What is a trimmer and trimming joist?

When cutting a joist to create a loft hatch opening, you need trimming timbers to maintain structural integrity. A trimmer runs across the opening (perpendicular to the joists) supporting the cut joist ends. Trimming joists run parallel to the existing joists on each side of the opening. These are typically the same depth as the existing joists.

What size loft hatch do I need?

Standard loft hatches are 726×566mm (fitting between joists at 600mm centres with a trimmer). For loft ladder access, 726×566mm is the minimum. If you plan to board the loft for storage or convert it, a larger opening of 900×600mm or more may be needed. Fire-rated hatches are available in standard sizes.

Does my loft hatch need to be fire rated?

If the loft hatch opens into a protected escape route (hallway or landing), Building Regulations require it to be FD30 fire rated (30-minute resistance). This means using a fire-rated hatch, 12.5mm fire-rated plasterboard lining around the opening, and intumescent strip. The hatch lid must also meet the FD30 standard.

Should I insulate the loft hatch?

Yes, loft hatches are a significant source of heat loss. The hatch lid should be insulated to the same standard as the surrounding loft insulation — typically 270mm of mineral wool or equivalent. A rigid insulation board (such as 100mm Celotex) can be bonded to the hatch lid. Also install draught strip around the frame to prevent air leakage.

How do I install a loft hatch in an existing ceiling?

Mark the opening between joists, cut the ceiling plasterboard, cut one joist if spanning more than one bay, install trimmer timbers using joist hangers, fit the lining boards, install the hatch frame and lid, add insulation and draught seal. If cutting a joist, use the same section size for trimmers and support with joist hangers at each end.

TradeCalculator

Loft Hatch Calculator

Calculate opening trimming timbers, fire-rated board lining, insulation blanket and draught seal for loft hatch installation.

Opening Width

Standard: 726mm

Opening Length

Standard: 566mm

Joist Depth

Depth of existing ceiling joists

Fire Rating

FD30 if on protected escape route

Include insulation blanket

Insulate lid to match loft insulation

Include draught seal

Continuous seal around frame

Loft Hatch Price (optional)

Pre-made hatch unit cost

How We Calculate This

This calculator determines the materials needed to form a loft hatch opening: trimming timbers, joist hangers, fire-rated lining (if required), insulation and draught seal.

The formula

Trimming: 2 trimmers + 2 trimming joists
Joist hangers: 4 (one per corner connection)
Fire lining = Perimeter × Joist depth + Lid area
Draught seal = Opening perimeter

Installation guide

Use joist hangers for all trimmer-to-joist connections
Line opening with fire-rated plasterboard if on escape route
Insulate the hatch lid to match loft insulation depth
Install continuous draught seal around the frame

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.