?
Yes, OSB3 tongue and groove boards around 18mm thick are sufficiently strong for use as loft boarding in domestic spaces when installed correctly on suitable joists. The boards spread loads effectively while providing a smooth surface.
Should insulation go on top of loft boards?
Insulation is normally installed on top of the finished loft boarding. Lay mineral wool insulation rolls between the joists, then overlay it with a rigid insulation board to provide a flat surface. This helps maximise warmth and prevent condensation.
How thick should loft boards be?
For domestic loft spaces, 18mm is generally the minimum thickness recommended for overlay boards. At this thickness OSB3 or plywood boards are robust enough to prevent sagging over the joist spans. Thicker boards up to 25mm can be used for heavier storage loads if needed.
How do you secure loft boards?
Loft boards should be screwed down into the joists below. Use countersunk wood screws at least 30mm longer than the board thickness. Space the screws at around 300-450mm intervals staggered across the boards for a secure fixing. Some adhesive can also be applied for extra strength.
Should loft boards be nailed or screwed?
Screws provide a more secure and long-lasting fixing for loft boards compared to nails. They allow the boards to be fastened snugly down onto the joists without the high impact force of hammering. Pre-drilling pilot holes prevents any splitting.
How far apart should loft joists be for boarding?
For 18mm thick OSB3 or plywood loft boards, the maximum joist spacing should be around 400-450mm. Wider than this and the boards could risk excessive sagging unless noggins are inserted between the joists for support. Closer joist spacing provides extra strength.
Do loft boards need treated timber?
In most cases, loft boarding does not need to be treated timber. Regular OSB3 or plywood boards are suitable when installed in a dry, ventilated loft space. The joists and roof structure provide the main damp and weather protection.
Should loft boards be supported on batons?
Installing loft boards on timber battens or noggins screwed to the joists below is another option, providing a 150mm clear void beneath. This allows easier fitting of insulation over and between the joists before fixing boards over the top.
Key Considerations for Boarding Lofts
Here is a summary of the key considerations when planning to install loft boards directly over joists:
Joist Sizes and Spacing
- Joist size and spacing must be within limits to support loads without sagging
- Max 400-450mm spacing for 18mm boards on domestic floor joists
- Solid timber joists normally suitable if in good condition
- Some engineered joists may require additional support
Board Types
- OSB3 and plywood boards at least 18mm thick are suitable
- OSB3 is cost effective and widely available
- Chipboard is prone to sagging so avoid
Preparation
- Check joists are level and pack if needed
- Include vapour barrier on underside
- Allow perimeter expansion gaps
Fixing Boards
- Screw fix into joists at 300-450mm spacings
- Use countersunk wood screws, predrill boards
- Stagger board joints between joists
Finishing Touches
- Insulate between and over the joists
- Install skirting boards around perimeter
- Optional floor coverings for comfort
Carefully checking these factors will help ensure a structurally sound, durable and thermally efficient boarded loft space.
Installing Loft Boards: Step-by-Step
Follow these key steps for safely installing loft boards directly over joists:
1. Calculate Joist Capacities
Check sizes, centres and material to confirm joists can support estimated loading from boards and storage.
2. Install Vapour Barrier
Staple 500 gauge polythene sheeting to underside of joists as a vapour barrier.
3. Check Levels
Use a long spirit level to identify any uneven joists or perimeter walls. Pack if needed.
4. Dry Lay Boards
Dry layout boards across joists to determine optimal layout. Number and mark boards.
5. Fix Boards
Fix boards using countersunk wood screws at 300-450mm centres into joists. Stagger joints.
6. Trim Openings
Measure and neatly cut boards to fit around any loft access points or openings.
7. Leave Expansion Gaps
Maintain 5-10mm gaps between boards/walls to allow for expansion.
8. Insulate
Fill between joists with insulation. Overlay with rigid board.
9. Fit Skirting
Install timber skirting off perimeter walls to cover board edges after fixing.
10. Floor Coverings
Top with underlay and flooring as desired for improved comfort.
Pro Tips
Allow for access requirements
Consider where fixed and retractable loft ladders will be located and ensure full floorboards can be installed around them.
ventilation
Maintain airflow under boards by keeping eaves vents clear. This prevents damp and condensation.
Safety rails
Consider fitting guard rails around any lightwells or drops to improve safety.
Joist top-up
If joists are uneven, packers glued and screwed underneath can raise them level, rather than packing down between joists.
Soundproofing
Use acoustic insulation between joists to reduce noise transmission to rooms below the boarded loft space.
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Safety Warnings
There are some important safety precautions to take when boarding loft spaces:
- Use scaffold boards and crawling boards to safely move around the loft
- Wear dust masks when cutting boards to avoid inhaling particles
- Use safety eyewear when using power tools
- Take care when working at heights near openings or stairwells
- Distribute weight when loading stacks of boards into loft space
- Ensure adequate lighting or task lighting
Proper precautions will help avoid potential risks of falls, injuries or inhalation of dust when working in loft spaces.
Conclusion
Laying loft boards directly over joists can provide a cost-effective and practical solution for utilising loft space. But care needs to be taken to ensure the joists are suitable and boards are fixed securely.
Key factors include checking joist sizes, spacing and condition, selecting appropriate board materials, allowing for expansion gaps, and fixing boards with suitable wood screws into joists.
With good preparation and installation, boarding over joists can maximise usable storage space. But always check the specific joists used on a project are adequate before proceeding. Seeking professional advice is recommended if unsure.
