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Aim To evaluate fracture resistance and gap/void presence of root‐filled mandibular molars restored with 2 bulk‐fill and 1 conventional resin composites, with or without a glass‐ionomer cement (GIC) base. Methods Coronal access and mesio‐occlusal (MO) cavities were prepared, then root canal treatment was performed on 30 mol/L. The teeth were randomly divided, according to the cavity volume, into 6 experimental groups (N = 5) and restored with conventional/light‐cured (Ceram‐X), bulk‐fill/light‐cured (SureFil SDR) or bulk‐fill/dual‐cured (Core‐X Flow) with/without a 2‐mm thick GIC base. Gaps and voids (%) were determined using microcomputed tomography. Intact teeth and unrestored teeth were used as negative and positive controls. Fracture load (N) was determined using a universal testing machine. Results No significant difference in fracture resistance or gap/void formation was found among the 3 resin composites. GIC‐base groups revealed significantly lower fracture strength than intact teeth, while fracture strengths of no GIC‐base groups were not significantly different from intact teeth. GIC‐base groups revealed significantly more gaps and voids in the area of the GIC than the resin composite. Conclusion Conventional and bulk‐fill resin composites provided similar fracture resistance and gaps/voids in root‐filled molars with MO cavities. Placing a GIC base decreased fracture resistance and increased gap/void formation.